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Versions: (draft-perlman-trill-rbridge-protocol) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 RFC 6325

TRILL Working Group                                        Radia Perlman
INTERNET-DRAFT                                          Sun Microsystems
Intended status: Proposed Standard                   Donald Eastlake 3rd
Expires: September 5, 2009                              Stellar Switches
                                                          Dinesh G. Dutt
                                                           Cisco Systems
                                                             Silvano Gai
                                                           Nuova Systems
                                                          Anoop Ghanwani
                                                           March 6, 2009

                 Rbridges: Base Protocol Specification

Status of This Document

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79. This document may contain material
   from IETF Documents or IETF Contributions published or made publicly
   available before November 10, 2008.  The person(s) controlling the
   copyright in some of this material may not have granted the IETF
   Trust the right to allow modifications of such material outside the
   IETF Standards Process.  Without obtaining an adequate license from
   the person(s) controlling the copyright in such materials, this
   document may not be modified outside the IETF Standards Process, and
   derivative works of it may not be created outside the IETF Standards
   Process, except to format it for publication as an RFC or to
   translate it into languages other than English.

   Distribution of this document is unlimited.  Comments should be sent
   to the TRILL working group mailing list <rbridge@postel.org>.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
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   The list of current Internet-Drafts can be accessed at

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R. Perlman, et al                                               [Page 1]

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   RBridges provide optimal pair-wise forwarding with zero
   configuration, safe forwarding even during periods of temporary
   loops, and support for multipathing of both unicast and multicast
   traffic. They achieve these goals using IS-IS routing and
   encapsulation of traffic with a header that includes a hop count.

   RBridges are compatible with previous IEEE 802.1 customer bridges as
   well as IPv4 and IPv6 routers and end nodes. They are as invisible to
   current IP routers as bridges are and, like routers, they terminate
   the bridge spanning tree protocol.

   The design supports VLANs and optimization of the distribution of
   multi-destination frames based on VLAN and IP derived multicast
   groups.  It also allows forwarding tables to be sized according to
   the number of RBridges (rather than the number of end nodes), which
   allows internal forwarding tables to be substantially smaller than in
   conventional bridges.


   Many people have contributed to this design, including, in alphabetic
   order, Alia Atlas, Ayan Banerjee, Suresh Boddapati, Caitlin Bestler,
   Stewart Bryant, James Carlson, Dino Farinacci, Don Fedyk, Bill
   Fenner, Eric Gray, Joel Halpern, Andrew Lange, Israel Meilik, David
   Melman, Erik Nordmark, Sanjay Sane, Pekka Savola, Matthew Thomas, Joe
   Touch, and Mark Townsley.  We invite you to join the mailing list at

R. Perlman, et al                                               [Page 2]

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Table of Contents

      Status of This Document....................................1

      1. Introduction............................................7
      1.1 Algorhyme V2, by Ray Perlner...........................8
      1.2 Normative Content and Precedence.......................8
      1.3 Terminology and Notation in this document..............8
      1.4 Acronyms...............................................9

      2. RBridges...............................................12
      2.1 End Station Addresses.................................13
      2.2 RBridge Encapsulation Architecture....................14
      2.2.1 Known-Unicast.......................................16
      2.2.2 Multi-destination...................................16
      2.3 RBridges and VLANs....................................17
      2.3.1 Link VLAN Assumptions...............................17
      2.4 RBridges and IEEE 802.1 Bridges.......................18
      2.4.1 RBridge and 802.1 Layering..........................18
      2.4.2 Incremental Deployment..............................19

      3. Details of the TRILL Header............................21
      3.1 TRILL Header Format...................................21
      3.2 Version (V)...........................................21
      3.3 Reserved (R)..........................................22
      3.4 Multi-destination (M).................................22
      3.5 TRILL Header Options..................................22
      3.6 Hop Count.............................................23
      3.7 RBridge Nicknames.....................................24
      3.7.1 Egress RBridge Nickname.............................24
      3.7.2 Ingress RBridge Nickname............................25
      3.7.3 RBridge Nickname Selection..........................25

      4. Other RBridge Design Details...........................27
      4.1 Ethernet Data Encapsulation...........................27
      4.1.1 VLAN Tag Information................................29
      4.1.2 Inner VLAN Tag......................................30
      4.1.3 Outer VLAN Tag......................................30
      4.1.4 Frame Check Sequence (FCS)..........................31
      4.2 Link State Protocol (IS-IS)...........................31
      4.2.1 IS-IS RBridge Identity..............................31
      4.2.2 IS-IS Instances.....................................32
      4.2.3 TRILL IS-IS Frames..................................32
      4.2.4 TRILL IS-IS Link Protocol...........................33 Hellos and MTU....................................34 Hello VLAN Tagging................................34 Hello Contents....................................36 VLAN Mapping Within a Link........................38

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Table of Contents Continued P2P Hello Links...................................39 Designated RBridge................................39 Appointed VLAN-x Forwarder........................40 TRILL LSP Information.............................41
      4.2.5 TRILL ESADI.........................................43 TRILL ESADI Participation.........................46 TRILL ESADI Information...........................46
      4.3 Distribution Trees....................................46
      4.3.1 Distribution Tree Calculation and Checks............48
      4.3.2 Pruning the Distribution Tree.......................49
      4.3.3 Tree Distribution Optimization......................50
      4.3.4 Forwarding Using a Distribution Tree................51
      4.4 Frame Processing Behavior.............................52
      4.4.1 Receipt of a Native Frame...........................52 Native Unicast Case...............................53 Native Multicast and Broadcast Frames.............53
      4.4.2 Receipt of a TRILL Frame............................54 TRILL IS-IS Frames................................55 TRILL ESADI Frames................................55 TRILL Data Frames.................................55
      4.4.3 Receipt of a Control Frame..........................57
      4.5 IGMP, MLD, and MRD Learning...........................57
      4.6 End Station Address Details...........................58
      4.6.1 Learning End Station Addresses......................58
      4.6.2 Forgetting End Station Addresses....................60
      4.6.3 Shared VLAN Learning................................61
      4.7 RBridge Ports.........................................61
      4.7.1 RBridge Port Configuration..........................62
      4.7.2 RBridge Port Structure..............................63
      4.7.3 BPDU Handling.......................................65 Receipt of BPDUs..................................66 Root Bridge Changes...............................66 Transmission of BPDUs.............................66
      4.7.4 Dynamic VLAN Registration...........................67

      5. Addresses, Configuration Parameters, and Constants.....68

      6. Security Considerations................................70
      6.1 VLAN Security Considerations..........................70
      6.2 BPDU/Hello Denial of Service Considerations...........71

      7. Assignment Considerations..............................72
      7.1 IANA Considerations...................................72
      7.2 IEEE Registration Authority Considerations............72

      8. Normative References...................................74

      9. Informative References.................................75

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Table of Contents Continued

      Appendix A: Incremental Deployment Considerations.........76
      A.1 Link Cost Determination...............................76
      A.2 Appointed Forwarders and Bridged LANs.................76
      A.3 Wiring Closet Topology................................78
      A.3.1 The RBridge Solution................................79
      A.3.2 The VLAN Solution...................................79
      A.3.3 The Spanning Tree Solution..........................79
      A.3.4 Comparison of Solutions.............................80

      Appendix B: Trunk and Access Port Configuration...........81

      Appendix C: Multipathing..................................82

      Appendix D: Determination of VLAN and Priority............84

      Appendix Z: Revision History..............................85
      Changes from -03 to -04...................................85
      Changes from -04 to -05...................................86
      Changes from -05 to -06...................................87
      Changes from -06 to -07...................................87
      Changes from -07 to -08...................................89
      Changes from -08 to -09...................................90
      Changes from -09 to -10...................................91
      Changes from -10 to -11...................................92
      Changes from -11 to -12...................................92

      Authors' Addresses........................................94
      Copyright and IPR Provisions..............................95

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Table of Figures

      Figure 2.1: Interconnected RBridges.......................14
      Figure 2.2: An Ethernet Encapsulated TRILL Frame..........14
      Figure 2.3: A PPP Encapsulated TRILL Frame................15
      Figure 2.4: RBridge Port Model............................19
      Figure 3.1: TRILL Header..................................21
      Figure 3.2: Options Area Initial Flags Octet..............22
      Figure 4.1: TRILL Data Encapsulation over Ethernet........28
      Figure 4.2: VLAN Tag Information..........................29
      Figure 4.3: TRILL Core IS-IS Frame Format.................33
      Figure 4.4: RBridge Link with Intervening Device..........34
      Figure 4.5: TRILL ESADI Frame Format......................45
      Figure 4.6: Detailed RBridge Port Model...................64
      Figure A.1: Link Cost of a Bridged Link...................76
      Figure A.2: Wiring Closet Topology........................78
      Figure C.1: Multi-Destination Multipath...................82
      Figure C.2: Known Unicast Multipath.......................83

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1. Introduction

   In traditional IPv4 and IPv6 networks, each subnet has a unique
   prefix. Therefore, a node in multiple subnets has multiple IP
   addresses, typically one per interface. This also means that when an
   interface moves from one subnet to another, it changes its IP
   address. Administration of IP networks is complicated because IP
   routers require significant configuration. Careful IP address
   management is required to avoid creating subnets that are sparsely
   populated, wasting addresses.

   IEEE 802.1 bridges avoid these problems by transparently gluing many
   physical links into what appears to IP to be a single LAN [802.1D].
   However, 802.1 bridge forwarding using the spanning tree protocol has
   some disadvantages:

   o  The spanning tree protocol blocks ports, limiting the number of
      forwarding links, and therefore creates bottlenecks by
      concentrating traffic onto selected links.

   o  Forwarding is not pair-wise shortest path, but is instead whatever
      path remains after the spanning tree eliminates redundant paths.

   o  The Ethernet header does not contain a hop count (or TTL) field.
      This is dangerous when there are temporary loops such as when
      spanning tree messages are lost or components such as repeaters
      are added.

   o  VLANs can partition when spanning tree reconfigures due to a node
      failure or topology change.

   This document presents the design for RBridges (Routing Bridges
   [RBridges]) which implement the TRILL protocol and are poetically
   summarized below.  Rbridges combine the advantages of bridges and
   routers and in most cases they can incrementally replace IEEE
   [802.1Q] or [802.1D] customer bridges.  While they can be applied to
   a variety of link protocols, this specification focuses on IEEE
   [802.3] links.

   For further discussion of the problem domain addressed by RBridges
   see [PAS].

R. Perlman, et al                                               [Page 7]

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1.1 Algorhyme V2, by Ray Perlner

      I hope that we shall one day see
      A graph more lovely than a tree.

      A graph to boost efficiency
      While still configuration-free.

      A network where RBridges can
      Route packets to their target LAN.

      The paths they find, to our elation,
      Are least cost paths to destination!

      With packet hop counts we now see,
      The network need not be loop-free!

      RBridges work transparently,
      Without a common spanning tree.

1.2 Normative Content and Precedence

   The bulk of the normative material in this specification appears in
   Sections 2, 3, and 4 as follows:

      Section 2: general RBridge description
      Section 3: the TRILL header
      Section 4: other TRILL protocol details

   In case of conflict, the order of precedence of these section is as
   follows, with those appearing earlier in this list having precedence
   over those that appear later:

         4 > 3 > 2

1.3 Terminology and Notation in this document

   "TRILL" normally refers to the protocol specified herein while
   "RBridge" refers to the devices that implement that protocol.  The
   second letter in Rbridge is case insensitive. Both Rbridge and
   RBridge are correct.

   This document uses Hexadecimal Notation for MAC addresses. Two
   hexadecimal digits represent each octet (that is, 8-bit byte), giving
   the value of the octet as an unsigned integer. A hyphen separates
   successive octets. This document consistently uses IETF bit ordering

R. Perlman, et al                                               [Page 8]

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   although the physical order of bit transmission within an octet on an
   IEEE [802.3] link is from the lowest order bit to the highest order
   bit, the reverse.

   In this document, Layer 2 frames are divided into three categories
   (control frames, TRILL frames, and native frames) as follows:

      o "control" frames are those with a multicast destination address
         in the range 01-80-C2-00-00-00 to 01-80-C2-00-00-0F or equal to
         01-80-C2-00-00-21. There are two sub-categories of control
         frames as follows:
         - "high level control" frames are those with a destination
            address of 01-80-C2-00-00-00 (BPDU) or 01-80-C2-00-00-21
         -  "low level control" frames are all other control frames.

      o  "TRILL" frames are those that are not control frames and either
         (1) have the TRILL Ethertype or (2) have a multicast
         destination address allocated to the TRILL protocol (see
         Section 7.2). There are three sub-categories of TRILL frames as
         listed below. RBridges silently discard any TRILL frame that
         does not fit one of these three subcategories.
         -  "TRILL IS-IS" frames have the All-IS-IS-RBridges multicast
            address as their destination address.
         -  "TRILL ESADI" frames have the TRILL Ethertype and the All-
            ESADI-RBridges multicast address as the encapsulated
            destination address.
         -  "TRILL data" frames have the TRILL Ethertype but are not
            TRILL ESADI frames.

      o "native" frames are all frames other than TRILL and control

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

1.4 Acronyms

      AllL1ISs - All Level 1 Intermediate Systems

      AllL2ISs - All Level 2 Intermediate Systems

      BPDU - Bridge PDU

      CHbH - Critical Hop-by-Hop

      CItE - Critical Ingress-to-Egress

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      CSNP - Complete Sequence Number PDU

      DA - Destination Address

      EAP - Extensible Authentication Protocol

      ECMP - Equal Cost Multi-Path

      EISS - Extended Internal Sublayer Service

      ESADI - End Station Address Distribution Information

      DRB - Designated RBridge

      FCS - Frame Check Sequence

      GARP - Generic Attribute Registration Protocol

      GVRP - GARP VLAN Registration Protocol

      IEEE - Institute of Electrical and Electronics Engineers

      IGMP - Internet Group Management Protocol

      IP - Internet Protocol

      IS-IS - Intermediate System to Intermediate System

      ISS - Internal Sublayer Service

      LAN - Local Area Network

      LSP - Link State PDU

      MAC - Media Access Control

      MLD - Multicast Listener Discovery

      MRD - Multicast Router Discovery

      MTU - Maximum Transmission Unit

      MVRP - Multiple VLAN Registration Protocol

      NSAP - Network Service Access Point

      P2P - Point-to-point

      PDU - Protocol Data Unit

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      PPP - Point-to-Point Protocol

      RBridge - Routing Bridge

      RPF - Reverse Path Forwarding

      SA - Source Address

      SPF - Shortest Path First

      TLV - Type, Length, Value

      TRILL - TRansparent Interconnection of Lots of Links

      VLAN - Virtual Local Area Network

      VRP - VLAN Registration Protocol

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2. RBridges

   This section provides a high level overview of RBridges, which
   implement the TRILL protocol, omitting some details. Sections 3 and 4
   below provide the main specification.

   RBridges run a link state protocol amongst themselves. This gives
   them enough information to compute pair-wise optimal paths for
   unicast, and calculate distribution trees for delivery of frames
   either to unknown MAC destinations or to multicast/broadcast groups.
   [RBridges] [RP1999]

   To mitigate temporary loop issues, RBridges forward based on a header
   with a hop count. RBridges also specify the next hop RBridge as the
   frame destination when forwarding unicast frames across a shared-
   media link, which avoids spawning additional copies of frames during
   a temporary loop.  A Reverse Path Forwarding Check and other checks
   are performed on multi-destination frames to further control
   potentially looping traffic (see Section 4.3.1).

   The first RBridge that a unicast frame encounters in a campus, RB1,
   encapsulates the received frame with a TRILL header that specifies
   the last RBridge, RB2, where the frame is decapsulated. RB1 is known
   as the "ingress RBridge" and RB2 is known as the "egress RBridge".
   To save room in the TRILL header, a dynamic nickname acquisition
   protocol is run among the RBridges to select a 2-octet nickname for
   each RBridge, unique within the campus, which is an abbreviation for
   the 6-octet IS-IS system ID of the RBridge.  The 2-octet nicknames
   are used to specify the ingress and egress RBridges in the TRILL

   Multipathing of multi-destination frames through alternative
   distribution tree roots and ECMP (Equal Cost MultiPath) of unicast
   frames are supported (see Appendix C).

   RBridges run the IS-IS [ISO10589] election protocol to elect a
   "Designated RBridge" (DRB) on each bridged LAN ("link").  As with an
   IS-IS router, the DRB may give a pseudonode name to the link, issue
   an LSP (Link State PDU) on behalf of the pseudonode, and issues CSNPs
   (Complete Sequence Number PDUs) on the link.  Additionally, the DRB
   specifies which VLAN will be the Designated VLAN used for
   communication between RBridges on that link.

   The DRB either encapsulates/decapsulates all data traffic to/from the
   link, or, for load splitting, delegates this responsibility, for one
   or more VLANs, to other RBridges on the link.  There must at all
   times be at most one RBridge on the link that
   encapsulates/decapsulates traffic for a particular VLAN. We will
   refer to the RBridge appointed to forward VLAN-x traffic on behalf of
   the link as the "appointed VLAN-x forwarder".  (Section 2.3 discusses

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   VLANs further.)

   Rbridges SHOULD support SNMPv3 [RFC3411].  The Rbridge MIB will be
   specified in a separate document.  If IP service is available to an
   RBridge, it SHOULD support SNMPv3 over IP; however, management can be
   used, within a campus, even by an RBridge that lacks an IP or other
   Layer 3 transport stack or which has zero configuration and thus no
   Layer 3 address, by transporting SNMP with Ethernet [RFC4789].

2.1 End Station Addresses

   An RBridge, RB1, which is the VLAN-x forwarder on any of its links
   MUST learn the location of VLAN-x end nodes, both on the links for
   which it is VLAN-x forwarder, and on other links in the campus. RB1
   learns the port and Layer 2 (MAC) addresses of end nodes on links for
   which it is VLAN-x forwarder from the source address of frames
   received, as bridges do (for example, see section 8.7 of [802.1Q]),
   or through a Layer 2 explicit registration protocol such as IEEE
   802.11 association and authentication. RB1 learns the Layer 2 address
   of distant VLAN-x end nodes, and the corresponding RBridge to which
   they are attached, by looking at the ingress RBridge nickname in the
   TRILL header and the VLAN and source address of the inner frame of
   TRILL data frames that it decapsulates.

   Additionally, an RBridge that is the appointed VLAN-x forwarder on
   one or more links MAY use the End Station Address Distribution
   Information (ESADI) protocol to announce some or all of the attached
   VLAN-x end nodes on those links. An ESADI could be used to announce
   end nodes that have been explicitly enrolled. Such information might
   be more authoritative than that learned from data frames being
   decapsulated onto the link.  Also, it can be more secure because not
   only might the enrollment be authenticated (for example by
   cryptographically based EAP methods via [802.1X]), but ESADI also
   supports cryptographic authentication of its messages [RFC5304].  But
   even if an ESADI is used to announce attached end nodes, RBridges
   MUST still learn from decapsulating data frames unless configured not
   to do so.

   Advertising end nodes using an ESADI is optional, as is learning from
   these announcements.

   (See Section 4.6 for further end station address details.)

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2.2 RBridge Encapsulation Architecture

   The Layer 2 technology used to connect Rbridges may be either IEEE
   [802.3] or some other technology such as PPP [RFC1661].  This is
   possible since the RBridge relay functionality is layered on top of
   the Layer 2 technologies.  However, this document specifies only an
   IEEE 802.3 encapsulation.

   Figure 2.1 shows two RBridges RB1 and RB2 interconnected through an
   Ethernet cloud. The Ethernet cloud may include hubs, point-to-point
   or shared media, IEEE 802.1D bridges, or 802.1Q bridges.

                              /            \
                 +-----+     /   Ethernet   \    +-----+
                 | RB1 |----<                >---| RB2 |
                 +-----+     \    Cloud     /    +-----+
                              \            /

                    Figure 2.1: Interconnected RBridges

   Figure 2.2 shows the format of a TRILL data or ESADI frame traveling
   through the Ethernet cloud between RB1 and RB2.

                    |     Outer Ethernet Header      |
                    |          TRILL Header          |
                    |     Inner Ethernet Header      |
                    |        Ethernet Payload        |
                    |         Ethernet FCS           |

             Figure 2.2: An Ethernet Encapsulated TRILL Frame

   In the case of media different from Ethernet, the outer Ethernet
   header is replaced by the header specific to that media. For example,
   Figure 2.3 shows a TRILL encapsulation over PPP.

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                    |           PPP Header           |
                    |          TRILL Header          |
                    |     Inner Ethernet Header      |
                    |        Ethernet Payload        |
                    |         Ethernet FCS           |

                Figure 2.3: A PPP Encapsulated TRILL Frame

   The outer header is link-specific and, although this document
   specifies only Ethernet links, other links are allowed.

   In both cases the Inner Ethernet Header and the Ethernet Payload come
   from the original frame and are encapsulated with a TRILL Header as
   they travel between RBridges. Use of a TRILL header offers the
   following benefits:

   1. loop mitigation through use of a hop count field;

   2. elimination of the need for original source and destination MAC
      address learning in transit RBridges;

   3. direction of frames towards the egress RBridge (this enables
      forwarding tables of RBridges to be sized with the number of
      RBridges rather than the total number of end nodes); and,

   4. provision of a separate VLAN tag for forwarding traffic between
      RBridges, independent of the VLAN of the native frame.

   When forwarding unicast frames between RBridges across a shared-
   media, the outer header has the MAC destination address of the next
   hop Rbridge, to avoid frame duplication. Having the outer header
   specify the transmitting RBridge as source address ensures that any
   bridges inside the Ethernet cloud will not get confused, as they
   might be if multipathing is in use and they were to see the original
   source or ingress RBridge in the outer header.

   From a forwarding standpoint, transit frames may be classified into
   two main categories: known-unicast and multi-destination.

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2.2.1 Known-Unicast

   These frames have a unicast inner MAC destination address
   (Inner.MacDA) and are those for which ingress RBridge knows the
   egress RBridge for that destination MAC address.

   Such frames are forwarded Rbridge hop by Rbridge hop to their egress

2.2.2 Multi-destination

   These are frames that must be delivered to multiple destinations.

   Multi-destination frames include the following:

   1. unicast frames for which the destination is unknown: the
      Inner.MacDA is unicast, but the ingress RBridge does not know its

   2. multicast frames for which the Layer 2 destination address is
      derived from an IP multicast address: the Inner.MacDA is
      multicast, from the set of Layer 2 multicast addresses derived
      from IPv4 [RFC1112] or IPv6 [RFC2464] multicast addresses; these
      frames are handled somewhat differently in different subcases:

      2.1 IGMP [RFC3376] and MLD [RFC2710] multicast group membership

      2.2 IGMP [RFC3376] and MLD [RFC2710] queries and MRD [RFC4286]
          announcement messages;

      2.3 other IP derived Layer 2 multicast frames;

   3. multicast frames for which the Layer 2 destination address is not
      derived from an IP multicast address: the Inner.MacDA is
      multicast, and not from the set of Layer 2 multicast addresses
      derived from IPv4 or IPv6 multicast addresses;

   4. broadcast frames: the Inner.MacDA is broadcast (FF-FF-FF-FF-FF-

   RBridges build distribution trees (see Section 4.3) and use these
   trees for forwarding multi-destination frames. These distribution
   trees are pruned in different ways for different cases to avoid
   unnecessary propagation of the frame.

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2.3 RBridges and VLANs

   A VLAN is a way to partition end nodes in a campus into different
   Layer 2 communities [802.1Q]. Use of VLANs requires configuration.
   By default, the port on which it is initially received determines the
   VLAN of a frame sent by an end station.  End stations can also
   explicitly insert this information in a frame.

   IEEE 802.1Q bridges can be configured to support multiple VLANs over
   a single link by inserting/removing a VLAN tag in the frame.  VLAN
   tags used by TRILL have the same format as VLAN tags defined in IEEE
   [802.1Q]. As shown in Figure 2.2 there are two places where such tags
   may be present in a TRILL-encapsulated frame sent over an IEEE 802.3
   link: one in the outer header (Outer.VLAN) and one in the inner
   header (Inner.VLAN). Inner and outer VLANs are further discussed in
   Section 4.1.

   RBridges enforce delivery of a native frame originating in a
   particular VLAN only to other links in the same VLAN; however, there
   are a few differences in the handling of VLANs between an RBridge
   campus and an 802.1 bridged LAN as described below.

   (See Section 4.2.4 for further discussion of TRILL IS-IS operation on
   a link.)

2.3.1 Link VLAN Assumptions

   In a network with a mix of bridges and RBridges, certain
   configurations of bridges or ports may prevent some nodes in some
   VLANs from communicating with each other.

   TRILL requires that on each link in the campus there is at least one
   VLAN that gives full connectivity to all the RBridges on that link
   and that the RBridges are configured to know which VLAN that is if it
   is not the default VLAN 1.

   Since there will be only one appointed forwarder for any VLAN, say
   VLAN A, on a link, if bridges are configured to cause VLAN A to be
   partitioned on a link, some VLAN A end nodes on that link may be
   orphaned (unable to communicate with the rest of the campus).

   It is possible for bridge and port configuration to cause VLAN
   mapping on a link (where a VLAN A frame turns into a VLAN B frame).
   TRILL detects this through IS-IS Hellos, by inserting the initial
   VLAN tag into the Hello and checking it on receipt, and takes steps
   to ensure that there is at most a single appointed forwarder on the
   link, to avoid possible frame duplication or loops (see Section

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   TRILL behaves as conservatively as possible, avoiding loops rather
   than avoiding partial connectivity. As a result, lack of connectivity
   may result from bridge or port misconfiguration.

2.4 RBridges and IEEE 802.1 Bridges

   As described below, RBridge ports are, for the most part, layered on
   top of IEEE [802.1Q] port facilities and RBridges can be
   incrementally deployed into a bridged LAN.

2.4.1 RBridge and 802.1 Layering

   RBridges make use of 802.1Q port VLAN processing and lower level
   802.1 protocols as well as the protocols for the link in use, such as
   port based access control [802.1X] or link aggregation (Clause 43 of
   [802.3]). The exceptions are (1) those protocols related to high
   level control frames including spanning tree and (2) that the discard
   of frames that have exceeded the Maximum Transit Delay is optional.
   (There may in the future be additional lower level 802.1 protocols
   that require different handling in an RBridge than in an 802.1

   RBridges do not use spanning tree and do not block ports in the way
   that spanning tree blocks ports.  Figure 2.4 shows a high level
   diagram of an RBridge port connected to an IEEE 802.3 link. Single
   lines represent the flow of control information, double lines the
   flow of both frames and control information.

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                          |                RBridge
                          |     Forwarding Engine, IS-IS, Etc.
                          | Processing of native and TRILL frames
                               |   |        ||         other ports...
                 +-------------+   |        ||
                 |                 |        ||
    +------------+-------------+   |        ||
    |         RBridge          |   |        ||
    |                          |   |   +----++------+ <- EISS
    | High level Control Frame |   |   |            |
    |  Processing (BPDU, VRP)  |   |   |   802.1Q   |
    |                          |   |   | Port VLAN  |
    +-----------++-------------+   |   | Processing |
                ||                 |   |            |
      +---------++-----------------+---+------------+ <-- ISS
      |                                             |
      |    802.1/802.3 Low Level Control Frame      |
      |    Processing, Port/Link Control Logic      |
      |                                             |
                  ||        +------------+
                  ||        | 802.3 PHY  |
                  |+--------+ (Physical  +--------- 802.3
                  +---------+ Interface) +--------- Link
                            |            |

                      Figure 2.4: RBridge Port Model

   The upper interface to the lower level port/link control logic
   corresponds to the Internal Sublayer Service (ISS) in [802.1Q]. In
   RBridges, high level control frames are processed above the ISS

   The upper interface to the port VLAN processing corresponds to the
   Extended Internal Sublayer Service (EISS) in [802.1Q]. In RBridges,
   native and TRILL frames are processed above the EISS interface and
   are subject to port VLAN and priority processing.

2.4.2 Incremental Deployment

   Because RBridges are generally compatible with current IEEE [802.1Q]
   customer bridges, a bridged LAN can be upgraded by incrementally

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   replacing such bridges with RBridges. Bridges that have not yet been
   replaced are transparent to RBridge traffic. The physical links
   directly interconnected by such bridges, together with the bridges
   themselves, constitute bridged LANs. These bridged LANs appear to
   RBridges to be multi-access links.  If the bridges replaced by
   RBridges were unmanaged, zero configuration bridges, then their
   RBridge replacements will not require configuration.

   The RBridge campus will work best if all IEEE 802.1 bridges are
   replaced with RBridges, assuming the RBridges have the same speed and
   capacity as the bridges. However, there may be intermediate states,
   where only some bridges have been replaced by RBridges, with inferior

   See Appendix A for further discussion of incremental deployment.

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3. Details of the TRILL Header

   The section specifies the TRILL header. Section 4 below provides
   other RBridge design details.

3.1 TRILL Header Format

   The TRILL header is shown in Figure 3.1 and is independent of the
   data link layer used. When that layer is IEEE [802.3], it is prefixed
   with the 16-bit TRILL Ethertype [RFC5342], making it 64 bit aligned.

                                   | V | R |M|Op-Length| Hop Count |
   |   Egress RBridge Nickname     |  Ingress RBridge Nickname     |

                         Figure 3.1: TRILL Header

   The header contains the following fields that are described in the
   sections referenced:

   o  V (Version): 2-bit unsigned integer. See Section 3.2.

   o  R (Reserved): 2 bits. See Section 3.3.

   o  M (Multi-destination): 1 bit. See Section 3.4.

   o  Op-Length (Options Length): 5-bit unsigned integer.  See Section

   o  Hop Count: 6-bit unsigned integer. See Section 3.6.

   o  Egress RBridge Nickname: 16-bit identifier. See Section 3.7.1.

   o  Ingress RBridge Nickname: 16-bit identifier. See Section 3.7.2.

3.2 Version (V)

   A single Ethertype is granted to a protocol and, under IEEE
   guidelines, it is the protocol's responsibility to structure itself
   to support future revisions.  Adhering to this guideline, there is a
   two bit Version field in the TRILL header. Version zero of TRILL is
   specified in this document. An RBridge that sees a message with a
   Version value it does not understand MUST silently discard the
   message because it may not be able to parse it.

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3.3 Reserved (R)

   The two R bits are reserved for future use in extensions to this
   version zero of the TRILL protocol. They MUST be zero on transmission
   and are ignored on receipt.

3.4 Multi-destination (M)

   The Multi-destination bit (see Section 2.2.2) indicates that the
   frame is to be delivered to a class of destination end stations via a
   distribution tree and that the egress RBridge nickname field
   specifies the root RBridge for this tree.  In particular:

   o  M = 0 (FALSE) - The egress RBridge nickname contains the nickname
      of the egress Rbridge for a known unicast TRILL data frame;

   o  M = 1 (TRUE) - The egress RBridge nickname field contains the
      nickname of the RBridge that is the root of a distribution tree.
      This tree is selected by the ingress RBridge for a TRILL data
      frame or by the source RBridge for a TRILL ESADI frame.

3.5 TRILL Header Options

   The TRILL Protocol includes an option capability in the TRILL Header.
   The Op-Length header field gives the length of the options in units
   of 4 octets, which allows up to 124 octets of options area.  If Op-
   Length is zero there are no options present.  If options are present,
   they follow immediately after the Ingress Rbridge Nickname field.

   All Rbridges MUST be able to skip the number of 4-octet chunks
   indicated by the Op-Length field in order to find the inner frame,
   since RBridges must be able to find the destination MAC address and
   VLAN tag in the inner frame.  (Transit RBridges need such information
   to filter VLANs, IP multicast, and the like. Egress Rbridges need to
   find the inner header to correctly decapsulate and handle the inner

   To ensure backward compatible safe operation, when Op-Length is non-
   zero indicating that options are present, the top two bits of the
   first octet of the options area are specified as follows:

               | CHbH | CItE |          Reserved           |

               Figure 3.2: Options Area Initial Flags Octet

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   If the CHbH (Critical Hop by Hop) bit is one, one or more critical
   hop-by-hop options are present so transit RBridges that support no
   options MUST drop the frame. If the CHbH bit is zero, the frame is
   safe, from the point of view of options processing, for a transit
   RBridge to forward, even if the forwarding RBridge doesn't understand
   any options. A transit RBridge that supports no options and forwards
   a frame MUST transparently forward the options area.

   If the CItE (Critical Ingress to Egress) bit is a one, one or more
   critical ingress-to-egress options are present. If it is zero, no
   such options are present.  If either CHbH or CItE is non-zero, egress
   RBridges that support no options MUST drop the frame.  If both CHbH
   and CItE are zero, the frame is safe, from the point of view of
   options, for any egress RBridge to process, even if it doesn't
   understand any options.

   Options will be further specified in other documents and are expected
   to include provisions for hop-by-hop and ingress-to-egress options as
   well as critical and non-critical options.

   Note: Most RBridges implementations are expected to be optimized for
      the simplest and most common cases of frame forwarding and
      processing. The inclusion of any options may, and the inclusion of
      complex or lengthy options very likely will, cause frame
      processing using a "slow path" with markedly inferior performance
      to "fast path" processing. Limited slow path throughput may cause
      such frames to be lost.

3.6 Hop Count

   The Hop Count field is a 6-bit unsigned integer. Each RBridge that is
   about to forward a TRILL data or ESADI frame MUST check this field
   and discard the frame if this field is zero. If this field is greater
   than or equal to 1, it MUST be decremented in the forwarded frame.

   For known unicast frames, the ingress RBridge MUST set the Hop Count
   to at least the number of RBridge hops it expects to the egress
   RBridge and SHOULD set it in excess of that number to allow for
   alternate routing later in the path.

   For multi-destination frames, the Hop Count MUST be set by the
   ingress RBridge (or source RBridge for a TRILL ESADI frame) to at
   least the expected number of hops to the most distant RBridge. To
   accomplish this, RBridge RBn calculates, for each branch from RBn of
   the distribution tree rooted at RBi, the maximum number of hops in
   that branch. When forwarding a multi-destination frame onto a branch,
   transit RBridge RBm MAY decrease the hop count by more than 1 unless
   decreasing the hop count by more than 1 would result in a Hop Count

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   insufficient to reach all destinations in that branch of the tree
   rooted at RBi. Using a Hop Count close to the minimum on multi-
   destination frames reduces potential problems with temporary loops
   when forwarding.

   Although the RBridge MAY decrease the hop count by more than 1, under
   the circumstances described above, the RBridge forwarding a frame
   MUST decrease the hop count by at least 1, and discards the frame if
   it cannot do so because the hop count is 0.

3.7 RBridge Nicknames

   Nicknames are 16-bit dynamically assigned abbreviations for each
   RBridge's 48-bit IS-IS System ID to achieve a more compact encoding.
   This assignment allows specifying up to 2**16 RBridges; however, the
   value 0x0000 is reserved to indicate that a nickname is not
   specified, the values 0xFFC0 through 0xFFFE are reserved for future
   specification, and the value 0xFFFF is permanently reserved.
   RBridges piggyback a nickname acquisition protocol on the link state
   protocol (see Section 3.7.3) to acquire a nickname unique within the

3.7.1 Egress RBridge Nickname

   There are two cases for the contents of the egress RBridge nickname
   field, depending on the M-bit (see Section 3.4). It is filled in by
   the ingress RBridge for TRILL data frames and by the source RBridge
   for TRILL ESADI frames.

   o  For known unicast TRILL data frames, M == 0 and the egress RBridge
      nickname field specifies the egress RBridge i.e. it specifies the
      RBridge that needs to remove the TRILL encapsulation and forward
      the native frame. Once the egress nickname field is set, it MUST
      NOT be changed by any subsequent transit RBridge.

   o  For multi-destination TRILL data frames and for TRILL ESADI
      frames, M == 1. The egress RBridge nickname field contains the
      nickname of the root RBridge of the distribution tree selected to
      be used to forward the frame. This root MUST NOT be changed by
      transit RBridges.

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3.7.2 Ingress RBridge Nickname

   The ingress RBridge nickname is set to the nickname of the ingress
   RBridge for TRILL data frames and to the nickname of the source
   RBridge for TRILL ESADI frames.

   Once the ingress nickname field is set, it MUST NOT be changed by any
   subsequent transit RBridge.

3.7.3 RBridge Nickname Selection

   The nickname selection protocol is piggybacked on the core TRILL IS-
   IS instance as follows:

   o  The nickname being used by an RBridge is carried in an IS-IS TLV
      (type-length-value data element) along with a priority of use
      value.  Each RBridge chooses its own nickname.

   o  The nickname value MAY be configured. An RBridge that has been
      configured with a nickname value will have priority for that
      nickname value over all Rbridges with non-configured nicknames.

   o  The nickname values 0x0000 and 0xFFC0 through 0xFFFF are reserved
      and MUST NOT be selected by or configured for an RBridge. The
      value 0x0000 is used to indicate that a nickname is not known.

   o  The priority of use field reported with a nickname is an unsigned
      8-bit value, where the most significant bit (0x80) indicates that
      the nickname value was configured. The bottom 7 bits have the
      default value 0x40, but MAY be configured to be some other value.
      Additionally, an RBridge MAY increase its priority after holding
      the nickname for some amount of time. However, the most
      significant bit of the priority MUST NOT be set unless the
      nickname value was configured.

   o  Once an RBridge has successfully acquired a nickname it SHOULD
      attempt to reuse it in the case of a reboot.

   o  Each RBridge is responsible for ensuring that its nickname is
      unique.  If RB1 chooses nickname x, and RB1 discovers, through
      receipt of RB2's LSP, that RB2 has also chosen x, then the RBridge
      with the numerically higher priority keeps the nickname, or if
      there is a tie in priority, the RBridge with the numerically
      higher IS-IS System ID keeps the nickname, and the other RBridge
      MUST select a new nickname. This can require an RBridge with a
      configured nickname to select a different nickname.

   o  To minimize the probability of nickname collisions, when an

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INTERNET-DRAFT                                          RBridge Protocol

      RBridge selects a new nickname, it does so by randomly hashing
      some of its parameters, e.g., interface MAC addresses, time and
      date, and other entropy sources such as those given in [RFC4086].
      There is no reason for all Rbridges to use the same algorithm for
      selecting nicknames.

   o  If two RBridge campuses merge, then transient nickname collisions
      are possible. As soon as each RBridge receives the LSPs from the
      other RBridges, the RBridges that need to change nicknames select
      new nicknames that do not, to the best of their knowledge, collide
      with any existing nicknames. Some RBridges may need to change
      their nickname more than once before the situation is resolved.

   o  To minimize the probability of a new RBridge usurping a nickname
      already in use, an RBridge whose nickname is not configured SHOULD
      wait to acquire the link state database from a neighbor before it
      announces its own nickname.

   An RBridge that will not act as an ingress, egress, or tree root need
   not have a nickname.

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4. Other RBridge Design Details

   Section 3 above specifies the TRILL header, while this Section
   specifies other RBridge design details.

4.1 Ethernet Data Encapsulation

   TRILL data and ESADI frames in transit on Ethernet links are
   encapsulated with an outer Ethernet header (see Figure 2.2). This
   outer header looks, to a bridge on the path between two RBridges,
   like the header of a regular Ethernet frame and therefore bridges
   forward the frame as they normally would. To enable RBridges to
   distinguish such TRILL frames, a new TRILL Ethertype (see Section
   7.2) is used in the outer header.

   Figure 4.1 details a TRILL data frame with an outer VLAN tag
   traveling on an Ethernet link as shown at the top of the Figure, that
   is, between transit RBridges RB3 and RB4. The native frame originated
   at end station ESa, was encapsulated by ingress RBridge RB1 and will
   ultimately be decapsulated by egress RBridge RB2 and delivered to
   destination end station ESb. The encapsulation shown has the
   advantage, in the absence of TRILL options, of aligning the original
   Ethernet frame at a 64-bit boundary.

   When a TRILL data frame is carried over an Ethernet cloud it has
   three pairs of addresses:

   o  Outer Ethernet Header: Outer Destination MAC Address (Outer.MacDA)
      and Outer Source MAC Address (Outer.MacSA): These addresses are
      used to specify the next hop RBridge and the transmitting RBridge,

   o  TRILL Header: Egress Nickname and Ingress Nickname. These specify
      the nickname values of the egress and ingress RBridges,
      respectively, unless the frame is multi-destination in which case
      the Egress Nickname specified the root of the distribution tree on
      which the frame is being sent.

   o  Inner Ethernet Header: Inner Destination MAC Address (Inner.MacDA)
      and Inner Source MAC Address (Inner.MacSA): These addresses are as
      transmitted by the original end station, specifying, respectively,
      the destination and source of the inner frame.

   A TRILL data frame also potentially has two VLAN tags that can carry
   two different VLAN Identifiers and specify priority.

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INTERNET-DRAFT                                          RBridge Protocol

      +----+  +-------+   +-------+       +-------+   +-------+  +----+
      |ESa +--+  RB1  +---+  RB3  +-------+  RB4  +---+  RB2  +--+ESb |
      +----+  |ingress|   |transit|   ^   |transit|   |egress |  +----+
              +-------+   +-------+   |   +-------+   +-------+
   Outer Ethernet Header:             |
      |             Outer Destination MAC Address  (RB4)              |
      | Outer Destination MAC Address | Outer Source MAC Address      |
      |                Outer Source MAC Address  (RB3)                |
      | Ethertype = C-Tag [802.1Q]    | Outer.VLAN Tag Information    |
   TRILL Header:
      | Ethertype = TRILL             | V | R |M|Op-Length| Hop Count |
      | Egress (RB2) Nickname         | Ingress (RB1) Nickname        |
   Inner Ethernet Header:
      |             Inner Destination MAC Address  (ESb)              |
      | Inner Destination MAC Address | Inner Source MAC Address      |
      |                  Inner Source MAC Address  (ESa)              |
      | Ethertype = C-Tag [802.1Q]    | Inner.VLAN Tag Information    |
      | Ethertype of Original Payload |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
      |                                  Original Ethernet Payload    |
      |                                                               |
   Frame Check Sequence:
      |               New FCS (Frame Check Sequence)                  |

            Figure 4.1: TRILL Data Encapsulation over Ethernet

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4.1.1 VLAN Tag Information

   A "VLAN Tag" (formerly known as a Q-tag), also known as a "C-tag" for
   customer tag, includes a VLAN ID and a priority field as shown in
   Figure 4.2.  The "VLAN ID" may be zero, indicating the no VLAN is
   specified, just a priority, although such frames are called "priority
   tagged" rather than a "VLAN tagged" [802.1Q].

   (802.1Q S-tags or service tags are beyond the scope of this

     | Priority  | C |                  VLAN ID                      |

                     Figure 4.2: VLAN Tag Information

   As recommended in [802.1Q], Rbridges SHOULD be implemented so as to
   allow use of the full range of VLAN IDs from 0x001 through 0xFFE.
   VLAN ID zero is the null VLAN identifier and indicates that no VLAN
   is specified while VLAN ID 0xFFF is reserved.  Rbridges MAY support a
   smaller number of simultaneously active VLAN IDs than the total
   number of different VLAN IDs they allow.

   The VLAN ID 0xFFF is reserved and MUST NOT be used. Rbridges MUST
   discard any frame they receive with an Outer.VLAN ID of 0xFFF.
   Rbridges MUST discard any frame for which they examine the Inner.VLAN
   ID and find it to be 0xFFF; such examination is required at all
   egress Rbridges which decapsulate a frame.

   The "C" bit shown in Figure 4.2 is not used in TRILL. It MUST be set
   to zero and is ignored by receivers.

   As specified in [802.1Q], the priority field contains an unsigned
   value from 0 through 7 where 1 indicates the lowest priority, 7 the
   highest priority, and the default priority zero is considered to be
   higher than priority 1 but lower than priority 2. The [802.1ad]
   amendment to [802.1Q] permits mapping some adjacent pairs of priority
   levels into a single priority level with and without drop
   eligibility. Ongoing work in IEEE 802.1 suggests the ability to
   configure "priority groups" that have a certain guaranteed bandwidth.
   RBridges MAY also implement such options.  RBridges are not required
   to implement any particular number of distinct priority levels but
   may treat one or more adjacent priority levels in the same fashion.

   Frames with the same source address, destination address, VLAN, and
   priority that are received on the same port as each other and are
   transmitted on the same port MUST be transmitted in the order
   received. (Such frames might not be sent out the same port if
   multipath is implemented. See Appendix C.)  Differing priorities can

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   cause frame re-ordering.

   The C-Tag Ethertype [RFC5342] is 0x8100.

4.1.2 Inner VLAN Tag

   The "Inner VLAN Tag Information" (Inner.VLAN) field contains the VLAN
   tag information associated with the native frame or the VLAN tag
   information associated with a TRILL ESADI frame when that frame was
   created.  When a TRILL frame passes through a transit RBridge, the
   Inner.VLAN MUST NOT be changed except when VLAN mapping is being
   intentionally performed within that RBridge.

   When a native frame arrives at an RBridge, the associated VLAN ID and
   priority are determined as specified in [802.1Q] (see Appendix D and
   [802.1Q] Section 6.7). If the RBridge is an appointed forwarder for
   that VLAN and the delivery of the frame requires transmission to one
   or more other links, this ingress RBridge forms a TRILL data frame
   with the associated VLAN ID and priority placed in the Inner.VLAN
   information.  Thus, in TRILL data frames, the Inner.VLAN tag always
   specifies a VLAN ID.

   The VLAN ID is required at the ingress Rbridge as one element in
   determining the appropriate egress Rbridge for a known unicast frame
   and is needed at the ingress and every transit Rbridge for multi-
   destination frames to correctly prune the distribution tree.

4.1.3 Outer VLAN Tag

   TRILL frames sent by an RBridge, except for some TRILL IS-IS Hellos,
   use an Outer.VLAN ID specified by the Designated RBridge (DRB) for
   the link onto which they are being sent, referred to as the
   Designated VLAN. For TRILL data and ESADI frames, the priority in the
   Outer.VLAN tag SHOULD be set to the priority in the Inner.VLAN tag.

   TRILL frames forwarded by a transit RBridge use the priority present
   in the Inner.VLAN of the frame as received.  TRILL data frames are
   sent with the priority associated with the corresponding native frame
   when received (see Appendix D).  TRILL IS-IS frames SHOULD be sent
   with priority 7.

   Whether an Outer.VLAN tag actually appears on the wire when a TRILL
   frame is sent depends on the configuration of the RBridge port
   through which it is sent in the same way as the appearance of a VLAN
   tag on a frame sent by an [802.1Q] frame depends on the configuration
   of the bridge port (see Section 4.7.2).

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4.1.4 Frame Check Sequence (FCS)

   Each Ethernet frame has a single Frame Check Sequence (FCS) that is
   computed to cover the entire frame, for detecting frame corruption
   due to bit errors on the link. Any received frame for which the FCS
   check fails MUST be discarded. The FCS normally changes on
   encapsulation, decapsulation, and every TRILL hop due to changes in
   the outer destination and source addresses, the decrementing of the
   hop count, etc.

   Although the FCS is normally calculated just before transmission, it
   is desirable, when practical, for an FCS to accompany a frame within
   an RBridge after receipt. That FCS could then be dynamically updated
   to account for changes to the frame during Rbridge processing and
   used for transmission or checked against the FCS calculated for frame
   transmission.  This optional, more continuous use of an FCS would be
   helpful in detecting some internal RBridge failures such as memory

4.2 Link State Protocol (IS-IS)

   TRILL uses an extension of IS-IS [ISO10589] as its routing protocol.
   IS-IS has the following advantages:

   o  it runs directly over Layer 2, so therefore it may be run with
      zero configuration (no IP addresses need to be assigned);

   o  it is easy to extend by defining new TLV (type-length-value) data
      elements and sub-elements for carrying TRILL information;

4.2.1 IS-IS RBridge Identity

   Each RBridge has a unique unsigned 48-bit (6-octet) IS-IS System ID.
   This ID may be derived from any of the RBridge's unique MAC

   A pseudonode is assigned a 7-octet ID by the DRB that created it,
   usually by taking a 6-octet ID owned by the DRB, and appending
   another octet. The only constraint is that the 7-octet ID be unique
   within the campus, and that the 7th octet be nonzero. An RBridge has
   a 7-octet ID consisting of its 6-octet system ID concatenated with a
   zero octet.

   In this document we use the term "IS-IS ID" to refer to the 7-octet
   quantity that can either be the ID of an RBridge or a pseudonode.

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4.2.2 IS-IS Instances

   TRILL implements separate IS-IS instances from any used by Layer 3,
   that is, different from the one used by routers.  Layer 3 IS-IS
   frames must be distinguished from TRILL IS-IS frames even when those
   Layer 3 IS-IS frames are transiting an RBridge campus.

   Layer 3 IS-IS native frames have special multicast destination
   addresses specified for that purpose, such as AllL1ISs or AllL2ISs.
   When they are TRILL encapsulated, these multicast addresses appear as
   the Inner.MacDA and the Outer.MacDA will be either unicast or the
   All-RBridges multicast address.

   Within TRILL, there is an IS-IS instance across all Rbridges in the
   campus as described in Section 4.2.3. This instance uses TRILL IS-IS
   frames that are distinguished by having a multicast destination
   address of All-IS-IS-RBridges. TRILL IS-IS frames have the IS-IS
   protocol type and do not have a TRILL Header.

   (In addition, there can be optional ESADI frames between the RBridges
   on each supported VLAN as described in Section 4.2.5. They are
   similar to TRILL data frames where the encapsulated frame is
   formatted as an IS-IS protocol frame but can be distinguished from
   other TRILL data frames by the presence of a multicast Inner.MacDA of

4.2.3 TRILL IS-IS Frames

   All Rbridges must participate in the core TRILL IS-IS instance. TRILL
   IS-IS frames are never forwarded by an RBridge but are locally
   processed on receipt. (Such processing may cause the RBridge to send
   additional TRILL IS-IS frames.)

   A TRILL IS-IS frame on an 802.3 link is structured as shown below.
   The port out which such a frame is sent may strip the outer VLAN tag.

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   Outer Ethernet Header:
      |             All-IS-IS-RBridges Multicast Address              |
      | All-IS-IS-RBridges continued  | Source RBridge MAC Address    |
      |             Source RBridge MAC Address continued              |
      | Ethertype = C-Tag [802.1Q]    | Outer.VLAN Tag Information    |
   NSAP Header:
      |         Frame Length          |           0xFEFE              |
      |     0x03      |     0x83      |
   IS-IS Payload:
      | IS-IS Common Header, IS-IS PDU Specific Fields, IS-IS TLVs    |

   Frame Check Sequence:
      |                 FCS (Frame Check Sequence)                    |

                 Figure 4.3: TRILL Core IS-IS Frame Format

   The VLAN specified in the Outer.VLAN information will be the
   Designated VLAN for the link on which the frame is sent, except in
   the case of some IS-IS Hellos.

4.2.4 TRILL IS-IS Link Protocol

   RBridges send TRILL IS-IS Hello frames on a link in order to discover
   adjacency to RBridge neighbors and to protect against having multiple
   appointed forwarders on a link for the same VLAN. The adjacency Hello
   function is essentially the same for RBridges as it is for Layer 3
   IS-IS. However, the protection function is only required because of
   the ingress and egress functions that are only present for RBridges.
   The adjacency and protection functions require different Hello

   As with Layer 3 IS-IS, one RBridge on each LAN link is elected DRB
   (Designated RBridge), based on configured priority (most significant
   field), and system ID, as communicated by both types of TRILL IS-IS
   Hellos.  The DRB, as described in Section, designates the
   VLAN to be used on the link for inter-RBridge communication and
   appoints itself or other RBridges on the link as appointed forwarder

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INTERNET-DRAFT                                          RBridge Protocol

   (see Section for VLANs on the link.

   RBridges default to using LAN IS-IS Hellos PDUs unless, on a per port
   basis, they are configured to use P2P IS-IS Hello PDUs. See Section for further details on differences for RBridges using P2P
   Hellos. Hellos and MTU

   Consider two RBridges connected via some intervening device "Dev" as
   shown in Figure 4.4. This device could be a hub, repeater, or IEEE
   802.1 bridge. For purposes of constructing the campus routing
   topology, RB1 and RB2 should only be considered adjacent if the
   largest TRILL data and IS-IS frames occurring in the campus can make
   it through Dev. Thus Adjacency Hellos need to be padded to make them
   big enough.

   Assume that the MTU for Dev is less that the size of the Adjacency
   Hellos in use in the campus. RB1 and RB2 would then not be considered
   adjacent for purposes of TRILL IS-IS routing. If no other Hellos were
   sent, this would be unsafe because both RB1 and RB2 would become a
   DRB and would then appoint themselves (or some other RBridge on their
   side of Dev) as appointed forwarder for each VLAN they are aware of
   on the link. We would then almost certainly have two appointed
   forwarders on the same link for the same VLAN, albeit a link with a
   lower MTU than desired by the campus. Very likely RB1 and RB2 would
   egress some native frames that were smaller than Dev's MTU. These
   frames would get through to the other side of Dev, be picked up and
   ingressed resulting in a loop.  For this reason, Protective Hellos
   must also be sent which are small enough to get through intervening
   devices with the smallest MTU likely to be encountered. Further
   details on Adjacency and Protective Hellos are given below.

                  +-------+                   +-------+
                  |       |      +-----+      |       |
                  |  RB1  +------+ Dev +------+  RB2  |
                  |       |      +-----+      |       |
                  +-------+                   +-------+

             Figure 4.4: RBridge Link with Intervening Device Hello VLAN Tagging

   By default, RBridges tag TRILL IS-IS Hello frames with VLAN 1.
   Because a link may be a bridged LAN with different connectivity for
   different VLANs, and since an RBridge may be configured so that it

R. Perlman, et al                                              [Page 34]

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   cannot use VLAN 1 on a port, Hellos may need to be sent out a port
   with additional and/or other VLANs for connectivity and safety.

   An RBridge RBn maintains for each port the same VLAN information as a
   customer IEEE [802.1Q] bridge, including the set of VLANs enabled for
   output through that port (see Section 4.7.2).  In addition, RBn
   maintains the following TRILL specific VLAN parameters per port:

      a) Desired Designated VLAN: the VLAN that RBn, if it is DRB, will
         specify in its Hellos to be used by all RBridges on the link to
         communicate all TRILL frames, except some Hellos.  This MUST be
         a VLAN enabled on RBn's port. It defaults to the numerically
         lowest enabled VLAN ID, which is VLAN 1 for a zero
         configuration RBridge.

      b) Designated VLAN: the VLAN being used on the link for all TRILL
         frames except some Hellos.  This is RBn's Desired Designated
         VLAN if RBn believes it is the DRB or the Designated VLAN in
         the DRB's Hellos if RBn is not the DRB.

      c) Announcing VLANs set. This defaults to the enabled VLANs set on
         the port but may be configured to be a subset of the enabled

      d) Forwarding VLANs set: the set of VLANs for which an RBridge
         port is appointed VLAN forwarder on the port. This MUST contain
         only enabled VLANs for the port, possibly all enabled VLANs.

   On each of its ports an RBridge sends Protective Hellos Outer.VLAN
   tagged with each VLAN in a set of VLANs. For each port, this set
   depends on the RBridge's DRB status and the above VLAN parameters.
   All RBridges send both Adjacency and Protective Hellos Outer.VLAN
   tagged with the Designated VLAN, unless that VLAN is not enabled.  In
   addition, the DRB sends Protective Hellos Outer.VLAN tagged with each
   enabled VLAN in its Announcing VLANs set. All non-DRB RBridges send
   Protective Hellos Outer.VLAN tagged with all enabled VLANs that are
   in the intersection of their Forwarding VLANs set and their
   Announcing VLANs set. More symbolically, Protective Hellos are sent
   as follows:

      If it is DRB
         intersection ( Enabled VLANs,
            union ( Designated VLAN, Announcing VLANs ) )

      If it is not DRB
         intersection ( Enabled VLANs,
            union ( Designated VLAN,
               intersection ( Forwarding VLANs, Announcing VLANs ) ) )

   Configuring the Announcing VLANs set to be null minimizes the number

R. Perlman, et al                                              [Page 35]

INTERNET-DRAFT                                          RBridge Protocol

   of Protective Hellos. In that case, Protective Hellos are only tagged
   with the Designated VLAN.

   The number of Hellos is maximized, within this specification, by
   configuring the Announcing VLANs set to be the set of all enabled
   VLAN IDs, which is the default.  In that case, the DRB will send
   Protective Hellos tagged with all its Enabled VLAN tags and any non-
   DRB RBridge RBn will send Protective Hellos tagged with the
   Designated VLAN, if enabled, and tagged with all VLANs for which RBn
   is an appointed forwarder. (It is possible to send even more
   Protective Hellos. In particular, non-DRB RBridges could send
   Protective Hellos on enabled VLANs for which they are not an
   appointed forwarder and which are not the Designated VLAN. This would
   not cause harm, other than a further communications and processing

   All RBridges send Adjacency Hellos on the Designated VLAN at a port
   if that VLAN is enabled at the port.

   When an RBridge port comes up, until it has heard a Hello from a
   higher priority RBridge, it considers itself to be DRB on that port
   and sends Hellos on that basis. Similarly, even if it has at some
   time recognized some other RBridge on the link as DRB, if it receives
   no Hellos on that port from an RBridge with higher priority as DRB
   for a long enough time, as specified by IS-IS, it will revert to
   believing itself DRB. Hello Contents

   A TRILL IS-IS Hello can include, in addition to the standard IS-IS
   Hello header, the following information depending on the type of
   Hello and DRB status of the sender. The actual encoding of this
   information and the IS-IS Type or sub-Type values for the TLV or sub-
   TLV data elements are specified in separate documents. Additional
   TLVs or sub-TLVs, such as the Security TLV, may also be included.

   1. The VLAN ID of the Designated VLAN for the link.

   2. In connection with VLAN mapping (see Section

      2.a A copy of the Outer.VLAN ID with which the Hello was tagged on

      2.b A flag which, if set, indicates that the sender has detected
          VLAN mapping on the link, within the past five Hello times.

   3. The set of VLANs for which end station service is enabled on the
      port. This MAY be omitted if the sender is DRB.

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   4. Several flags as follows:

      4.a A flag which, if set, indicates that the sender believes it is
          appointed forwarder for the VLAN and port on which the Hello
          was sent.

      4.b An access port flag which, if set, indicates that the sender's
          port was configured as an access port. When it is asserted in
          Hellos sent by the DRB, all ports on that link that recognize
          the DRB act as access ports.

      4.c A bypass pseudonode flag, as described below in this section.

   5. If the sender is DRB, the Rbridges (excluding itself) that it
      appoints as forwarders for that link and the VLANs for which it
      appoints them.

   6. The IS Neighbor TLV.

   7. On ports configured to use P2P Adjacency Hellos (see Section, the Adjacency State TLV (#240).

   8. Sufficient padding to assure that the largest frame to be
      forwarded through the campus will not be blocked by MTU
      restrictions on the link. Note that the padding must be enough to
      compensate for the absence of a TRILL header from TRILL IS-IS
      Hellos on IEEE 802.3 media, or for any header differences that are
      specific on other media.

   Since Hellos cannot be fragmented at the IS-IS level, consideration
   must be given to their limited size when specifying the encoding of
   the above information in Hellos. In particular, because a port can
   have an arbitrary subset of VLANs enabled, an efficient encoding will
   be needed for item 3 above, such as bit encoding.

   Protective Hellos contain, after the standard IS-IS Hello header,
   only items 1, 2, and 4 of those listed above. In particular, in order
   to detect connectivity through limited MTU paths, they MUST NOT be
   padded. Protective Hellos are not send on port which are configured
   to use P2P Hellos or which are configured as trunk ports.

   Note that TRILL IS-IS Adjacencies are only formed when connectivity
   exists on the Designated VLAN. So the IS Neighbor TLV MUST be sent in
   Adjacency Hellos, which are only sent on the Designated VLAN, and
   MUST NOT be sent in Hellos on any other VLAN.

   The table below summarizes, in terms of the numbered items above, the
   contents of various types of TRILL IS-IS Hellos:

R. Perlman, et al                                              [Page 37]

INTERNET-DRAFT                                          RBridge Protocol

             Data |  Adjacency  |  Protective |
             Item |  LAN Hello  |  LAN Hello  | P2P Hellos
                  | DRB |nonDRB | DRB |nonDRB |
               1  |  Y  |   Y   |  Y  |   Y   |    N
               2  |  Y  |   Y   |  Y  |   Y   |    N
               3  |  -  |   Y   |  N  |   N   |    N
               4  |  Y  |   Y   |  N  |   N   |    N
               5  |  Y  |   N   |  N  |   N   |    N
               6  |  Y  |   Y   |  N  |   N   |    N
               7  |  N  |   N   |  N  |   N   |    Y
               8  |  Y  |   Y   |  N  |   N   |    Y

   RBridges default to using IS-IS LAN Hellos, but it is anticipated
   that many links between RBridges will be point-to-point, in which
   case using a pseudonode merely adds to the complexity. If the DRB
   specifies the bypass pseudonode bit, the RBridges on the link just
   report their adjacencies as point-to-point.  This has no effect on
   how LSPs are flooded on a link. It only affects what LSPs are

   For example, if RB1 and RB2 are the only RBridges on the link and RB1
   is DRB, then if RB1 creates a pseudonode that is used, there are 3
   LSPs: for, say, RB1.25 (the pseudonode), RB1, and RB2, where RB1.25
   reports connectivity to RB1 and RB2, and RB1 and RB2 each just say
   they are connected to RB1.25.  Whereas if DRB R1 set the bypass
   pseudonode bit in its Hellos, then there will be only 2 LSPs: RB1 and
   RB2 each reporting connectivity to each other.

   A DRB SHOULD set the bypass pseudonode bit for its links unless, for
   a particular link, it has seen at least two simultaneous adjacencies
   on the link at some point since it last re-booted. VLAN Mapping Within a Link

   IEEE [802.1Q] does not provide for bridges changing the C-tag VLAN ID
   for a tagged frame they receive, that is, mapping VLANs.
   Nevertheless, some bridge products provide this capability and, in
   any case, bridged LANs can be configured to display this behavior.
   For example, a bridge port can be configured to strip certain VLAN
   tags on output and send the resulting untagged frames onto a link
   leading to another bridge's port configured to tag these frames with
   a different VLAN.  Although each port's configuration is legal under
   [802.1Q], in the aggregate they perform manipulations not permitted
   to a single customer 802.1Q bridge.  Since RBridge ports have the
   same VLAN capabilities as customer 802.1Q bridges, this can occur
   even in the absence of bridges. (VLAN mapping is referred to in IEEE

R. Perlman, et al                                              [Page 38]

INTERNET-DRAFT                                          RBridge Protocol

   802 as VLAN ID translation.)

   RBridges include the Outer.VLAN ID inside a TLV within every TRILL
   Hello message. When a Hello is received, RBridges compare this saved
   copy with the Outer.VLAN ID information associated with the received
   frame. If these differ and the VLAN ID inside the Hello is X and the
   Outer.VLAN is Y, it can be assumed that VLAN ID X is being mapped
   into VLAN ID Y.

   When a VLAN mapping is detected, the RBridge detecting it sets a flag
   in all Hellos it sends on the link for the subsequent five Hellos
   times. (Of course, if the RBridge continues to received Hellos
   showing mapping, it will continue to set the mapping detected flag in
   its Hellos indefinitely.) This notifies the DRB if the detecting
   RBridge is not the DRB. The DRB then assures that only one RBridge
   (either the DRB itself or some RBridge it appoints) is appointed
   forwarder for any VLANs on the link. This avoids loops and
   duplication of frames with different VLAN tags. P2P Hello Links

   RBridge ports may be configured to use IS-IS P2P Hellos. This implies
   that the port is a point-to-point link to another RBridge and that no
   end station service should be provided through that port. As with
   Layer 3 IS-IS, such P2P ports do not participate in a DRB election.
   They send all frames VLAN tagged as being in the Desired Designated
   VLAN configured for the port. Since all traffic through the port
   should be TRILL or control frames, such a port cannot be an appointed
   forwarder and no Protective Hellos are required. RBridge P2P ports
   MUST use the IS-IS three-way handshake so that an extended circuit ID
   is associated with the link for tie breaking purposes.

   Note that, even if all links in a network are physically point-to-
   point, if some of the nodes are bridges, the bridged LANs including
   those bridges appear to be multi-access link to attached RBridges.
   This would necessitate using LAN Hellos for proper operation in some

   While it is safe to erroneously configure ports as P2P, this may
   result in lack of connectivity. Designated RBridge

   TRILL IS-IS elects one RBridge for each LAN link to be the Designated
   RBridge (DRB), that is, to have special duties. This election is
   based on the information in both Adjacency and Protective Hellos. The

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INTERNET-DRAFT                                          RBridge Protocol

   Designated RBridge:

   o  Chooses, for the link, and announces in its Hellos, the Designated
      VLAN ID to be used for inter-RBridge communication. This VLAN is
      used for all TRILL data and ESADI frames and all non-Hello TRILL
      IS-IS frames.  TRILL IS-IS Adjacency and Protective Hellos are
      sent on this VLAN but additional Protective Hellos are usually
      sent on other VLANs (see Section

   o  If the link is represented in the IS-IS topology as a pseudonode,
      chooses a pseudonode ID and announces that in its Hello and issues
      an LSP on behalf of the pseudonode.

   o  Issues CSNPs.

   o  For each VLAN-x appearing on the link, chooses an RBridge on the
      link to be the appointed VLAN-x forwarder (the DRB MAY choose
      itself to be the appointed VLAN-x forwarder for all or some of the

   o  Before appointing a VLAN-x forwarder (including appointing
      itself), wait at least 5 Hello intervals (to ensure it is DRB).

   o  Continues sending IS-IS Protective Hellos on all its enabled VLANs
      that have been configured in the Announcing VLANs set of the DRB,
      which defaults to all enabled VLANs. Appointed VLAN-x Forwarder

   The appointed VLAN-x forwarder for a link is responsible for the

   o  Loop avoidance:

      -  Inhibiting itself for a configurable time from 30 to zero
         seconds, which defaults to 30 second, after it sees a root
         bridge change on the link (see Section An inhibited
         appointed forwarder for a port drops any native frames it
         receives and does not transmit any native frames in the VLAN
         for which it is appointed.

      -  Inhibiting itself, as described above, for VLAN-x if, within
         the past five Hello times, it has received a Hello in which the
         sender asserts that it is appointed forwarder and which is
            received on VLAN-x (has VLAN-x as its Outer.VLAN) or
            was originally sent on VLAN-x as indicated inside the body
            of the Hello.

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INTERNET-DRAFT                                          RBridge Protocol

      -  Optionally, not decapsulating a frame from ingress RBridge RBm
         unless it has RBm's LSP, and the root bridge on the link it is
         about to forward onto is not listed in RBm's list of root
         bridges for VLAN-x. This is known as the "decapsulation check"
         or "root bridge collision check".

   o  Unless inhibited (see above), receiving VLAN-x native traffic from
      the link and, forwarding it as appropriate.

   o  Receiving VLAN-x traffic for the link and, if uninhibited,
      transmitting it in native form after decapsulating it as

   o  Learning the MAC address of local VLAN-x nodes by looking at the
      source address of VLAN-x frames from the link.

   o  Optionally learning the port of local VLAN-x nodes based on any
      sort of Layer 2 registration protocols such as IEEE 802.11
      association and authentication.

   o  Keeping track of the { egress RBridge, VLAN, MAC address } of
      distant VLAN-x end nodes, learned by looking at the fields {
      ingress RBridge, Inner.VLAN ID, Inner.MacSA } from VLAN-x frames
      being received for decapsulation onto the link.

   o  Optionally observe native IGMP [RFC3376], MLD [RFC2710], and MRD
      [RFC4286] frames to learn the presence of local multicast
      listeners and multicast routers.

   o  Optionally listening to TRILL ESADI messages for VLAN-x to learn {
      egress RBridge, VLAN-x, MAC address } triplets and the confidence
      level of such explicitly advertised end nodes.

   o  Optionally advertising VLAN-x end nodes, on links for which it is
      appointed VLAN-x forwarder, in ESADI messages.

   o  Listening to BPDUs on the common spanning tree to learn the root
      bridge, if any, for that link and to report in its LSP the
      complete set of root bridges seen on any of its links for which it
      is appointed forwarder for VLAN-x. TRILL LSP Information

   The information in the TRILL IS-IS LSP for the mandatory core
   instance is listed below.  The actual encoding of this information
   and the IS-IS Type or sub-Type values for any new IS-IS TLV or sub-
   TLV data elements are specified in a separate document.

R. Perlman, et al                                              [Page 41]

INTERNET-DRAFT                                          RBridge Protocol

   1. The IS-IS IDs of neighbors (pseudonodes as well as RBridges) of
      RBridge RBn, and the cost of the link to each of those neighbor.
      RBridges MUST use the Extended IS Reachability TLV (#22, also know
      as "wide metric") and not the IS Reachability TLV (#2, also known
      as "narrow metric"). Among other things, this enables them to
      encode a metric for links that are not to be used for through
      traffic in connection with access ports (see Section 4.7.1).

   2. If an RBn is to be able to act as an ingress, egress, or tree
      root, the nickname of RBridge RBn (2 octets) and the unsigned
      8-bit priority for RBn to have that nickname (see Section 3.7.3).

   3. The TRILL Header Versions supported by RBridge RBn (4 bits).

   4. The following information in connection with distribution tree
      determination and announcement. See Section 4.3 for further
      details on how this information is used.

      4.1 The 16-bit unsigned priority of RBn for becoming a
          distribution tree root.

      4.2 A second unsigned 16-bit number that is the number of trees
          all RBridges in the campus calculate if RBn is highest

      4.3 A third unsigned 16-bit number that is the number of trees RBn
          would like to use.

      4.4. A list of RBridge nicknames. These are either intended roots
          of distribution trees all RBridges in the campus must
          calculate or roots that RBn would like to use, depending on
          whether RBn is highest priority or not.

   5. The list of VLAN IDs of VLANs directly connected to RBn for links
      on which RBn is the appointed forwarder for that VLAN.  (Note: an
      RBridge may advertise that it is connected to additional VLANs in
      order to receive additional frames to support certain VLAN based
      features beyond the scope of this specification as mentioned in
      Section 4.6.3 and in a separate document concerning VLAN mapping
      within RBridges.)  In addition, the LSP contains the following
      information on a per-VLAN basis:

      5.1 Per VLAN Multicast Router attached flags: This is two bits of
          information that indicate whether there is an IPv4 and/or IPv6
          multicast router attached to the Rbridge on that VLAN. An
          RBridge that does not do IP multicast control snooping MUST
          set both of these bits (see Section 4.3.3). This information
          is used because IGMP [RFC3376] and MLD [RFC2710] Membership
          Reports MUST be transmitted to all links with IP multicast
          routers, and SHOULD NOT be transmitted to links without such

R. Perlman, et al                                              [Page 42]

INTERNET-DRAFT                                          RBridge Protocol

          routers. Also, all frames for IP-derived multicast addresses
          MUST be transmitted to all links with IP multicast routers
          (within a VLAN), in addition to links from which an IP node
          has explicitly asked to join the group the frame is for,
          except for some IP multicast addresses that MUST be treated as

      5.2 Per VLAN Other Multicast flag. This is a flag bit that
          indicates that the RBridge wishes to receive non-IP derived
          multicast for that VLAN.  It defaults to true (one). Within
          each VLAN, all non-IP derived multicast traffic MUST be sent
          to an RBridge that asserts this flag.

      5.3 Per VLAN mandatory announcement of the set of IDs of Root
          bridges for any of RBn's links on which RBn is forwarder for
          that VLAN. Where MSTP (Multiple Spanning Tree Protocol) is
          running on a link, this is the root bridge of the CIST (Common
          and Internal Spanning Tree).  This is to quickly detect cases
          where two Layer 2 clouds accidentally get merged, and where
          there might otherwise temporarily be two DRBs for the same
          VLAN on the same link. (See Section

      5.4 Optionally, per VLAN Layer 2 multicast addresses derived from
          IPv4 IGMP or IPv6 MLD notification messages received from
          attached end nodes on that VLAN, indicating the location of
          listeners for these multicast addresses (see Section 4.3.4).

      5.5 Per VLAN ESADI participation flag, priority, and holding time.
          If this flag is one, it indicates that the RBridge wishes to
          receive such TRILL ESADI frames (see Section

      5.6 Per VLAN appointed forwarder status lost counter (see Section

   6. Optionally, a list of VLAN groups where address learning is shared
      across that VLAN group (see Section 4.6.3).  Each VLAN group is a
      list of VLAN IDs, with the first VLAN ID listed in a group, if
      present, is the "primary" and the others are "secondary". This is
      to detect misconfiguration of features outside the scope of this
      document. RBridges that do not support features such as "shared
      VLAN learning" ignore this field.


   RBridges that are the appointed VLAN-x forwarder for a link MAY
   participate in the TRILL end station address distribution information
   (ESADI) protocol for that VLAN. But all transit RBridges MUST
   properly forward TRILL ESADI frames as if they were multicast TRILL

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INTERNET-DRAFT                                          RBridge Protocol

   data frames. TRILL ESADI frames are structured like IS-IS frames but
   are always TRILL encapsulated on the wire.

   Because of this forwarding, it appears to an ESADI at an RBridge that
   it is directly connected by a shared virtual link to all other
   RBridges in the campus running EASDI for that VLAN.  RBridges that do
   not implement that ESADI or are not appointed forwarder for that VLAN
   do not decapsulate or locally process any TRILL ESADI frames they
   receive for that VLAN. In other words, these frames are transparently
   tunneled through transit RBridges. Such transit RBridges treat them
   exactly as multicast TRILL data frames and no special processing is
   invoked due to such forwarding.

   TRILL ESADI frames sent on an IEEE 802.3 link are structured as shown
   below. The port out which such a frame is sent may strip the outer
   VLAN tag.

R. Perlman, et al                                              [Page 44]

INTERNET-DRAFT                                          RBridge Protocol

   Outer Ethernet Header:
      |                Next Hop Destination Address                   |
      | Next Hop Destination Address  | Sending RBridge MAC Address   |
      |               Sending RBridge Port MAC Address                |
      | Ethertype = C-Tag [802.1Q]    | Outer.VLAN Tag Information    |
   TRILL Header:
      | Ethertype = TRILL             | V | R |M|Op-Length| Hop Count |
      | Egress (Dist. Tree) Nickname  | Ingress (Origin) Nickname     |
   Inner Ethernet Header:
      |             All-ESADI-RBridges Multicast Address              |
      | All-ESADI-RBridges continued  | Origin RBridge MAC Address    |
      |                Origin RBridge MAC Address continued           |
      | Ethertype = C-Tag [802.1Q]    | Inner.VLAN Tag Information    |
   NSAP Header:
      |         Frame Length          |           0xFEFE              |
      |     0x03      |     0x83      |
   ESADI Payload (formatted as IS-IS):
      | IS-IS Common Header, IS-IS PDU Specific Fields, IS-IS TLVs    |

   Frame Check Sequence:
      |               New FCS (Frame Check Sequence)                  |

                   Figure 4.5: TRILL ESADI Frame Format

   The VLAN specified in the Outer.VLAN information will always be the
   Designated VLAN for the link on which the frame is sent. The V and R
   fields will be zero while the M field will be one. The VLAN specified
   in the Inner.VLAN information will be the VLAN to which the ESADI
   applies. The Next Hop Destination address is normally the All-
   RBridges multicast but MAY be the unicast address of the next hop
   RBridge if there is only one.

R. Perlman, et al                                              [Page 45]

INTERNET-DRAFT                                          RBridge Protocol TRILL ESADI Participation

   An RBridge participating in an ESADI does not send any additional
   Hellos. The information available in the core TRILL IS-IS link state
   database is sufficient to determine the ESADI DRB on the virtual link
   for each VLAN's ESADI. In particular, the core link state database
   information for each RBridge includes the VLANs, if any, for which
   that RBridge is participating in an ESADI, its priority for being
   selected as DRB for each of those ESADIs, its holding time, and its
   IS-IS system ID for breaking ties in priority.

   The DRB sends TRILL ESADI CSNP frames on the ESADI virtual link. For
   robustness, a participating RBridge that determines that some other
   RBridge should be ESADI DRB on such a virtual link and has not
   received or sent a CSNP in at least the DRB holding time MAY also
   send a CSNP on the virtual link. A participating RBridge that
   determines that no other RBridges are participating in an ESADI for a
   particular VLAN SHOULD NOT send TRILL ESADI LSPs or CSNPs on the
   virtual link. TRILL ESADI Information

   The information in the LSP for an optional TRILL ESADI is the list of
   local end station MAC addresses known to the originating RBridge and,
   for each such address, a one octet unsigned "confidence" rating in
   the range 0-254 (see Section 4.6). In order to make it practical to
   optionally provide for VLAN ID translation, as specified in a
   separate document, TRILL ESADI frames MUST NOT contain the VLAN ID in
   the body of the frame after the Inner.VLAN tag.

4.3 Distribution Trees

   RBridges use distribution trees to forward multi-destination frames
   (see Section 2.2.2). Distribution Trees are bidirectional.  Although
   a single tree is logically sufficient for the entire campus, the
   computation of additional distribution trees is warranted for the
   following reasons: it enables multipathing of multi-destination
   frames and enables the choice of a tree root closer to or, in the
   limit, identical with the ingress RBridge. Such a closer tree root
   reduces out-of-order delivery when a unicast address transitions
   between unknown and known and improves the efficiency of the delivery
   of multi-destination frames that are being delivered to a subset of
   the links in the campus.

   In connection with distribution tree determination and selection,
   each RBridge RBn advertises in the core IS-IS link state database its

R. Perlman, et al                                              [Page 46]

INTERNET-DRAFT                                          RBridge Protocol

   priority to be chosen as a tree root, a second number k (related to
   the number of trees to be calculated), a third number j (related to
   the number of trees to be used by RBn), and an ordered list of
   RBridge nicknames. The uses of these data items are described below.

   The priority is a 16-bit unsigned integer that defaults, for a zero
   configuration RBridge to 0x8000. All RBridge in a campus can be
   ordered by priority with ties broken by system ID. Numerically lower
   priority numbers and system IDs are considered higher priority to be
   a tree root.

   The highest priority RBridge in the campus dictates the number of
   distribution trees every RBridges MUST calculate as one plus the
   second number (k) advertised by that highest priority RBridge. The
   specific root RBridges for which trees are calculated are determined
   as follows:
   1. Assume that the highest priority RBridge has dictated, through the
      value of the second number k that it advertises, that distribution
      trees be calculated for k+1 roots. k is a 16-bit configurable
      unsigned integer that defaults, for a zero configuration RBridge,
      to 1.
   2. Is the ordered list of nicknames advertised by the highest
      priority RBridge empty? If so, do 2.a below, if not, do 2.b.
   2.a Take the highest priority k+1 nicknames from the ordered list of
      all campus RBridges. Calculate trees rooted at them (or just at
      all the RBridges in the campus if there are k or fewer RBridges in
      the campus). You are done.
   2.b Ignore any nicknames in the ordered list advertised by the
      highest priority bridge that do not appear in the campus. Take the
      remaining nicknames in that list and re-order the priority order
      list of all RBridges by placing these nicknames, in the order they
      are advertised, as if they were the highest priority RBridges.
      Then do 2.a immediately above but using this re-ordered list of
      all RBridges in the campus.

   Except during transient conditions, the above method will cause all
   RBridges to calculate distribution trees for the same set of roots.

   In order to determine Reverse Path Forwarding Check filters (see
   Section 4.3.1) for multi-destination frames, each RBridge needs to
   know what trees every other RBridge might use as root when it is
   constructing TRILL encapsulated frames. The tree roots that a
   particular RBridge RBm might use can be determined as follows:
   3. Assume that the number j of different roots that RBm would like to
      choose from is indicated through the value of the third number j
      that RBm advertises. j is a 16-bit configurable unsigned integer
      that defaults, for a zero configuration RBridge, to 0 (which has a
      special meaning as described below).
   4. Is the ordered list of nicknames advertised by RBm empty? If so,
      do 4.a below, if not, do 4.b.

R. Perlman, et al                                              [Page 47]

INTERNET-DRAFT                                          RBridge Protocol

   4.a RBm can uses as tree roots only the first j RBridges in the
      ordered list of roots resulting from the procedure given earlier
      in this section for determining what trees need be calculated
      except that, if j is zero or larger than the number of tree roots
      being calculated, RBm may choose from all trees being calculated
      for the campus. You are done.
   4.b Ignore any nicknames in the ordered list advertised by RBm that
      are not the root of a tree being calculated by all RBridges in the
      campus. Take the remaining nicknames in that list and re-order the
      list being used in 4.a immediately above by placing these
      nicknames first in the order RBm advertises them. Then do 4.a
      immediately above but using this re-ordered list of tree roots.

   It is RECOMMENDED that RBridges prefer, when constructing TRILL
   encapsulated multi-destination frames, trees whose roots are low cost
   from the encapsulating RBridge and, among those with equal cost, to
   prefer those with higher priority to be a tree root.

4.3.1 Distribution Tree Calculation and Checks

   RBridges do not use the spanning tree protocol to calculate
   distribution trees. Instead, distribution trees are calculated based
   on the link state information, selecting a particular RBridge as the
   root.  Each RBridge RBn independently calculates a tree rooted at RBi
   by performing the SPF (Shortest Path First) calculation with RBi as
   the root without requiring any additional exchange of information.

   When a node RBn has two or more minimal equal cost paths toward the
   Root RBi, a deterministic tiebreaker is needed to guarantee that all
   Rbridges calculate the same distribution tree. This is obtained by
   selecting the path that goes to the parent that has the lower 7-octet
   IS-IS ID.

   If there are two or more equal cost adjacencies between two RBridges,
   ties are broken as follows: between adjacencies established by P2P
   Hellos and adjacencies established by LAN Hellos, the P2P adjacencies
   are preferred; between LAN links, the adjacency with the lowest
   Designated IS LAN ID (pseudonode) is preferred; and between P2P
   links, the adjacency with the lowest Extended Circuit ID is
   preferred. Such tie breaking only affects the two RBridges connected
   by such equal cost adjacencies. The tie breaking determines which of
   the tied links to send multi-destination traffic on and which and on
   which of them to permit receipt of such TRILL frames via the RPF
   Check. Such adjacencies MUST BE reported in both RBridges LSPs as a
   single adjacency so this situation is invisible to other RBridges.
   For tree consistency, if an RBridge sees multiple adjacencies between
   two other RBridges erroneously included in an LSP, it MUST assume the
   cost for the link is that for the least cost adjacency or the cost

R. Perlman, et al                                              [Page 48]

INTERNET-DRAFT                                          RBridge Protocol

   for one of them if there are multiple minimal cost adjacencies).

   Each RBridge RBn keeps a set of adjacencies ( { port, neighbor} pairs
   ) for each distribution tree it is calculating.  One of these
   adjacencies is toward the tree root RBi and the others are toward the
   leaves. Once the adjacencies are chosen, it is irrelevant which ones
   are towards the root RBi, and which are away from RBi. Let's suppose
   that RBn has calculated that adjacencies a, c, and f are in the RBi
   tree. A multi-destination frame for the distribution tree RBi is
   received only from one of the adjacencies a, c, or f (otherwise it is
   discarded) and forwarded to the other two adjacencies.

   To further avoid temporary multicast loops during topology changes,
   RBridges MUST do a sanity check that a multi-destination frame
   arrives on the expected link. This is called the Reverse Path
   Forwarding (RPF) Check and is done as follows: When RBn calculates
   the RBi tree, for each adjacency in the RBi tree, RBn lists the
   possible ingress RBridge nicknames for multi-destination frames that
   might be received on that adjacency. The only ingress RBridges that
   appear on any of the adjacencies are RBridges that have indicated, in
   their LSP, that they may select RBi as a distribution tree root. If a
   multi-destination frame is received on a particular adjacency and
   with egress RBridge nickname indicating that it is being distributed
   on the RBi-tree, then RBn MUST NOT forward it if the ingress RBridge
   is not listed in the allowed list of ingress RBridges for that
   adjacency for the RBi-tree.

   When a topology change occurs (including apparent changes during
   start up), an RBridge MUST adjust its input distribution tree filters
   no later than it adjusts its output forwarding.

4.3.2 Pruning the Distribution Tree

   Each distribution tree SHOULD be pruned per-VLAN, eliminating
   branches that have no potential receivers downstream. Multi-
   destination TRILL data frames SHOULD only be forwarded on branches
   that are not pruned.

   Further pruning SHOULD be done in several cases: (1) IGMP [RFC3376],
   MLD [RFC2710], and MRD [RFC4286] messages, where these are to be
   delivered only to links with IP Multicast routers; (2) other
   multicast frames derived from an IP multicast address that should be
   delivered only to links that have registered listeners, plus links
   which have IP Multicast routers, except for IP multicast addresses
   which must be broadcast; and (3) other multicast traffic not derived
   from an IP address that is only delivered to links for which the
   appointed forwarder has the Other Multicast requested flag set. All
   of these cases are scoped per-VLAN.

R. Perlman, et al                                              [Page 49]

INTERNET-DRAFT                                          RBridge Protocol

   Let's assume that RBridge RBn knows that adjacencies (a, c, and f)
   are in the RBi-distribution tree.  RBn marks pruning information for
   each of the adjacencies in the RBi-tree. For each adjacency and for
   each tree, RBn marks:

   o  the set of VLANs reachable downstream,

   o  for each one of those VLANs, flags indicating whether there are
      IPv4 or IPv6 multicast routers downstream, and whether there are
      one or more RBridges downstream with the Other Multicast flag set,

   o  the set of Layer 2 multicast addresses derived from IP multicast
      groups for which there are receivers downstream.

4.3.3 Tree Distribution Optimization

   RBridges MUST determine the VLAN associated with all native frames
   they ingress and properly enforce VLAN rules on the emission of
   native frames at egress RBridge ports according to how those ports
   are configured and appointed forwarders. They SHOULD also prune the
   distribution tree of multi-destination frames according to VLAN.
   But, since they are not required to do such pruning, they may receive
   TRILL data or ESADI frames that should have been VLAN pruned earlier
   in the tree distribution. They silently discard such frames. A campus
   may contain some Rbridges that prune on VLAN and some that do not.

   The situation is more complex for multicast. RBridges SHOULD analyze
   IP derived native multicast frames, and learn and announce listeners
   and IP multicast routers for such frames as discussed in Section 4.5
   below. And they SHOULD prune the distribution of IP derived multicast
   frames based on such learning and announcements. But, they are not
   required to prune based on IP multicast listener and router
   attachment state. And, unlike VLANs, where VLAN attachment state of
   ports MUST be maintained and honored, RBridges are not required to
   maintain IP multicast listener and router attachment state.

   An RBridge that does not examine native IGMP [RFC3376], MLD
   [RFC2710], and MRD [RFC4286] frames that it ingresses MUST advertise
   that it has IPv4 and IPv6 IP multicast routers attached for all the
   VLANs for which it is an appointed forwarder.  It need not advertise
   any IP derived multicast listeners.  This will cause all IP derived
   multicast traffic to be sent to this RBridge for those VLANs. It then
   egresses that traffic onto the links for which it is appointed
   forwarder where the VLAN of the traffic matches the VLAN for which it
   is appointed forwarder on that link.  (This may cause the suppression
   of certain IGMP membership report messages from end stations but that
   is not significant as any multicast traffic such reports would be

R. Perlman, et al                                              [Page 50]

INTERNET-DRAFT                                          RBridge Protocol

   requesting will be sent to such end stations under these

   A campus may contain a mixture of Rbridges with different levels of
   IP derived multicast optimization. An RBridge may receive IP derived
   multicast frames that should have been pruned earlier in the tree
   distribution. It silently discards such frames.

   See also "Considerations for Internet Group Management Protocol
   (IGMP) and Multicast Listener Discovery (MLD) Snooping Switches"

4.3.4 Forwarding Using a Distribution Tree

   With full optimization, forwarding a multi-destination data frame is
   done as follows:

   o The RBridge RBn receives a multi-destination TRILL data frame with
      inner VLAN-x and a TRILL header indicating that the selected tree
      is the RBi-tree;

   o  if the adjacency from which the frame was received is not one of
      the adjacencies in the RBi-tree for the specified ingress RBridge,
      the frame is dropped (see Section 4.3.1);

   o  else, if the frame is an IGMP or MLD announcement message or an
      MRD query message, then the encapsulated frame is forwarded onto
      adjacencies in the RBi-tree that indicate there are downstream
      VLAN-x IPv4 or IPv6 multicast routers as appropriate;

   o else, if the frame is for a Layer 2 multicast address derived from
      an IP multicast group, but its IP address is not the range of IP
      multicast addresses that must be treated as broadcast, the frame
      is forwarded onto adjacencies in the RBi-tree that indicate there
      are downstream VLAN-x IP multicast routers of the corresponding
      type (IPv4 or IPv6), as well as adjacencies that indicate there
      are downstream VLAN-x receivers for that group address;

   o  else, if the frame is for a Layer 2 multicast address not derived
      from an IP multicast group, then the frame is forwarded onto
      adjacencies in the RBi-tree that indicate there are downstream
      RBridges in VLAN-x with the Other Multicast flag set;

   o  else (the inner frame is for an unknown destination or broadcast
      or an IP multicast address which is required to be treated as
      broadcast) the frame is forwarded onto an adjacency if and only if
      that adjacency is in the RBi-tree, and marked as reaching VLAN-x

R. Perlman, et al                                              [Page 51]

INTERNET-DRAFT                                          RBridge Protocol

   For each link for which RBn is appointed forwarder, RBn additionally
   checks to see if it should decapsulate the frame and send it to the
   link in native form, or process the frame locally.

   TRILL ESADI frames will be delivered only to RBridges that are
   appointed forwarders for their VLAN. Such frames will be multicast
   throughout the campus, like other non-IP-derived multicast data
   frames, on the distribution tree chosen by the RBridge which created
   the TRILL ESADI frame, and pruned according to the Inner.VLAN ID.
   Thus all the RBridges that are appointed forwarders for a link in
   that VLAN receive them unless the RBridge has cleared its Other
   Multicast bit for that VLAN and has no appointed forwarders
   downstream in the tree with the Other Multicast bit set.

4.4 Frame Processing Behavior

   This section describes RBridge behavior for all varieties of received
   frames, including how they are forwarded when appropriate. Section
   4.4.1 covers native frames, Section 4.4.2 covers TRILL frames, and
   section 4.4.3 covers control frames. Processing may be organized or
   sequenced in a different way than described here as long as the
   result is the same.

   Corrupt frames, for example frames that are not a multiple of 8 bits,
   are too short or long for the link protocol/hardware in use, or have
   a bad FCS are discarded on receipt by an RBridge port just as they
   are discarded on receipt at an IEEE 802.1 bridge port.

   Source address information ( { VLAN, Outer.MacSA, port } ) is learned
   from any frame with a unicast sources address (see Section 4.6).

4.4.1 Receipt of a Native Frame

   If the port is configured as disabled or if end station service is
   disabled on the port by configuring it as a trunk port or configuring
   it to use P2P Hellos, the frame is discarded.

   The ingress Rbridge RB1 determines the VLAN ID for a native frame
   according to the same rules as IEEE 802.1Q bridges do (see Appendix D
   and Section 4.7.2). Once the VLAN is determined, if RB1 is not the
   appointed forwarder for that VLAN on the port where the frame was
   received, the frame is discarded. If it is appointed forwarder for
   that VLAN and is not inhibited (see Section, then the native
   frame is forwarded according to if it is unicast and
   according to if it is multicast or broadcast.

R. Perlman, et al                                              [Page 52]

INTERNET-DRAFT                                          RBridge Protocol Native Unicast Case

   If the destination MAC address of the native frame is a unicast
   address, the following steps are performed.

   The Layer 2 destination address and VLAN are looked up in the ingress
   RBridge's Encapsulation Database to find the egress RBridge RBm or
   the local egress port or to discover that the destination is unknown.
   One of the following three cases will apply:

   1. If the destination is known to be on the same link from which the
      native frame was received, the RBridge silently discards the
      frame, since the destination should already have received it.

   2. If the destination is known to be on a different local link for
      which RBm is the appointed forwarder, then RB1 converts the native
      frame to a TRILL data frame with an Outer.MacDA of the next hop
      RBridge towards RBm, a TRILL header with M = 0, the ingress
      nickname for RB1, and the egress nickname for RBm. If RBm is RB1,
      processing then proceeds as in; otherwise, the
      Outer.MacSA is set to the MAC address of the RB1 port on the path
      to the next hop RBridge towards RBm and the frames is queued for
      transmission out that port.

   3. If a unicast destination address is unknown, RB1 handles the frame
      as described in Section for a broadcast frame except that
      the Inner.MacDA is the original native frame's unicast destination
      address. Native Multicast and Broadcast Frames

   If the frame is a native IGMP [RFC3376], MLD [RFC2710], or MRD
   [RFC4286] frame, then RB1 SHOULD analyze it, learn any group
   membership or IP multicast router presence indicated, and announce
   that information for the appropriate VLAN in its LSP (see Section

   For all multi-destination native frames, RB1 forwards the frame in
   native form to its links where it is appointed forwarder for the
   frame's VLAN, subject to further pruning and inhibition. In addition,
   it converts the native frame to a TRILL data frame with Outer.MacSA
   of RB1 and the All-RBridges multicast address as Outer.MacDA, a TRILL
   header with the multi-destination bit M = 1, the ingress nickname for
   RB1, and the egress nickname for the root of the distribution tree it
   decides to use. It then forwards the frame on the pruned distribution
   tree (see Section 4.3).

   The default is for RB1 to write into the egress nickname field the

R. Perlman, et al                                              [Page 53]

INTERNET-DRAFT                                          RBridge Protocol

   nickname for the distribution tree, from the set of distribution
   trees RB1 has announced it might use, whose root is least cost from
   RB1. However, RB1 MAY choose a different distribution tree if RB1 has
   been configured to path-split multicast.  In that case RB1 MUST
   select a tree by specifying an RBridge that is a distribution tree
   root (see Section 4.3).  Also, RB1 MUST select a tree that RB1 has
   announced (in RB1's own LSP) to be one of those that RB1 might use.

   Although the Outer.MacDA is normally the All-RBridges multicast
   address if, for any particular frame sent out a particular port,
   there is only one next hop RBridge of interest, the frame MAY be sent
   with the unicast Outer.MacDA of the target RBridge. (Using a unicast
   Outer.MacDA is of no benefit on a point-to-point link but may result
   in substantial savings if the link is actually a bridged LAN with
   many bridged branches and end stations, to all of which the frame may
   get flooded if a multicast destination is used.)

4.4.2 Receipt of a TRILL Frame

   A TRILL frame has either a multicast Outer.MacDA allocated to TRILL
   (see Section 7.2) or is a non-control frame with an outer TRILL
   Ethertype. Processing proceeds in the following order:

   If the Outer.MacDA is All-IS-IS-RBridges, the frame is handled as
   described in Section  If the Outer.MacDA is All-RBridges,
   the frame is handled as below.  If the Outer.MacDA is any other
   multicast address allocated to TRILL (including All-ESADI-RBridges),
   the frame is discarded..

   If the Ethertype is not TRILL, the frame is discarded.

   If the Outer.MacDA is a unicast address, the frame is discarded
   unless that address is the address of the receiving Rbridge.  (Such
   discarded frames are most likely addressed to another RBridge on a
   multi-access link and that other Rbridge will handle them.)

   After the above checks, further processing of TRILL frames is
   independent of the Outer.MacDA.

   If the Version field in the TRILL Header is greater than 0, the frame
   is discarded. The Inner.MacDA is then tested. If it is the All-ESADI-
   RBridges multicast address and RBn implements the ESADI feature,
   processing proceeds as in Section below. If it is any other
   address or RBn does not implement the ESADI feature, processing
   proceeds as in Section

R. Perlman, et al                                              [Page 54]

INTERNET-DRAFT                                          RBridge Protocol TRILL IS-IS Frames

   If the frame protocol is not IS-IS, it is discarded. Otherwise, it is
   processed by the core IS-IS instance on RBn and is not forwarded. TRILL ESADI Frames

   The port on which the frame was received is first checked and the
   frame discarded if there is no TRILL core IS-IS adjacency on that

   If M == 0, the frame is silently discarded.

   The egress nickname designates the distribution tree. The frame is
   forwarded as described in Section

   In addition, if the forwarding Rbridge is an appointed forwarder for
   a link in the specified VLAN and implements a TRILL ESADI for that
   VLAN and ESADI is enabled, the inner frame is decapsulated and
   provided to that local ESADI. TRILL Data Frames

   The port on which the frame was received is first checked and the
   frame discarded if there is no TRILL IS-IS adjacency on that port.

   The M flag is then checked. If it is zero, processing continues as
   described in Section, if it is one, processing continues as
   described in Section Known Unicast TRILL Data Frames

   The egress nickname in the TRILL header is examined and, if it is
   unknown or reserved, the frame is discarded.

   If the egress RBridge indicated is the RBridge performing the
   processing (RBn), the frame being forwarded is decapsulated to native
   form. The Inner.MacDA is checked: if it is not unicast, the frame is
   silently discarded; if it is unicast, the frame is then either (1)
   sent onto the link containing the destination if the RBridge is
   appointed forwarder for that link for the frame's VLAN and is not
   inhibited (or discarded if it is inhibited), (2) locally processed if
   the RBridge itself is the destination, or (3) processed as in the
   following paragraph.

   A known unicast TRILL data frame can arrive at the egress Rbridge

R. Perlman, et al                                              [Page 55]

INTERNET-DRAFT                                          RBridge Protocol

   only to find that the Inner.MacDA is not actually known by that
   RBridge. One way this can happen is that the Inner.MacDA may have
   timed out in the egress RBridge MAC address cache. In this case, the
   egress RBridge sends the native frame out on all links that are in
   the frame's VLAN for which the RBridge is appointed forwarder and has
   not been inhibited, except that it MAY refrain from sending the frame
   on links where it knows there cannot be an end station with the
   destination MAC address, for example the link port is configured as a
   trunk (see Section 4.7.1).

   If RBn is a transit RBridge and the hop count is zero, the frame is
   silently discarded. Otherwise the hop count is decremented by one and
   the frame forwarded to the next hop RBridge towards the egress
   RBridge, using the Forwarding Database. The Inner.VLAN is not
   examined by a transit RBridge forwarding a known unicast TRILL data
   frame. Multi-Destination TRILL Data Frames

   The egress nickname in the TRILL header is examined and, if it is
   unknown or reserved, the frame is discarded.

   The Outer.MacSA is checked and the frame discarded if it is not a
   tree adjacency for the tree indicated by the egress RBridge nickname
   or the reverse path forwarding check fails (see Section 4.3.1).

   If the RBridge is an appointed forwarder for the VLAN of the frame, a
   copy of the frame is decapsulated, sent in native form on those links
   in its VLAN for which the RBridge is appointed forwarder subject to
   additional pruning and inhibition as described in Section,
   and/or locally processed as appropriate.

   If the hop count in the frame is zero, it is then silently discarded.
   If non-zero, the hop count is decreased (see Section 3.6) and the
   frame is forwarded down the tree specified by the egress RBridge
   nickname pruned as described in Section 4.3.

   In the forwarded frame, the Outer.MacSA is set to that of the port on
   which the frame is being transmitted and the Outer.MacDA is normally
   the All-RBridges multicast address; however, if for any particular
   frame transmitted on a particular port there is only one next hop
   RBridge of interest, the frame MAY be sent with a unicast Outer.MacDA
   of that next hop RBridge. (Using a unicast Outer.MacDA is of no
   benefit on a point-to-point link but may result in substantial
   savings if the link is actually a bridged LAN with many bridged
   branches and end stations, to all of which the frame may get flooded
   if a multicast destination is used.)

R. Perlman, et al                                              [Page 56]

INTERNET-DRAFT                                          RBridge Protocol

4.4.3 Receipt of a Control Frame

   Low-level control frames received by an RBridge are handled within
   the port where they are received as described in Section 4.7.

   There are two types of high-level control frames, distinguished by
   their destination address, which are handled as described in the
   sections referenced below.

      Name   Section   Destination Address

      BPDU   4.7.3     01-80-C2-00-00-00
      VRP    4.7.4     01-80-C2-00-00-21

4.5 IGMP, MLD, and MRD Learning

   Ingress RBridges SHOULD learn, based on seeing native IGMP [RFC3376],
   MLD [RFC2710], and MRD [RFC4286] frames, which IP derived multicast
   messages should be forwarded onto which links. Such frames are also,
   in general, encapsulated as TRILL data frames and distributed as
   described below and in Section 4.3.

   An IGMP or MLD membership report received in native form from a link
   indicates a multicast group listener for that group on that link. An
   IGMP or MLD query or an MRD advertisement received in native form
   from a link indicates the presence of an IP multicast router on that

   IP multicast group membership reports have to be sent throughout the
   campus and delivered to all IP multicast routers, distinguishing IPv4
   and IPv6. All IP-derived multicast traffic must also be sent to all
   IP multicast routers for the same version of IP.

   IP multicast data SHOULD only be sent on links where there is either
   an IP multicast router for that IP type (IPv4 or IPv6) or an IP
   multicast group listener for that IP multicast derived MAC address,
   unless the IP multicast address is in the range required to be
   treated as broadcast.

   RBridges do not need to announce themselves as listeners to the All-
   Snoopers multicast group (the group used for MRD reports [RFC4541]),
   because the IP multicast address for that group is in the range where
   all frames sent to that IP multicast addresses must be broadcast.

   See also "Considerations for Internet Group Management Protocol
   (IGMP) and Multicast Listener Discovery (MLD) Snooping Switches"

R. Perlman, et al                                              [Page 57]

INTERNET-DRAFT                                          RBridge Protocol

4.6 End Station Address Details

   RBridges have to learn the MAC addresses and VLANs of their locally
   attached end stations for link/VLAN pairs for which they are the
   appointed forwarder so they can

   o  forward the native form of incoming known unicast TRILL data
      frames onto the correct link and

   o  decide, for an incoming native unicast frame from a link, where
      the RBridge is the appointed forwarder for the frame's VLAN,
      whether the frame is

      -  known to have been destined for another end station on the same
         link, so the RBridge need do nothing, or

      -  has to be converted to a TRILL data frame and forwarded.

   RBridges need to learn the MAC addresses, VLANs, and remote RBridges
   of remotely attached end stations for VLANs for which they and the
   remote RBridge are an appointed forwarder, so they can efficiently
   direct native frames they receive which are unicast to those
   addresses and VLANs.

4.6.1 Learning End Station Addresses

   There are five independent ways an RBridge can learn end station
   addresses as follows:

   1. From the observation of VLAN-x frames received on ports where it
      is appointed VLAN-x forwarder, learning the { source MAC, VLAN,
      port } triplet of received frames.

   2. The { source MAC, VLAN, ingress RBridge nickname } triplet of any
      native frames that it decapsulates.

   3. By Layer 2 registration protocols learning the { source MAC, VLAN,
      port } of end stations registering at a local port.

   4. By running one or more TRILL ESADIs that receive remote address
      information and transmit local address information.

   5. By management configuration.

   RBridges MUST implement capabilities 1 and 2 above. RBridges use
   these capabilities unless configured, for one or more particular
   VLANs and/or ports, to not learn from either received frames or from
   decapsulating native frames to be transmitted or both.

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   RBridges MAY implement capabilities 3 and 4 above. If capability 4 is
   implemented, such ESADIs are run only when the RBridge is configured
   to do so on a per-VLAN basis.

   RBridges SHOULD implement capability 5.

   Entries in the table of learned MAC addresses and associated
   information also have a one octet unsigned confidence level
   associated with each entry. Such information learned from the
   observation of data has a confidence of 0x20 unless configured to
   have a different confidence. This confidence level can be configured
   on a per RBridge basis separately for information learned from local
   native frames and that learned from remotely originated encapsulated
   frames.  Such information received via TRILL ESADI is accompanied by
   a confidence level in the range 0 to 254. Such information configured
   by management defaults to a confidence level of 255 but may be
   configured to have another value.

   The table of learned MAC addresses includes (1) { confidence, VLAN,
   MAC address, local port } for addresses learned from local native
   frames and local registration protocols, (2) { confidence, VLAN, MAC
   address, egress RBridge nickname } for addresses learned from remote
   encapsulated frames and ESADI link state databases, and (3)
   additional information to implement timeout of learned addresses,
   statically configured addresses, and the like.

   When a new learned address and related information are to be entered
   into the local database there are three possibilities:

   A. If this is a new { address, VLAN } pair, the information is
      entered accompanied by the confidence level.

   B. If there is already an entry for this { address, VLAN } pair with
      the same accompanying delivery information, the confidence level
      in the local database is set to the maximum of its existing
      confidence level and the confidence level with which it is being
      learned. In addition, if the information is being learned with the
      same or a higher confidence level than its existing confidence
      level, timer information is reset.

   C. If there is already an entry for this { address, VLAN } pair with
      different information, the learned information replaces the older
      information only if it is being learned with higher or equal
      confidence than that in the database entry. If it replaces older
      information, timer information is also reset.

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4.6.2 Forgetting End Station Addresses

   While RBridges need to learn end station addresses as described
   above, it is equally important that they be able to forget such
   information. Otherwise, frames for end stations that have moved to a
   different part of the campus could be indefinitely black holed by
   RBridges with stale information as to the link to which the end
   station is attached.

   For end station address information locally learned from frames
   received, the time out from the last time a native frame was received
   or decapsulated with the information conforms to the recommendations
   of [802.1Q]. It is referred to as the "Aging Time" and is
   configurable per RBridge with a range of from 10 seconds to 1,000,000
   seconds and a default value of 300 seconds.

   The situation is different for end station address information
   acquired via TRILL ESADI. It is up to the originating RBridge to
   decide when to remove such information from the ESADI LSP (or up to
   ESADI timeouts if the originating RBridge becomes inaccessible).

   When an RBridge ceases to be appointed forwarder for VLAN-x on a
   port, it forgets all end station address information learned from the
   observation of VLAN-x native frames received on that port.  It also
   increments a per VLAN counter of the number of times it lost
   appointed forwarder status for that VLAN.

   When, for all of its ports, RBridge RBn is no longer appointed
   forwarder for VLAN-x, it forgets all end station address information
   learned from decapsulating VLAN-x native frames. Also, if RBn is
   participating in TRILL ESADI for VLAN-x, it ceases to so participate
   after sending a final LSP nulling out the end station address
   information for that VLAN which it had been originating. In addition,
   all other RBridges that are VLAN-x forwarder on at least one of their
   ports notice that the link state data for RBn has changed to show
   that it no longer has a link on VLAN-x. In response, they forget all
   end station address information they have learned from decapsulating
   VLAN-x frames that show RBn as the ingress RBridge.

   When the appointed forwarder lost counter for RBridge RBn for VLAN-x
   is observed to increase via the core TRILL IS-IS link state database
   but RBn continues to be an appointed forwarder for VLAN-x on at least
   one of its ports, every other RBridge that is an appointed forwarder
   for VLAN-x modifies the aging of all the addresses it has learned by
   decapsulating native frames in VLAN-x from ingress RBridge RBn as
   follows: The time remaining for each entry is adjusted to be no
   larger than a per RBridge configuration parameter called (to
   correspond to [802.1Q]) "Forward Delay". This parameter is in the
   range of 4 to 30 seconds with a default value of 15 seconds.

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4.6.3 Shared VLAN Learning

   RBridges can map VLAN IDs into a smaller number of identifiers for
   purposes of address learning, as [802.1Q] bridges can. Then, when a
   lookup is done in learned address information, this identifier is
   used for matching in place of the VLAN ID. If the ID of the VLAN on
   which the address was learned is not retained, then there are the
   following consequences:

   o  The RBridge no longer has the information needed to participate in
      TRILL ESADI for the VLANs who's ID is not being retained.

   o  In cases where 4.6.2 above requires the discarding of learned
      address information based on a particular VLAN, when the VLAN ID
      is not available for entries under a shared VLAN identifier,
      instead the time remaining for each entry under that shared VLAN
      identifier is adjusted to be no longer than the RBridge's "Forward

   Although outside the scope of this specification, there are some
   Layer 2 features in which a set of VLANs has shared learning, where
   one of the VLANs is the "primary" and the other VLANs in the group
   are "secondaries". An example of this is where traffic from different
   communities are separated using VLAN tags, and yet some resource
   (such as an IP router or DHCP server) is to be shared by all the
   communities. A method of implementing this feature is to give a VLAN
   tag, say Z, to a link containing the shared resource, and have the
   other VLANs, say A, C, and D, be part of the group { primary = Z,
   secondaries = A, C, D }. An RBridge, aware of this grouping, attached
   to one of the secondary VLANs in the group also claims to be attached
   to the primary VLAN. So an RBridge attached to A would claim to also
   be attached to Z. An RBridge attached to the primary would claim to
   be attached to all the VLANs in the group.

   This document does not specify how VLAN groups might be used.  Only
   RBridges that participate in a VLAN group will be configured to know
   about the VLAN group. However, to detect misconfiguration, an RBridge
   configured to know about a VLAN group SHOULD report the VLAN group in
   its LSP.

4.7 RBridge Ports

   Section 4.7.1 below describes the several RBridge port configuration
   bits, Section 4.7.2 give a logical port structure in terms of frame
   processing, and Sections 4.7.3 and 4.7.4 describe the handling of
   high-level control frames.

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4.7.1 RBridge Port Configuration

   There are four per port configuration bits as follows:

   o  Disable port bit. When this bit is set, all frames received are
      discarded and no frames are sent, with the possible exception of
      some low-level control frames that may be received and processed

   o  End station service disable (trunk port) bit. When this bit is
      set, all native frames received on the port and all native frames
      that would have been sent on the port are discarded. (See Appendix
      B.)  (Note that, for this document, "native frames" does not
      include control frames.) Because the services of Protective Hellos
      are only needed in connection with the ingress and egress of
      native frames, trunk ports need not send any Protective Hellos.

      A port with end station service disabled reports, in the Adjacency
      Hellos it sends out that port, that it has no VLANs for which it
      provides end station support. As a result, such a port will not be
      appointed forwarder for any VLAN. Thus a port with end station
      service disabled cannot contribute to the VLANs that the RBridge
      reports in its LSP as being "connected" to that RBridge. Unless
      there is at least one port on an RBridge for which VLAN-x is
      appointed forwarder, that RBridge does not normally advertise
      itself in the link state as connected to VLAN-x. As a consequence,
      it will not normally receive any traffic for VLAN-x except as
      TRILL data frames to forward as a transit RBridge.

   o  TRILL traffic disable (access port) bit. If this bit is set, the
      goal is to avoid sending TRILL fames, except TRILL Hellos, on the
      port since it is intended for end station traffic (see Appendix
      B). This bit is reported in Hellos sent out the port and the bit
      for the DRB controls the link. If the DRB asserts the access port
      bit in its Hello, then the link is an RBridge access link and all
      RBridge ports which acknowledge that DRB act as access ports.  If
      there are no TRILL IS-IS adjacencies on the access port, no
      special action need be taken.

      If there are adjacencies, and the RBridge is DRB, it normally does
      not create a pseudonode for the link. In this case, the cost
      metric for such adjacencies over the access link is reported as
      2**24 - 1 in the Extended IS Reachability TLV in their LSPs by any
      of the RBridges connected to the link. Since links with this
      metric are never used for SPF paths, it is never necessary to tie
      break between two or more of them. In addition, no TRILL IS-IS
      frames, except Hellos, are send out the port.

      Alternatively, the DRB MAY choose to creates a pseudonode for the
      access link, which indicates a slightly more relaxed policy. If it

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      does create a pseudonode, it reports metrics normally but sets the
      IS-IS overflow bit in the pseudonode. This will cause IS-IS SPF
      routing to avoid sending transit data on the link if any other
      path is available but it will still be available as a path of last
      resort; however, TRILL IS-IS frames such as LSPs and CSNPs will be
      sent over the link. In addition, if a pseudonode is created by the
      DRB for an access link, all the RBridges on the access link report
      connectivity to the pseudonode as usual and the DRB reports
      connectivity in the LSP it creates for the pseudonode.

   o Use IS-IS P2P Hellos bit. If this bit is set, TRILL IS-IS Hellos
      sent on this port are IS-IS P2P Hellos, not the default IS-IS LAN
      Hellos. In addition, the IS-IS P2P three-way handshake MIST be
      used on P2P RBridge links.

   The dominance relationship of these four configuration bits is as
   follows where configuration bits to the left dominate those to the
   right. That is to say, when any pair of bits are asserted,
   inconsistencies in behavior they mandate are resolved in favor of the
   bit to the left in the this list.

         Disable > P2P > Access > Trunk

4.7.2 RBridge Port Structure

   An RBridge port can be modeled as having a lower level structure
   similar to that of an [802.1Q] bridge port as shown in Figure 4.6. In
   this figure, the double lines represent the general flow of the
   frames and information while single lines represent information flow
   only. The dashed lines in connection with VRP (GVRP/MVRP) are to show
   that VRP support is optional.  An actual RBridge port implementation
   may be structured in any way that provides the correct behavior.

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                     |                RBridge
                     | Interport Forwarding, IS-IS. Management, ...
                          ||                      |             ||
                    Trill || Data                 |             ||
                          ||                   +--+---------+   ||
            +-------------++-----+             |   TRILL    |   ||
            |    Encapsulation   |      +------+ IS-IS Hello|   ||
            |    Decapsulation   |      |      | Processing |   ||
            |     Processing     |      |      +-----++-----+   ||
            +--------------------+      |            ||         ||
            |  RBridge Appointed +------+            ||         ||
        +---+   Forwarder and    |                   ||         ||
        |   |  Inhibition Logic  +==============\\   ||   //====++
        |   +---------+--------+-+   Native       \\ ++ //
        |             |        |     Frames         \++/
        |             |        |                     ||
   +----+-----+  +- - + - - +  |                     ||
   |  RBridge |  |  RBridge |  |                     || All TRILL and
   |   BPDU   |  |    VRP   |  |                     || Native Frames
   |Processing|  |Processing|  |                     ||
   +-----++---+  + - - -+- -+  |                     ||
         ||             |      |            +--------++--+ <- EISS
         ||            |       |            |   802.1Q   |
         ||             |      |            | Port VLAN  |
         ||            |       |            | Processing |
     +---++------------++------+------------+------------+ <-- ISS
     |        802.1/802.3 Low Level Control Frame        |
     |        Processing, Port/Link Control Logic        |
                  ||        +------------+
                  ||        | 802.3 PHY  |
                  ++========+ (Physical  +======== 802.3
                            | Interface) |         Link

                  Figure 4.6: Detailed RBridge Port Model

   Low-level control frames are handled in the lower level port/link
   control logic in the same way as in an 802.1Q bridge.  This can
   optionally include a variety of 802.1 or link specific protocols such
   as link layer discovery, link aggregation (Clause 43 of [802.3]), MAC
   security [802.1AE], or port based access control [802.1X]. While
   handled at a low level, these frames may affect higher level
   processing. For example, a Layer 2 registration protocol may affect
   the confidence in learned addresses. The upper interface to this

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   lower level port control logic corresponds to the Internal Sublayer
   Service (ISS) in 802.1Q.

   High-level control frames (BPDUs and, if supported, VRP frames) are
   not VLAN tagged. Although they extend through the ISS interface, they
   are not subject to port VLAN processing. Behavior on receipt of a
   VLAN tagged BPDU or VLAN tagged VRP frame, is unspecified. If a VRP
   is not implemented, then all VRP frames are discarded.  Handling of
   BPDUs is described in Section 4.7.3. Handling of VRP frames is
   described in Section 4.7.4.

   Non-control frames, that is, all TRILL and native frames, are subject
   to Port VLAN and priority processing which is the same as for an
   802.1Q bridge.  The upper interface to the port VLAN processing
   corresponds to the Extended Internal Sublayer Service (EISS) in

   In this model, RBridge port processing below the EISS layer is
   identical to an [802.1Q] bridge except for (1) the handling of high
   level control frames and (2) that the discard of frames that have
   exceeded the Maximum Transit Delay is not mandatory but MAY be done.

   Incoming native frames are only accepted if the RBridge is an
   uninhibited appointed forwarder for the frame's VLAN, after which
   they are normally encapsulated and forwarded. Outgoing native frames
   are usually obtained by decapsulation and are only output if the
   RBridge is an uninhibited appointed forwarder for the frame's VLAN.

   TRILL Hellos are handled per port and never forwarded. They can
   affect the appointed forwarder and inhibition logic as well as the
   RBridge's LSP.

   TRILL IS-IS frames except Hellos and TRILL data and ESADI frames are
   passed up to higher level RBridge processing on receipt and
   transmitted on creation or forwarding. Note that these frames are
   never blocked due to the appointed forwarder and inhibition logic,
   which affects only native frames, but there are additional filters on
   some of them such as the Reverse Path Forwarding Check.

4.7.3 BPDU Handling

   If RBridge campus topology were static, RBridges would simply be end
   stations from a bridging perspective, terminating but not otherwise
   interacting with spanning tree. However, there are reasons for
   RBridges to listen to and sometimes to transmit BPDUs as described
   below. Even when RBridges listen to and transmit BPDUs, this is a
   local RBridge port activity.  The ports of a particular RBridge never
   interact so as to make the RBridge as a whole a spanning tree node.

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INTERNET-DRAFT                                          RBridge Protocol Receipt of BPDUs

   Rbridges MUST listen to spanning tree BPDUs received on a port and
   keep track of the root bridge, if any, on that link.  If MSTP is
   running on the link, this is the CIST root. This information is
   reported per VLAN by the RBridge in its LSP. In addition, the receipt
   of spanning tree BPDUs is used as an indication that a link is a
   bridged LAN, which can affect the RBridge transmission of BPDUs.

   An RBridge MUST NOT encapsulate or forward any BPDU frame it

   RBridges discard any topology change BPDUs they receive, but note
   Section Root Bridge Changes

   A change in the root bridge seen in the BPDUs received at an RBridge
   port may indicate a change in bridged LAN topology, including the
   possibility of the merger of two bridged LANs or the like, without
   any physical level indication at the port. During topology
   transients, bridges may go into pre-forwarding states that block
   TRILL IS-IS Hellos. For these reasons, when an RBridge sees a root
   bridge change on a port for which it is appointed forwarder for one
   or more VLANs, it is inhibited (discards all native frames received
   from or which it would otherwise have sent to the link) for a period
   of time between 30 and zero seconds. This time period is configurable
   per RBridge and defaults to 30 seconds.

   For example, consider two bridged LANs carrying multiple VLANs, each
   with various RBridge appointed forwarders. Should they become merged,
   due to a cable being plugged in or the like, those RBridges attached
   to the original bridged LAN with the lower priority root will see a
   root bridge change while those attached to the other original bridged
   LAN will not. Thus all appointed forwarders in the first set will be
   inhibited for a time period while things are sorted out by BPDUs
   within the merged bridged LAN and TRILL IS-IS Hellos between the
   RBridges involved. Transmission of BPDUs

   When an RBridge ceases to be appointed forwarder for one or more
   VLANs out a particular port it SHOULD, as long as it continues to
   receive spanning tree BPDUs on that port, send topology change BPDUs
   until it sees the topology change acknowledged in a spanning tree

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   RBridges MAY support a capability for sending spanning tree BPDUs for
   the purpose of attempting to force a bridged LAN to partition as
   discussed in Section A.3.3.

4.7.4 Dynamic VLAN Registration

   Dynamic VLAN registration provides a means for bridges (and less
   commonly end stations) to request that VLANs be enabled or disabled
   on ports leading to the requestor. This is done by VLAN registration
   protocol (VRP) frames: GVRP or MVRP. RBridges MAY implement GVRP
   and/or MVRP as described below.

   VRP frames are never encapsulated as TRILL frames between RBridges or
   forwarded in native form by an RBridge. If an RBridge does not
   implement a VRP, it discards any VRP frames received and sends none.

   RBridge ports may have dynamically enabled VLANs. If an RBridge
   supports a VRP, the actual enablement of dynamic VLANs is determined
   by GVRP/MVRP frames received at the port as it would be for an
   [802.1Q] / [802.1ak] bridge.

   An RBridge that supports a VRP sends GVRP/MVRP frames as an [802.1Q]
   / [802.1ak] bridge would send on each port that is not configured as
   an RBridge trunk port. For this purpose, it sends VRP frames to
   request traffic in the VLANs for which it is appointed forwarder and
   in the Designated VLAN, unless the Designated VLAN is disabled on the
   port, and to not request traffic in any other VLAN.

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5. Addresses, Configuration Parameters, and Constants

   IS-IS requires each RBridge to have a unique 48-bit (6-octet) System
   ID. This is easily obtainable, for example, as any one of the MAC-48
   addresses owned by that RBridge.

   A new Ethertype must be assigned to indicate a TRILL encapsulated

   Three Layer 2 multicast addresses must be assigned:

      o  All-RBridges for use as Outer.MacDA in TRILL ESADI and multi-
         destination TRILL data frames.

      o  All-IS-IS-RBridges for use as the Outer.MacDA for TRILL IS-IS

      o  All-ESADI-RBridges for use as the Inner.MacDA for TRILL ESADI

   The following per RBridge parameters may be configured:

      o  A nickname and nickname selection priority.

      o  Priority to be a distribution tree root, a desired number of
         additional distribution trees for the campus, a desired number
         of distribution tree to use, and a list of RBridge nicknames,
         as discussed in Section 4.3.

      o  The per RBridge parameters Aging Timer, Forward Delay, and
         Maximum Transit Delay.

   RBridges may be configured to have ESADI (end station address
   distribution information) protocol instances and to send and/or learn
   end station address information via such instances. Static end
   address information and priority of such end station information
   statically configured and learned in various ways can also be

   The per RBridge per VLAN Other Multicast bit, which defaults to true,
   to request the receipt of non-IP derived multicast traffic.

   The following RBridge per port parameters:

      o  The same parameters as for an 802.1Q port in terms of VLAN C-
         tags and frame priority code points.

      o  Four per-port configuration bits: disable port, disable end
         station service (trunk), access port, and use IS-IS P2P Hellos
         (see Section 4.7.1).

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      o  Configuration for the optional send-BPDUs solution to the
         wiring closet topology problem (see Section A.3.3) consists of
         System ID of the RBridge with lowest System ID. If RB1 and RB2
         are part of a wiring closet topology, both need to be
         configured to know about this, and that RB1 is the ID that
         should be used in the spanning tree protocol on the specified

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6. Security Considerations

   Layer 2 bridging in not inherently secure.  It is, for example,
   subject to spoofing of source addresses and bridging control
   messages.  A goal for TRILL is that RBridges do not add new issues
   beyond those existing in current bridging technology.

   Countermeasures are available such as to configure the TRILL IS-IS
   and ESADI instances to use IS-IS security [RFC5304] and ignore
   unauthenticated TRILL IS-IS and ESADI frames received on a port.
   Since such authentication requires configuration, RBridges using it
   are no longer zero configuration.

   IEEE 802.1 port admission and link security mechanisms, such as
   [802.1X] and [802.1AE], can also be used. These are best thought of
   as being implemented within a port and are outside the scope of TRILL
   (just as they are generally out of scope for bridging standards
   [802.1D] and [802.1Q]); however, TRILL can make use of secure
   registration through the confidence level communicated in optional
   TRILL ESADI (see Section 4.6).

   TRILL encapsulates native frames inside the RBridge campus while they
   are in transit between ingress RBridge and egress RBridge(s).  Thus,
   TRILL ignorant devices with firewall features and which cannot be
   detected by RBridges as end stations will generally not be able to
   inspect the content of such frames for security checking purposes.
   This may render them ineffective.  Routers and hosts appear to
   RBridges to be end stations and native frames will be decapsulated
   before being sent to such devices. Thus they will not see the TRILL
   Ethertype. Firewall devices which do not appear to an RBridge to be
   an end station, for example bridges with co-located firewalls, should
   be modified to understand TRILL encapsulation.

   RBridges do not prevent nodes from impersonating other nodes, for
   instance, by issuing bogus ARP/ND replies.  However, RBridges do not
   interfere with any schemes that would secure neighbor discovery.

6.1 VLAN Security Considerations

   TRILL supports VLANs. These provide logical separation of traffic but
   care should be taken in using VLANs for security purposes. To have
   reasonable assurance of such separation, all the RBridges and links
   (including Bridged LANs) in a campus must be secured and configured
   so as to prohibit end stations from using dynamic VLAN registration
   frames or otherwise gaining access to any VLAN carrying traffic for
   which they are not authorized to read and/or inject.

   Furthermore, if VLANs were used to keep some information off links

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   where it might be observed in a Bridged LAN, this will no longer work
   in general when bridges are replaced with RBridges; with
   encapsulation and a different outer VLAN tag, the data will travel
   the least cost transit path regardless of VLAN. Appropriate counter
   measures are to use end-to-end encryption or an appropriate TRILL
   security option should one be specified.

6.2 BPDU/Hello Denial of Service Considerations

   The TRILL protocol requires that an appointed forwarder at an RBridge
   port be temporarily inhibited if it sees a Hello from another RBridge
   claiming to be the appointed forwarder for the same VLAN or sees a
   root bridge change out that port. Thus it would seem that forged
   BPDUs showing repeated root bridge changes and forged Hellos with the
   Appointed Forwarder flag set could represent a significant denial of
   service attack. However, the situation is not as bad as it seems.

   The best defense against forged Hellos or other IS-IS messages is the
   use of IS-IS security [RFC5304]. Rogue end-stations would not
   normally have access to the required IS-IS keying material needed to
   forge authenticatible messages.

   Authentication similar to IS-IS security is usually unavailable for
   BPDUs. However, it is also the case that in typical modern wired
   LANs, all the links are point-to-point. If you have an all RBridged
   point-to-point campus, then the worst that an end-station can do by
   forging BPDUs or Hellos is to deny itself service. This could be
   either through falsely inhibiting the forwarding of native frames by
   the RBridge to which it is connected or by falsely activating the
   optional decapsulation check (see Section

   However, when an RBridge campus contains bridged LANs, those bridged
   LANs appear to any connected RBridges to be multi-access links. The
   forging of BPDUs by an end-station attached to such a bridged LAN
   could affect service to other end-stations attached to the bridged
   LAN. Note that bridges never forward BPDUs but process them, although
   this processing may result in the issuance of further BPDUs. Thus,
   for an end-station to forge BPDUs to cause continuing changes in the
   root bridge as seen by an RBridge through intervening bridges would
   typically require it to cause root bridge thrashing throughout the
   bridged LAN that would be disruptive even in the absence of RBridges.

   Some bridges can be configured to not send BPDUs and/or to ignore
   BPDUs on particular ports and RBridges can be configured not to
   inhibit appointed forwarding on a port; however, such configuration
   should be used with caution as it can be unsafe.

R. Perlman, et al                                              [Page 71]

INTERNET-DRAFT                                          RBridge Protocol

7. Assignment Considerations

   This section discuses IANA and IEEE 802 assignment considerations.
   See [RFC5226].

7.1 IANA Considerations

   A new IANA registry is created for TRILL Versions, Nicknames, Version
   0 Header Reserved bits, and multicast addresses.

   The initial contents of the TRILL Version Registry is as follows:

      Version  Status
         0     As specified in <RFC-this-document>
        1-3    Available for allocation by IETF Standards Action

   The initial contents of the Version 0 Header Reserved Bits Registry
   is as follows:

      Bit  Status
      0x4  Available for allocation by IETF Standards Action
      0x2  Available for allocation by IETF Standards Action
      0x1  Multi-destination bit as specified in <RFC-this-document>

   The initial contents of the TRILL Nicknames Registry is as follows:

      0x0000 Reserved to indicate no nickname specified
      0x0001-0xFFBF Dynamically allocated within each TRILL campus
      0xFFC0-0xFFFE Available for allocation by IETF Review
      0xFFFF Permanently reserved

   The initial contents of the TRILL Multicast Address Registry is as

      01-80-C2-XX-XX-X0  Assigned as All-RBridges
      01-80-C2-XX-XX-X1  Assigned as All-IS-IS-RBridges
      01-80-C2-XX-XX-X2  Assigned as All-ESADI-RBridges
      01-80-C2-XX-XX-X3 to 01-80-C2-XX-XX-XF  Available for allocation
                      by IETF Review

7.2 IEEE Registration Authority Considerations

   The Ethertype <tbd> is assigned by the IEEE Registration Authority to
   the TRILL Protocol.

   The block of 16 multicast MAC addresses from <01-80-C2-XX-XX-X0> to

R. Perlman, et al                                              [Page 72]

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   <01-80-C2-XX-XX-XF> are assigned by the IEEE Registration Authority
   for IETF TRILL protocol use.

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INTERNET-DRAFT                                          RBridge Protocol

8. Normative References

   [802.1ak] "IEEE Standard for Local and metropolitan area networks /
      Virtual Bridged Local Area Networks / Multiple Registration
      Protocol", IEEE Standard 802.1ak-2007, 22 June 2007.

   [802.1D] "IEEE Standard for Local and metropolitan area networks /
      Media Access Control (MAC) Bridges", 802.1D-2004, 9 June 2004.

   [802.1Q] "IEEE Standard for Local and metropolitan area networks /
      Virtual Bridged Local Area Networks", 802.1Q-2005, 19 May 2006.

   [802.3] "IEEE Standard for Information technology /
      Telecommunications and information exchange between systems /
      Local and metropolitan area networks / Specific requirements Part
      3: Carrier sense multiple access with collision detection
      (CSMA/CD) access method and physical layer specifications",
      802.3-2005, 9 December 2005

   [ISO10589] ISO/IEC 10589:2002, "Intermediate system to Intermediate
      system routeing information exchange protocol for use in
      conjunction with the Protocol for providing the Connectionless-
      mode Network Service (ISO 8473)," ISO/IEC 10589:2002.

   [RFC1112]  Deering, S., "Host Extensions for IP Multicasting", STD 5,
      RFC 1112, Stanford University, August 1989.

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
      Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet
      Networks", RFC 2464, December 1998.

   [RFC2710] Deering, S., Fenner, W., and B. Haberman, "Multicast
      Listener Discovery (MLD) for IPv6", RFC 2710, October 1999.

   [RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
      Thyagarajan, "Internet Group Management Protocol, Version 3", RFC
      3376, October 2002.

   [RFC4286] Haberman, B., Martin, J., "Multicast Router Discovery", RFC
      4286, December 2005.

   [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
      IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.

R. Perlman, et al                                              [Page 74]

INTERNET-DRAFT                                          RBridge Protocol

9. Informative References

   [802.1AE] "IEEE Standard for Local and metropolitan area networks /
      Media Access Control (MAC) Security", 802.1AE-2006, 18 August 2006

   [802.1X] "IEEE Standard for Local and metropolitan area networks /
      Port Based Network Access Control", 802.1X-2004, 13 December 2004.

   [PAS] Touch, J., & R. Perlman, "Transparent Interconnection of Lots
      of Links (TRILL) / Problem and Applicability Statement", draft-
      ietf-trill-prob-06.txt, March 2009, work in progress.

   [RBridges] Perlman, R., "RBridges: Transparent Routing", Proc.
      Infocom 2005, March 2004.

   [RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
      RFC 1661, July 1994.

   [RFC3411] Harrington, D., Presuhn, R., and B. Wijnen, "An
      Architecture for Describing Simple Network Management Protocol
      (SNMP) Management Frameworks", STD 62, RFC 3411, December 2002.

   [RFC4086] Eastlake, D., 3rd, Schiller, J., and S. Crocker,
      "Randomness Requirements for Security", BCP 106, RFC 4086, June

   [RFC4541] Christensen, M., Kimball, K., and F. Solensky,
      "Considerations for Internet Group Management Protocol (IGMP) and
      Multicast Listener Discovery (MLD) Snooping Switches", RFC 4541,
      May 2006.

   [RFC4789] Schoenwaelder, J. and T. Jeffree, "Simple Network
      Management Protocol (SNMP) over IEEE 802 Networks", RFC 4789,
      November 2006.

   [RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic
      Authentication", RFC 5304, October 2008.

   [RFC5342] Eastlake 3rd., D., "IANA Considerations and IETF Protocol
      Usage for IEEE 802 Parameters", BCP 141, RFC 5342, September 2008.

   [RP1999] Perlman, R., "Interconnection: Bridges, Routers, Switches,
      and Internetworking Protocols, 2nd Edition", Addison Wesley
      Longman, Chapter 3, 1999.

R. Perlman, et al                                              [Page 75]

INTERNET-DRAFT                                          RBridge Protocol

Appendix A: Incremental Deployment Considerations

   Some aspects of partial RBridge deployment are described below for
   link cost determination (Section A.1) and possible congestion due to
   appointed forwarder bottlenecks (Section A.2). A particular example
   of a problem related to the TRILL use of a single appointed forwarder
   per link per VLAN (the "wiring closet topology") is explored in
   detail in Section A.3.

A.1 Link Cost Determination

   With an RBridged campus having no bridges or repeaters on the links
   between RBridges, the RBridges can accurately determine the number of
   physical hops involved in a path and the line speed of each hop,
   assuming this is reported by their port logic. With intervening
   devices, this is no longer possible. For example, as shown in Figure
   A.1, the two bridges B1 and B2 can completely hide a slow link so
   that both Rbridges RB1 and RB2 incorrectly believe the link is

            +-----+        +----+        +----+        +-----+
            |     |  Fast  |    |  Slow  |    |  Fast  |     |
            | RB1 +--------+ B1 +--------+ B2 +--------+ RB2 |
            |     |  Link  |    |  Link  |    |  Link  |     |
            +-----+        +----+        +----+        +-----+

                  Figure A.1: Link Cost of a Bridged Link

   Even in the case of a single intervening bridge, two RBridges may
   know they are connected but each see the link as a different speed
   from how it is seen by the other.

   However, this problem is not unique to RBridges. For example, routers
   can encounter similar situations due to links hidden by bridges,
   repeaters or Rbridges.

A.2 Appointed Forwarders and Bridged LANs

   With partial RBridge deployment, the RBridges may partition a bridged
   LAN into a relatively small number of relatively large remnant
   bridged LANs, or possibly not partition it at all so a single bridged
   LAN remains.  Then two potential problems can occur as follows:

   1. The requirement that native frames enter and leave a link via the
      appointed forwarder for the link and VLAN of the frame can cause
      congestion or suboptimal routing. (Similar problems can occur

R. Perlman, et al                                              [Page 76]

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      within a bridged LAN due to the spanning tree algorithm.)  The
      extent to which such a problem will occur is highly dependent on
      the network topology. For example, if a bridged LAN had a star-
      like structure with core bridges that connected only to other
      bridges and peripheral bridges that connected to end stations and
      are connected to core bridges, the replacement of all of the core
      bridges by RBridges without replacing the peripheral bridges would
      generally improve performance without inducing any appointed
      forwarder congestion.

      Solutions to this problem are discussed below and a particular
      example explored in Section A.3.

   2. TRILL traffic sent to the All-RBridges multicast addresses will
      typically be flooded throughout a bridged LAN, which may create a
      greater burden than necessary. In cases where there is actually
      only one RBridge next hop recipient of interest, this problem can
      be eliminated by using the option of unicasting TRILL data or
      ESADI frames to that recipient rather than multicasting it to All-

   Inserting RBridges so that all the bridged portions of the LAN stay
   connected to each other and have multiple RBridge connections is
   generally the least efficient arrangement.

   There are four techniques that may help if problem 1 above occurs and
   which can, to some extent, be used in combination:

   1. Replace more IEEE 802.1 bridges with RBridges so as to minimize
      the size of the remnant bridged LANs between RBridges. This
      requires no configuration of the RBridges unless the bridges they
      replace required configuration.

   2. Re-arrange network topology to minimize the problem.  If the
      bridges and RBridges involved are configured, this may require
      changes in their configuration.

   3. Configure the RBridges and bridges so that end stations on a
      remnant bridged LAN are separated into different VLANs that have
      different appointed forwarders. If the end stations were already
      assigned to different VLANs, this is straightforward (see Section If the end stations were on the same VLAN and have to be
      split into different VLANs, this technique may lead to
      connectivity problems between end stations.

   4. Configure the RBridges such that their ports which are connected
      to the bridged LAN send BPDUs (see Section A.3.3) in such a way as
      to force the partition of the bridged LAN. (Note: A spanning tree
      is never formed through an RBridge but always terminates at
      RBridge ports.)  To use this technique, the RBridges must support

R. Perlman, et al                                              [Page 77]

INTERNET-DRAFT                                          RBridge Protocol

      this optional feature, and would need to be configured to make use
      of it, but the bridges involved would rarely have to be
      configured.  Warning: This technique makes the bridged LAN
      unavailable for TRILL through traffic because the bridged LAN

   Conversely to item 3 above, there may be bridged LANs which use
   VLANs, or use more VLANs than would otherwise be necessary, to
   support the Multiple Spanning Tree Protocol or otherwise reduce the
   congestion that can be caused by a single spanning tree. Replacing
   the IEEE 802.1 bridges in such LANs with RBridges may enable a
   reduction in or elimination of VLANs and configuration complexity.

A.3 Wiring Closet Topology

   If 802.1 bridges are present and RBridges are not properly
   configured, the bridge spanning tree or the DRB may make
   inappropriate decisions.  Below is a specific example of the more
   general problem that can occur when a bridged LAN is connected to
   multiple RBridges.

   In cases where there are two (or more) groups of end nodes, each
   attached to a bridge (say B1 and B2), and each bridge is attached to
   an RBridge (say RB1 and RB2 respectively), with an additional link
   connecting B1 and B2 (see Figure A.2), it may be desirable to have
   the B1-B2 link only as a backup in case one of RB1 or RB2 or one of
   the links B1-RB1 or B2-RB2 fail.

                  |             |          |      |
                  |  Data    +-----+    +-----+   |
                  | Center  -| RB1 |----| RB2 |-  |
                  |          +-----+    +-----+   |
                  |             |          |      |
                                |          |
                                |          |
                  |             |          |      |
                  |          +----+     +----+    |
                  | Wiring   | B1 |-----| B2 |    |
                  | Closet   +----+     +----+    |
                  | Bridged                       |
                  | LAN                           |

                    Figure A.2: Wiring Closet Topology

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INTERNET-DRAFT                                          RBridge Protocol

   For example, B1 and B2 may be in a wiring closet and it may be easy
   to provide a short, high bandwidth, low cost link between them while
   RB1 and RB2 are at a distant data center such that the RB1-B1 and
   RB2-B2 links are slower and more expensive.

   Default behavior might be that one of RB1 or RB2 (say RB1) would
   become DRB for the bridged LAN including B1 and B2 and appoint itself
   forwarder for the VLANs on that bridged LAN. As a result, RB1 would
   forward all traffic to/from the link, so end nodes attached to B2
   would be connected to the campus via the path B2-B1-RB1, rather than
   the desired B2-RB2. This wastes the bandwidth of the B2-RB2 path and
   cuts available bandwidth between the end stations and the data center
   in half. The desired behavior would be to make use of both the RB1-B1
   and RB2-B2 links.

   Three solutions to this problem are described below.

A.3.1 The RBridge Solution

   Of course, if B1 and B2 are replaced with RBridges, the right thing
   will happen with zero configuration (other than VLAN support), but
   this may not be immediately practical if bridges are being
   incrementally replaced by RBridges.

A.3.2 The VLAN Solution

   If the end stations attached to B1 and B2 are already divided among a
   number of VLANs, RB1 and RB2 could be configured so that which ever
   becomes DRB for this link will appoint itself forwarder for some of
   these VLANs and appoint the other RBridge for the remaining VLANs.
   Should either of the RBridges fail or become disconnected, the other
   will have only itself to appoint as forwarder for all the VLANs.

   If the end stations are all on a single VLAN, then it would be
   necessary to assign them between at least two VLANs to use this
   solution. This may lead to connectivity problems that might require
   further measures to rectify.

A.3.3 The Spanning Tree Solution

   Another solution is to configure RB1 and RB2 to be part of a "wiring
   closet group", with a configured System ID RBx (which may be RB1 or
   RB2's System ID). Both RB1 and RB2 emit BPDUs on their configured
   ports as highest priority root RBx. This causes the spanning tree to

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INTERNET-DRAFT                                          RBridge Protocol

   logically partition the bridged LAN as desired by blocking the B1-B2
   link at one end or the other (unless one of the bridges is configured
   to also have highest priority and has a lower ID, which we consider
   to be a misconfiguration).  With the B1-B2 link blocked, RB1 and RB2
   cannot see each other's TRILL Hellos via that link and each acts as
   Designated RBridge and appointed forwarder for its respective
   partition. Of course, with this partition, no TRILL through traffic
   can flow over the RB1-B1-B2-RB2 path.

   In the spanning tree BPDU, the Root is "RBx" with highest priority,
   cost to Root is 0, Designated Bridge ID is "RB1" when RB1 transmits
   and "RB2" when RB2 transmits, and port ID is a value chosen
   independently by each of RB1 and RB2 to distinguish each of its own
   ports. (If RB1 and RB2 were actually bridges on the same shared
   medium with no bridges between them, the result would be that the one
   with the larger ID sees "better" BPDUs (because of the tiebreaker on
   the third field: the ID of the transmitting RBridge), and would turn
   off its port.)

   Should either RB1 or the RB1-B1 link or RB2 or the RB2-B2 link fail,
   the spanning tree algorithm will stop seeing one of the RBx roots and
   will unblock the B1-B2 link maintaining connectivity of all the end
   stations with the data center.

   If the link RB1-B1-B2-RB2 is on the cut set of the campus and RB2 and
   RB1 have been configured to believe they are part of a wiring closet
   group, the campus becomes partitioned as the link is blocked.

A.3.4 Comparison of Solutions

   Replacing all 802.1 bridges with RBridges is usually the best
   solution with the least amount of configuration required, possibly

   The VLAN solution works well with a relatively small amount of
   configuration if the end stations are already divided among a number
   of VLANs. If they are not, it becomes more complex and problematic.

   The spanning tree solution does quite well in this particular case.
   But it depends on both RB1 and RB2 having implemented the optional
   feature of being able to configure a port to emit BPDUs as described
   in Section A.3.3 above. It also makes the bridged LAN whose partition
   is being forced unavailable for through traffic. Finally, while in
   this specific example it neatly breaks the link between the two
   bridges B1 and B2, if there were a more complex bridged LAN, instead
   of exactly two bridges, there is no guarantee that it would partition
   into roughly equal pieces. In such a case, you might end up with a
   highly unbalanced load on the RB1-B1 link and the RB2-B2 link.

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INTERNET-DRAFT                                          RBridge Protocol

Appendix B: Trunk and Access Port Configuration

   Many modern bridged LANs are organized into a core and access model,
   The core bridges have only point-to-point links to other bridges
   while the access bridges connect to end stations, core bridges, and
   possibly other access bridges.  It seems likely that some RBridge
   campuses will be organized in a similar fashion.

   An RBridge port can be configured as a trunk port, that is, a link to
   another RBridge or RBridges, by configuring it to disable end station
   support. There is no reason for such a port to have more than one
   VLAN enabled and in its Announcing Set on the port. Of course, the
   RBridge (or RBridges) to which it is connected must have the same
   VLAN enabled. There is no reason for this VLAN to be other than the
   default VLAN 1 unless, perhaps, the link is actually over carrier
   Ethernet facilities that only provide some other specific VLAN or the
   like. Such configuration minimizes wasted TRILL Hellos and eliminates
   useless decapsulation and transmission of multi-destination traffic
   in native form onto the link. (see Sections 4.2.4 and 4.7.1)

   An RBridge access port would be expected to lead to a link with end
   stations and possibly one or more bridges. Such a link might also
   have more than one RBridge connected to it to provide more reliable
   service to the end stations.  It would be a goal to minimize or
   eliminate transit traffic on such as link as it is intended for end
   station native traffic. This can be accomplished by turning on the
   access port configuration bit for the RBridge port or ports connected
   to the link as further detailed in Section 4.7.1.

   When designing RBridge configuration user interfaces, consideration
   should be given to making it convenient to configure ports as trunk
   and access ports.

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INTERNET-DRAFT                                          RBridge Protocol

Appendix C: Multipathing

   Rbridges support multipathing of both known unicast and multi-
   destination traffic. Implementation of multipathing is optional.

   Multi-destination traffic can be multipathed by using different
   distribution tree roots for different frames. For example, assume
   that in Figure C.1 end stations attached to RBy are the source of
   various multicast streams each of which has multiple listeners
   attached to various of RB1 through RB9. Assuming equal bandwidth
   links, the distribution tree rooted at RBy will predominantly use the
   vertical links among RB1 through RB9 while that rooted at RBz will
   predominantly use the horizontal.  If RBy chooses itself as the
   distribution tree root for half of this traffic and RBz the root for
   the other half, it may be able to substantially increase the
   aggregate bandwidth by making use of both the vertical and horizontal
   links among RB1 through RB9.

   Since the distribution trees an RBridge must calculate are the same
   for all RBridges and transit RBridges MUST respect the tree root
   specified by the ingress RBridge, a campus will operate correctly
   with a mix of RBridges some of which use different roots for
   different multi-destination frames they ingress and some of which use
   a single root for all such frames.

                              +---+               |
                             /  |  \              |
                           /    |    \            |
                         /      |      \          |
                      +---+   +---+   +---+       |
                      |RB1|---|RB2|---|RB3|       |
                      +---+   +---+   +---+\      |
                        |       |       |    \    |
                      +---+   +---+   +---+    \+---+
                      +---+   +---+   +---+    /+---+
                        |       |       |    /
                      +---+   +---+   +---+/
                      +---+   +---+   +---+

                  Figure C.1: Multi-Destination Multipath

   Known unicast equal cost multipathing (ECMP) can occur if, instead of
   using a tie-breaker criterion when building an SPF path between
   ingress and egress RBridges, information about equal cost paths is
   retained.  Different unicast frames can then be sent via different
   equal cost paths. For example, in Figure C.2, there are three equal

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   cost paths between RB1 and RB2 and two equal cost paths between RB2
   and RB5.

   A transit RBridge receiving a known unicast frame forwards it towards
   the egress RBridge and is not concerned with whether it believes
   itself to be on any particular path from the ingress RBridge or a
   previous transit RBridge.  Thus a campus will operate correctly with
   a mix of RBridges some of which implement ECMP and some of which do

   As an alternative to multipathing, it might be possible to combine
   the three paths between RB1 and RB2 into one logical link through the
   "link aggregation" feature of 802.3 (see Clause 43 of [802.3]).
   Rbridges MAY implement link aggregation. However, link aggregation
   requires multiple single hop equal bandwidth links (no intervening
   bridges). Equal cost multipathing is more general in that there can
   be multiple hops with intervening bridges and RBridges and links of
   different costs as long as the path cost is the same.  (Generally,
   the default estimate of the cost of a link is proportional to the
   reciprocal of its line speed.)

                               +---+       double line = 10 Gbps
                 -----      ===|RB3|---     single line = 1 Gbps
                /     \   //   +---+   \
            +---+     +---+            +---+
         ===|RB1|-----|RB2|            |RB5|===
            +---+     +---+            +---+
                \     /   \    +---+   //
                 -----     ----|RB4|===

                    Figure C.2: Known Unicast Multipath

   When multipathing is used, frames that follow different paths will be
   subject to different delays and may be re-ordered.  While some
   traffic may be order/delay insensitive, typically most traffic
   consists of flows of frames where re-ordering within a flow is
   damaging. How to determine flows or what granularity flows should
   have is beyond the scope of this document but, as an example, under
   many circumstances it would be safe to consider all the frames
   flowing between a particular pair of end station ports to be a flow.

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Appendix D: Determination of VLAN and Priority

   A high level informative summary of how VLAN ID and priority are
   determined for incoming native frames, omitting some details, is
   given in the bulleted items below. For more detailed information, see

   o  When an untagged native frame arrives, a zero configuration
      RBridge associates the default priority zero and the VLAN ID 1
      with it. It actually sets the VLAN for the untagged frame to be
      the "port VLAN ID" associated with that port. The port VLAN ID
      defaults to VLAN ID 1 but may be configured to be any other VLAN
      ID. An Rbridge may also be configured on a per port basis to
      discard such frames or to associate a different priority code
      point with them.  Determination of the VLAN ID associated with an
      incoming untagged non-control frame may also be made dependent on
      the Ethertype or NSAP (referred to in 802.1 as the Protocol) of
      the arriving frame, the source MAC address, or other local rules.

   o  When a priority tagged native frame arrives, a zero configuration
      RBridge associates with it both the port VLAN ID, which defaults
      to 1, and the priority code point provided in the priority tag in
      the frame.  An Rbridge may be configured on a per port basis to
      discard such frames or to associate them with a different VLAN ID
      as described in the point immediately above.  It may also be
      configured to map the priority code point provided in the frame by
      specifying, for each of the eight possible values that might be in
      the frame, what actual priority code point will be associated with
      the frame by the RBridge.

   o  When a C-tagged (formerly called Q-tagged) native frame arrives, a
      zero configuration RBridge associates with it the VLAN ID and
      priority in the C-tag.  An RBridge may be configured on a per port
      per VLAN basis to discard such frames. It may also be configured
      on a per port basis to map the priority value as specified above
      for priority tagged frames.

   In 802.1, the process of associating a priority code point with a
   frame, including mapping a priority provided in the frame to another
   priority, is referred to as priority "regeneration".

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Appendix Z: Revision History

   RFC Editor: Please delete this Appendix Z before publication. In
   addition, please replace the string "<RFC-this-document>" where it
   occurs in this document with "RFC xxxx" where xxxx is the RFC number
   assigned to this document.

Changes from -03 to -04

    1. Divide IANA Considerations section into IANA and IEEE parts. Add
       IANA considerations for TRILL Header variations and reserved bit
       and normative references to RFCs 2434 and 4020.

    2. Add note on the terms Rbridge and TRILL to section 1.2.

    3. Remove IS-IS marketing text.

    4. Split Section 3 into Sections 3 and 4.  Add a new top level
       section "5. Pseudo Code", renumbering following sections. Move
       pseudo code that was in old Section 3 into Section 5 and make
       section 3 more textural.  This idea is that Section 3 and 4 have
       more readable text descriptions with some corner cases left out
       for simplicity while section 5 has more structured and complete

    5. Revised and extended Security Considerations section.

    6. Move multicast router attachment bit and IGMP membership report
       information from the per-VLAN IS-IS instance to the core IS-IS
       instance so the information can be used by core RBridges to prune
       distribution trees.

    7. Remove ARP/ND optimization.

    8. Change TRILL Header to add option feature. Add option section.

    9. Change TRILL Header to expand Version field to the Variation
       field. Add TRILL message variations (8 bits) supported to the per
       RBridge link state information.

   10. Distinguish TRILL data and IS-IS messages by using Variation = 0
       and 1.

   11. Consistently state that VLAN pruning and IP derived multicast
       pruning of distribution trees are SHOULD.

   12. Add text and pseudo code to discard TRILL Ethertype data frames
       received on a port that does not have an IS-IS adjacency on it.

R. Perlman, et al                                              [Page 85]

INTERNET-DRAFT                                          RBridge Protocol

   13. Add end station address learning section.  Specify end station
       address learning from decapsulated native frames.

   14. Add nickname allocation priority and optional nickname
       configuration. Reserve nickname values zero and 0xFFFF.

   15. Explain about multiple Designated RBridges because of multiple

   16. Add Incremental Deployment Considerations Section incorporating
       expanded Wiring Closet Topology Section.

   17. Add more detail on VLAN tag information and material on frame

   18. Miscellaneous minor editing and terminology updates.

Changes from -04 to -05

   NOTE: Section 5 was NOT updated as indicated below but the remainder
   of the draft was so updated.

    1. Mention optional VLAN and multicast optimization in Abstract.

    2. Change to distinguish TRILL IS-IS from TRILL data frames based on
       the Inner.MacDA instead of a TRILL Header bit.

    3. Split IP multicast router attached bit in two so you can
       separately indicate attachment of IPv4 and IPv6 routers.  Provide
       that these bits must be set if an RBridge does not actually do
       multicast control snooping on ingressed traffic.

    4. Add the term "port VLAN ID" (PVID).

    5. Drop references to PIM. Improve discussions of IGMP, MLD, and MRD

    6. Move M bit over one and create two bit pruning field at the
       bottom of the "V" combined field.

    7. Add pruning control values of V and discussion of same.

    8. Permit optional unicast transmission of multi-destination frames
       when there is only one received out a port.

    9. Miscellaneous minor editing and terminology updates.

R. Perlman, et al                                              [Page 86]

INTERNET-DRAFT                                          RBridge Protocol

Changes from -05 to -06

    1. Revise Section 2 discussion of DRB determination in the presence
       of VLANs and move it to Section 2.2. Adjust VLAN handling

    2. Change "V" field to be a 2-bit version fields followed by 2
       reserved bits. Make corresponding changes to eliminate the
       inclusion in the header of frame analysis indicating type of
       multi-destination pruning which is proper for frame.  Thus all
       non-ingress RBridges that wish to perform such pruning are forced
       to do full frame analysis. Make further corresponding changes in
       IANA Considerations.

    3. The Inner.MacDA for TRILL IS-IS frames is changed to a second
       multicast address: All-IS-IS-RBridges. IEEE Allocation
       Considerations, etc., are correspondingly changed.

    4. Note in Section 6 that bridges can hide slow links and generally
       make it harder from RBridges to determine the cost of an RBridge
       to RBridge hop that is a bridged LAN.

    5. Add material noting that replacement of bridges by RBridges can
       cause connectivity between previously isolated islands of the
       same VLAN.

    6. Expand Security Considerations by mentioning RFC 3567 and
       indicating that TRILL enveloping may reduce the effectively of
       TRILL-ignorant firewall functionality.

    7. Extensive updates to pseudo code.

    8. Change to one DRB per physical link that dictates the inter-
       RBridge VLAN for the link, appoints forwarders per-VLAN, can be
       configured to send Hellos on multiple VLANs, etc.

    9. Add a minimal management by SNMP statement to Section 2.

   10. Delete explicit requirement to process TRILL frames arriving on a
       port even if the port implements spanning tree and is in spanning
       tree blocked state.

   11. Miscellaneous minor editing and terminology updates.

Changes from -06 to -07

   [WARNING: Section 5 of draft -07 was not fully updated to incorporate
   the changes below.]

R. Perlman, et al                                              [Page 87]

INTERNET-DRAFT                                          RBridge Protocol

    1. Drop recommendation to set "bridge" flags in some 802.1AB frame

    2. Add Section 2.5 giving an informative description of zero
       configuration behavior for 802.1D and 802.1Q bridges and

    3. Add Section 4.7 (renumbering the former 4.7 to be 4.9) on the
       receipt, handing, and transmission of MVRP and other MRP frames
       by RBridges. Add references to 802.1ak.

    4. Add Section 4.8 on Multipathing.

    5. Partial changes to Section 5 to correspond with changes elsewhere
       in the draft.

    6. Addition of frame category definitions in Section 1.2.

    7. Addition of Section 10, Acronyms.

    8. Add note in Section 6.2 that difficult in link cost determination
       due to intervening devices is not confined to RBridges.

    9. Re-ordered some sections in Section 6.

   10. Added a paragraph about taking care if trying to use VLANs for
       security to Security Considerations Section and re-ordered
       paragraphs in that section.

   11. Added mention of being able to configure a port so that native
       frames are not send and are dropped on receipt. Probably need to
       say more about this.

   12. Remove material about pseudo node suppression.

   13. Fix a few cases where hop count was off by one.

   14. Add option critical bits when option area length non-zero.

   15. Replace some remaining references to Q-tag with C-tag.

   16. Miscellaneous minor editing and terminology updates. Changed
       Figure numbers to be relative to major section. Added Table

R. Perlman, et al                                              [Page 88]

INTERNET-DRAFT                                          RBridge Protocol

Changes from -07 to -08

    1. Add "low" and "high" level control frame definitions to Section
       1.2 and note concerning frames that would qualify as both "TRILL"
       and "control" frames. Utilize these defined frame types more
       consistently through the document.

    2. Move substantial areas of tutorial, motivational, and
       informational text to Appendices, or a separate document,
       including Sections numbered 2.5, 4.8, 6.3, and 6.4 in version
       -07. Remove pseudo-code (Section 5 in version -07).

    3. Move link Hellos / VLAN specification and discussion to a new
       subsection of Section 4.

    4. Replace distribution tree root flag per RBridge with new logic
       which orders all RBridges in a campus as to their priority to be
       a distribution tree root and provides for the highest priority
       distribution tree root to dictate the numbers of trees in the
       campus. RBridges use the tree with least cost from themselves to
       the tree root for multi-destination frame distribution, or the n
       such trees if they multi-path multi-destination traffic.

    5. Add "Access" port configuration bit and Appendix on Trunk and
       Access Links.

    6. Add statement that use of S-tags in TRILL is outside the scope of
       this document.

    7. Add new section on RBridge port structure (Section 4.7) which
       includes discussion of RBridge interactions with BPDUs and
       revised interactions with VRP frames. Make provisions for dynamic
       VLAN registration a "MAY" implement and agnostic between GVRP and
       MVRP. Remove references to 802.1ak. Simplify text related to VRP.
       Remove related configuration option.

    8. Add requirement to adjust input filters no later than output

    9. Add requirement for configurable (default 30 second) inhibition
       on RBridge decapsulation out a port if a root bridge change has
       just been observed on that port.

   10. Add provisions for propagating topology change to attached
       bridged LAN when an RBridge is de-appointed forwarder. Also other
       end station addressing forgetting details including per VLAN
       forwarding status dropped counter.

   11. Delete requirement that appointed forwarder wait until it has
       received all the LSPs listed in the first CSNP (if any) it has

R. Perlman, et al                                              [Page 89]

INTERNET-DRAFT                                          RBridge Protocol

       received from its neighbors before forwarding frames off a link.

   12. Add explicit criterion for when an RBridge port defers to the DRB
       indicated in a Hello it receives even if that Hello is not from
       the DRB or even from an RBridge in direct communication with the

   13. Add provisions for pseudonode minimization.

   14. Update reference to RFC 2434 to be to RFC 5226.

   15. Miscellaneous minor editing and terminology updates. Add Figures
       index after Table of Contents.

Changes from -08 to -09

    1. Specify SHOULD as the implementation requirement for SNMPv3

    2. Change default confidence level to 0x20 for addresses learned
       from observing locally received native frames and from
       decapsulating TRILL data frames. This provides more space for
       lower confidence levels.

    3. Add security consideration for observation of traffic no longer
       constrained to links in its Inner.VLAN due to TRILL

    4. Updated bridge configuration assumptions in Section 2.3.1.

    5. Use "inhibited" to describe the status of an appointed forwarder
       when it is temporarily discarding all received native frames and
       not sending any native frames.

    6. In Section 4.3, there was an implication that the priority to be
       a tree root and the number of trees to be computed had not only
       default values for a zero configuration RBridge but could also be
       individually present or absent in the LSP for the RBridge. This
       tends to lead to a variable-length sub-TLV or multiple sub-TLVs
       in the LSP that leads to additional code paths to test. So
       various "if advertised" conditional clauses have been removed.

    7. Reserve nicknames 0xFFC0 through 0xFFFE as well as 0x0000 and
       0xFFFF and provide IANA Considerations for their allocation.

    8. Improve Figure 4.1, "TRILL Data Encapsulation over Ethernet" by
       generalizing it and adding an RBridge diagram.

R. Perlman, et al                                              [Page 90]

INTERNET-DRAFT                                          RBridge Protocol

    9. Add "access port" bit to Hello. Extend and clarify behavior for
       access ports and for the occurrence of the IS Neighbor TLV in
       TRILL Hellos.

   10. Miscellaneous minor editing.

Changes from -09 to -10

    1. Split Section 2.4 into two subsections inserting 2.4.1 with a
       simplified RBridge port diagram and discussion of how RBridges
       mostly use the mechanisms of IEEE 802.1Q bridges below the EISS

    2. Remove the "SHOULD" requirement that the hop count for multi-
       destination frames not be set by the ingress RBridge in excess of
       the distance through the distribution tree to the most remote

    3. Remove any implication that addresses received by ESADI are
       always better than those learned from the data plane.

    4. Rephrase language concerning the case where a known unicast
       native frame in receive by an RBridge to be output in native form
       on another link of that RBridge so that instead of describing
       this as logically forwarding the frame in native form it is
       described as logically encapsulating and then decapsulating the

    5. Remove language saying that a TRILL Ethertype frame with a
       broadcast outer destination address MAY be treated as if its
       outer destination address was All-RBridges.

    6. Clarify that all TRILL data frames with unknown or reserved
       egress nicknames are discarded.

    7. Substantially expand Figure 4.6 at the upper port layers and
       correspondingly expand the accompanying text that is now Section

    8. Change TRILL IS-IS frames so they are no longer encapsulated but
       have the All-IS-IS-RBridges Outer.MacDA. Change the Inner.MacDA
       of ESADI frames to be the new All-ESADI-RBridges multicast

    9. Update reference to RFC 3567 to be to RFC 5304.

   10. Miscellaneous minor editing changes.

R. Perlman, et al                                              [Page 91]

INTERNET-DRAFT                                          RBridge Protocol

Changes from -10 to -11

    1. Add BPDU/Hello denial of service section to Security

    2. Remove general prohibition on RBridges sending spanning tree

    3. Change ESADI from "End Station Address Distribution Instance" to
       "End Station Address Distribution Information".

    4. Delete redundant requirement that TRILL IS-IS Hellos be
       distinguished by the port from which they are sent.

    5. Add Maximum Transit Delay for RBridges with enforcement a MAY.

    6. Confused note re DRB deferral deleted.

    7. Update boilerplate and make miscellaneous minor editing changes.

Changes from -11 to -12

    1. Changes in the determination of the distribution trees to allow
       the highest priority RBridge to explicitly list some or all of
       the tree roots. Change the listing of distribution trees an
       RBridge can use in encapsulating multi-destination frames to
       allow the RBridge to not explicitly list all the roots it can

    2. Add figures and a little text illustrating the structure of TRILL
       IS-IS and TRILL ESADI frames.

    3. Add brief discussion of Hello size limitations.

    4. Extend appointed forwarder inhibition to also occur on receiving
       a Hello sent on VLAN-x as well as received on VLAN-x in cases of
       VLAN translation.

    5. Provide for the allocation of a block of 16 multicast addresses
       for TRILL use by the IETF Registration Authority. RBridges
       conforming to this specification discard frames sent to any of
       these addresses other than All-RBridges and All-IS-IS-RBridges.
       (All-ESADI-RBridges is only allowed as an Inner.MacDA.)

    6. Add text on MTU and add Protection Hellos so there are now two
       kinds of Hellos, Adjacency and Protection.

    7. Add text mandating the RBridges with the Extended IS Adjacency

R. Perlman, et al                                              [Page 92]

INTERNET-DRAFT                                          RBridge Protocol

       TLV (#22) and do not use the IS Adjacency TLV (#2).

    8. Add text requiring and specifying "tie-breaking" to select only
       one when sending multi-destination frames between RBridges
       connected by multiple parallel links. Mandate three-way handshake
       on links configured to use P2P Hellos to provide Extended Circuit

    9. Add section and material on using P2P Hellos.

   10. Miscellaneous minor editing changes.

R. Perlman, et al                                              [Page 93]

INTERNET-DRAFT                                          RBridge Protocol

Authors' Addresses

   Radia Perlman
   Sun Microsystems
   16 Network Circle
   Menlo Park, CA 94025

   Phone: +1-650-960-1300
   Email: Radia.Perlman@sun.com

   Donald E. Eastlake, 3rd
   Stellar Switches
   155 Beaver Street
   Milford, MA 01757 USA

   Phone: +1-508-333-2270
   Email: d3e3e3@gmail.com

   Dinesh G. Dutt
   Cisco Systems
   170 Tasman Drive
   San Jose, CA 95134-1706 USA

   Phone: +1-408-527-0955
   Email: ddutt@cisco.com

   Silvano Gai
   Nuova Systems
   2600 San Tomas Expressway
   Santa Clara, CA 95051 USA

   Phone: +1-408-387-6123
   Email: sgai@nuovasystems.com

   Anoop Ghanwani
   Brocade Communications Systems
   1745 Technology Drive
   San Jose, CA 95110 USA

   Phone: +1-408-333-7149
   Email: anoop@brocade.com

R. Perlman, et al                                              [Page 94]

INTERNET-DRAFT                                          RBridge Protocol

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R. Perlman, et al                                              [Page 95]

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