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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: January 13, 2009                           Eastlake Enterprises
                                                          Dinesh G. Dutt
                                                           Cisco Systems
                                                             Silvano Gai
                                                           Nuova Systems
                                                          Anoop Ghanwani
                                                                 Brocade
                                                           July 14, 2008


                 Rbridges: Base Protocol Specification

               <draft-ietf-trill-rbridge-protocol-08.txt>


Status of This Document

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   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-
   Drafts.

   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
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.












R. Perlman, et al                                               [Page 1]


INTERNET-DRAFT                                          RBridge Protocol


Abstract

   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 based on RBridge
   destinations (rather than end node destinations), which allows
   internal forwarding tables to be substantially smaller than in
   conventional bridge systems.



Acknowledgements

   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, David Melman, Erik
   Nordmark, Sanjay Sane, Pekka Savola, Matthew Thomas, Joe Touch, and
   Mark Townsley.  We invite you to join the mailing list at
   http://www.postel.org/rbridge.





















R. Perlman, et al                                               [Page 2]


INTERNET-DRAFT                                          RBridge Protocol


Table of Contents

      Status of This Document....................................1
      Abstract...................................................2
      Acknowledgements...........................................2

      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....................13
      2.2.1 Known-Unicast.......................................15
      2.2.2 Multi-destination...................................15
      2.3 RBridges and VLANs....................................16
      2.3.1 Link VLAN Assumptions...............................17
      2.4 RBridges and IEEE 802.1 Bridges.......................17

      3. Details of the TRILL Header............................19
      3.1 TRILL Header Format...................................19
      3.2 Version (V)...........................................19
      3.3 Reserved (R)..........................................20
      3.4 Multi-destination (M).................................20
      3.5 TRILL Header Options..................................20
      3.6 Hop Count.............................................21
      3.7 RBridge Nicknames.....................................22
      3.7.1 Egress RBridge Nickname.............................22
      3.7.2 Ingress RBridge Nickname............................23
      3.7.3 RBridge Nickname Allocation.........................23

      4. Other RBridge Design Details...........................25
      4.1 Ethernet Data Encapsulation...........................25
      4.1.1 VLAN Tag Information................................26
      4.1.2 Inner VLAN Tag......................................27
      4.1.3 Outer VLAN Tag......................................28
      4.1.4 Frame CheckSum (FCS)................................28
      4.2 Link State Protocol (IS-IS)...........................29
      4.2.1 IS-IS RBridge Identity..............................29
      4.2.2 IS-IS Instances.....................................29
      4.2.3 Core TRILL IS-IS....................................30
      4.2.3.1 Core IS-IS Link Protocol..........................30
      4.2.3.2 Designated RBridge................................33
      4.2.3.3 Appointed VLAN-x Forwarder........................34
      4.2.3.4 Core TRILL IS-IS LSP Information..................35
      4.2.4 End Station Address Distribution Instance (ESADI)
            IS-IS...............................................37
      4.2.4.1 ESADI Protocol....................................37


R. Perlman, et al                                               [Page 3]


INTERNET-DRAFT                                          RBridge Protocol


Table of Contents Continued

      4.2.4.2 ESADI Information.................................38
      4.3 Distribution Trees....................................38
      4.3.1 Distribution Tree Calculation and Checks............39
      4.3.2 Pruning the Distribution Tree.......................40
      4.3.3 Tree Distribution Optimization......................41
      4.3.4 Forwarding Using a Distribution Tree................42
      4.4 Frame Processing Behavior.............................43
      4.4.1 Receipt of a Native Frame...........................43
      4.4.1.1 Native Unicast Case...............................43
      4.4.1.2 Native Multicast and Broadcast Frames.............44
      4.4.2 Receipt of a TRILL Frame............................45
      4.4.2.1 TRILL IS-IS Frames................................45
      4.4.2.2 TRILL Data Frames.................................46
      4.4.3 Receipt of a Control Frame..........................47
      4.5 IGMP, MLD, and MRD Learning...........................48
      4.6 End Station Address Details...........................48
      4.6.1 Learning End Station Addresses......................49
      4.6.2 Forgetting End Station Addresses....................50
      4.6.3 Shared VLAN Learning................................51
      4.7 RBridge Port Structure................................52
      4.7.1 BPDU Handling.......................................55
      4.7.1.1 Receipt of BPDUs..................................55
      4.7.1.2 Root Bridge Changes...............................55
      4.7.1.3 Transmission of BPDUs.............................56
      4.7.2 Dynamic VLAN Registration...........................56

      5. RBridge Addresses, Parameters, and Constants...........57

      6. Security Considerations................................58

      7. Assignment Considerations..............................59
      7.1 IANA Considerations...................................59
      7.2 IEEE 802 Assignment Considerations....................59

      8. Normative References...................................60

      9. Informative References.................................61

      Appendix A: Incremental Deployment Considerations.........62
      A.1 Link Cost Determination...............................62
      A.2 Appointed Forwarders and Bridged LANs.................62
      A.3 Wiring Closet Topology................................64
      A.3.1 The RBridge Solution................................65
      A.3.2 The VLAN Solution...................................65
      A.3.3 The Spanning Tree Solution..........................65
      A.3.4 Comparison of Solutions.............................66

      Appendix B: Trunk and Access Port Configuration...........67


R. Perlman, et al                                               [Page 4]


INTERNET-DRAFT                                          RBridge Protocol


Table of Contents Continued

      Appendix C: Multipathing..................................68

      Appendix D: Determination of VLAN and Priority............70

      Appendix Z: Revision History..............................71
      Changes from -03 to -04...................................71
      Changes from -04 to -05...................................72
      Changes from -05 to -06...................................73
      Changes from -06 to -07...................................73
      Changes from -07 to -08...................................74

      Disclaimer................................................77
      Additional IPR Provisions.................................77
      Authors' Addresses........................................78




































R. Perlman, et al                                               [Page 5]


INTERNET-DRAFT                                          RBridge Protocol


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................14
      Figure 3.1: TRILL Header..................................19
      Figure 3.2: Options Area Initial Flags Byte...............21
      Figure 4.1: TRILL Data Encapsulation over Ethernet........26
      Figure 4.2: VLAN Tag Information..........................27
      Figure 4.3: RBridge Port Model............................54
      Figure A.1: Link Cost of a Bridged Link...................62
      Figure A.2: Wiring Closet Topology........................64
      Figure C.1: Multi-Destination Multipath...................68
      Figure C.2: Known Unicast Multipath.......................69






































R. Perlman, et al                                               [Page 6]


INTERNET-DRAFT                                          RBridge Protocol


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  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, perhaps unexpectedly, when spanning tree
      reconfigures due to a node failure or topology change.

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

   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] customer bridges.  While they can be applied to a variety of
   link protocols, this specification focuses on IEEE [802.3] links.















R. Perlman, et al                                               [Page 7]


INTERNET-DRAFT                                          RBridge Protocol


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 which 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.

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

      o  "TRILL" frames are those with the TRILL Ethertype. There are


R. Perlman, et al                                               [Page 8]


INTERNET-DRAFT                                          RBridge Protocol


         two sub-categories of TRILL frames as follows:
         -  "TRILL IS-IS" frames have the All-IS-IS-RBridges multicast
            address as the encapsulated destination address.
         -  "TRILL data" frames are all other TRILL frames.

      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
            (VRP).
         -  "low level control" frames are all control frames other than
            high level control frames.

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

   (Although, according to the definitions above, a frame could be both
   a TRILL frame and a control frame, such a frame would be invalid
   regardless of which view was taken and would be silently discarded.)

   This document uses Hexadecimal Notation for MAC addresses.  Each
   octet (that is, 8-bit byte) is represented by two hexadecimal digits
   giving the value of the octet as an unsigned integer and successive
   octets are separated by a hyphen. This document consistently uses
   IETF bit ordering 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.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].



1.4 Acronyms

      AllL1ISs - All Level 1 Intermediate Systems

      AllL2ISs - All Level 2 Intermediate Systems

      CHbH - Critical Hop-by-Hop

      CItE - Critical Ingress-to-Egress

      CSNP - Complete Sequence Number PDU

      DA - Destination Address



R. Perlman, et al                                               [Page 9]


INTERNET-DRAFT                                          RBridge Protocol


      EAP - Extensible Authentication Protocol

      ECMP - Equal Cost Multi-Path

      EISS - Extended Internal Sublayer Service

      ESADI - End Station Address Distribution Instance

      DRB - Designated RBridge

      FCS - Frame Check Sequence

      GARP - Generic Attribute Registration Protocol

      GVRP - GARP VLAN Registration Protocol

      IEEE - Institute of Electrical and Electronic 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

      MRP - Multiple Registration Protocol

      MVRP - Multiple VLAN Registration Protocol

      NSAP - Network Service Access Point

      PDU - Protocol Data Unit

      PPP - Point to Point Protocol

      RBridge - Routing Bridge

      RPF - Reverse Path Forwarding



R. Perlman, et al                                              [Page 10]


INTERNET-DRAFT                                          RBridge Protocol


      SA - Source Address

      SNP - Sequence Number PDU

      SPF - Shortest Path First

      TLV - Type, Length, Value

      TRILL - TRansparent Interconnection of Lots of Links

      VLAN - Virtual Local Area Network

      VRP - VLAN Registraion Protocol







































R. Perlman, et al                                              [Page 11]


INTERNET-DRAFT                                          RBridge Protocol


2. RBridges

   This section provides a high level overview of RBridges, which
   implement the TRILL protocol. 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. 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 header.

   Multipathing of multi-destination frames through alternative
   distribution tree roots and ECMP (Equal Cost MultiPath) of unicast
   frames are supported but not fully specified in this document (see
   Appendix C). Multipathing may introduce frame reordering as can
   differing frame priorities or changes in network topology.

   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, issues
   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 VLANs further.)


R. Perlman, et al                                              [Page 12]


INTERNET-DRAFT                                          RBridge Protocol


   Rbridges can be managed with SNMP [RFC3410].  The Rbridge MIB will be
   specified in a separate document.  This 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 has 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. 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 end station address distribution instance (ESADI) of
   IS-IS MAY be used by the appointed VLAN-x forwarder on a link to
   announce some or all of the attached VLAN-x end nodes on that link.
   The intention is that such an announcement would be used to announce
   end nodes that have been explicitly enrolled, and so such information
   would be more authoritative than simply learning 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 IS-IS also
   supports cryptographic authentication of its messages [RFC3567].  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 of IS-IS is optional, as is
   learning from these announcements.

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



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.


R. Perlman, et al                                              [Page 13]


INTERNET-DRAFT                                          RBridge Protocol


   Figure 2.1 shows two RBridges RB1 and RB2 interconnected through an
   Ethernet cloud. The Ethernet cloud may include point-to-point or
   shared media, hubs and IEEE 802.1 bridges. The Ethernet cloud may
   support VLAN tagging or not.

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

                    Figure 2.1: Interconnected RBridges

   Figure 2.2 shows the format of a TRILL frame traveling through the
   Ethernet cloud from RB1 to 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.

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

                Figure 2.3: A PPP Encapsulated TRILL Frame



R. Perlman, et al                                              [Page 14]


INTERNET-DRAFT                                          RBridge Protocol


   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 for several reasons:

   1. to mitigate loop issues a hop count field is included;

   2. to prevent original source MAC learning in the core from frames in
      transit;

   3. to direct 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. to provide a separate VLAN tag for forwarding traffic between
      RBridges, independent of the original VLAN of the 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 shared-media link will not get confused, as they
   might given multipathing, if they were to see the original source or
   ingress RBridge in the outer header.

   There are several types of transit frames between RBridges that are
   forwarded differently. They are here classified into two main
   categories: known-unicast and multi-destination.



2.2.1 Known-Unicast

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

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



2.2.2 Multi-destination

   These are frames that must be delivered to multiple destinations.

   The main cases of multi-destination frames are as follows:



R. Perlman, et al                                              [Page 15]


INTERNET-DRAFT                                          RBridge Protocol


   1. frames for unknown unicast destinations: the Inner.MacDA is
      unicast, but the ingress RBridge does not know its location;

   2. frames for Layer 2 multicast addresses derived from IP multicast
      addresses: 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
          reports;

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

      2.3 other IP derived Layer 2 multicast frames;

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

   4. frames for the Layer 2 broadcast address: the Inner.MacDA is
      broadcast.

   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.



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.
   The usual method of determining the community of a frame sent by an
   end station is based on the port on which it is initially received.
   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.  Some
   end nodes have the same capability.  VLAN tags used by TRILL are
   structured according to 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


R. Perlman, et al                                              [Page 16]


INTERNET-DRAFT                                          RBridge Protocol


   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 4.2.3.1 for further discussion of TRILL IS-IS operation
   on a link beyond that in the subsections below.)



2.3.1 Link VLAN Assumptions

   TRILL makes certain assumptions concerning the configuration of the
   links connected RBridges. Those links may be bridged LANs.  Campuses
   will still be safe if these assumptions do not hold, in that there
   will not be loops, unless RBridges are configured so as to disable
   their loop prevention features; but some end stations may not be
   provided with connectivity to the campus and some Rbridge ports may
   not be utilized. If lack of connectivity occurs due to violation of
   these assumptions, it can be restored by altering bridge
   configurations so as to conform to the assumptions. Such alteration
   could occur through management, dynamic VLAN registration (see
   Section 4.7.2), or a combination of these.

   TRILL assumes that for every link with two or more Rbridges on it,
   there will be at least one VLAN those RBridges are configured to use
   that will provide bi-directional connectivity between them. If this
   is not the case, one or more of the Rbridges can become orphaned from
   the link and not be usable for forwarding native traffic or transit
   TRILL data on that link.

   Even with partial RBridge deployment, TRILL will provide connectivity
   between all links in a particular VLAN that are connected to an
   RBridge, assuming TRILL connectivity between the RBridges themselves.
   For service to be provided to all end stations in a particular VLAN
   on a link, connectivity from those end stations to the appointed
   forwarder on that link for that VLAN is needed.



2.4 RBridges and IEEE 802.1 Bridges

   Because RBridges are generally compatible with current IEEE [802.1Q]
   customer bridges, a LAN can be upgraded by incrementally 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 un-
   managed, zero configuration bridges, then their RBridge replacements
   will not require configuration.


R. Perlman, et al                                              [Page 17]


INTERNET-DRAFT                                          RBridge Protocol


   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
   performance.

   See Appendix A for further discussion of incremental deployment.













































R. Perlman, et al                                              [Page 18]


INTERNET-DRAFT                                          RBridge Protocol


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, 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 which 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
      3.5.

   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.


R. Perlman, et al                                              [Page 19]


INTERNET-DRAFT                                          RBridge Protocol


3.3 Reserved (R)

   The two R bits are reserved for future use in extensions to 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 or is
      zero for a core instance TRILL IS-IS 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 an end station address
      distribution TRILL IS-IS 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 frame to correctly decapsulate and dispose of the
   inner frame.)

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






R. Perlman, et al                                              [Page 20]


INTERNET-DRAFT                                          RBridge Protocol


               +------+------+----+----+----+----+----+----+
               | CHbH | CItE |          Reserved           |
               +------+------+----+----+----+----+----+----+

                Figure 3.2: Options Area Initial Flags Byte

   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, for a transit RBridge. 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.

   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. A critical option is one
   which must be understood to safely process a frame.  A non-critical
   option can be safely ignored.

   Warning: 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 almost certainly 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 frame to another RBridge 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 an end station address


R. Perlman, et al                                              [Page 21]


INTERNET-DRAFT                                          RBridge Protocol


   distribution TRILL IS-IS frame) to at least the expected number of
   hops to the most distant RBridge and SHOULD NOT be set in excess of
   that amount. 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 so as to set the hop count to be no more than
   necessary to reach all destinations in that branch of the tree rooted
   at RBi. This minimal Hop Count on multi-destination frames is to
   reduce potential problems with temporary loops when forwarding.

   Although the RBridge MAY decrease the hop count by more than 1, 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
   and the value 0xFFFF is reserved for future specification.  RBridges
   piggyback a nickname acquisition protocol on the link state protocol
   (see Section 3.7.3) to acquire a nickname unique within the campus.



3.7.1 Egress RBridge Nickname

   There are three cases for the contents of the egress RBridge nickname
   field, depending on the M-bit (see Section 3.4) and the Inner.MacDA
   (see Section 4.1). It is filled in by the ingress RBridge for TRILL
   data frames and by the source RBridge for TRILL IS-IS frames. Once
   the egress nickname field is set, it MUST NOT be changed by any
   subsequent transit RBridge.

   o  For known unicast TRILL data frames, M == 0, the Inner.MacDA is
      not All-IS-IS-RBridges, and the egress RBridge nickname field
      specifies the egress RBridge i.e. it specifies the RBridge that
      needs to remove the TRILL encapsulation from the native frame.

   o  For core instance TRILL IS-IS frames, M == 0, the Inner.MacDA is
      All-IS-IS-Rbridge, and the egress RBridge nickname field is not
      used. Such frames may be sent before nicknames have been
      established and are only sent one hop.  The Egress RBridge
      Nickname MUST be set to zero by the source RBridge for such frames
      and is ignored by other RBridges.



R. Perlman, et al                                              [Page 22]


INTERNET-DRAFT                                          RBridge Protocol


   o  For multi-destination TRILL data frames and for ESADI (end station
      address distribution instance) TRILL IS-IS 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.



3.7.2 Ingress RBridge Nickname

   The ingress RBridge nickname is set to the nickname of the ingress
   RBridge for all TRILL data frames and to the nickname of the source
   RBridge for all ESADI (end station address distribution instance)
   TRILL IS-IS frames.

   For core TRILL IS-IS frames, which may be sent before nicknames have
   been established, the ingress nickname MUST be set to zero by the
   source RBridge and is ignored by receiving RBridges.

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



3.7.3 RBridge Nickname Allocation

   The nickname allocation 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 0xFFFF are reserved and MUST NOT be
      selected or configured. In some cases the value 0x0000 is used to
      indicate that the 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.



R. Perlman, et al                                              [Page 23]


INTERNET-DRAFT                                          RBridge Protocol


   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 choose a new nickname. This can require an RBridge with a
      configured nickname to select a different nickname.

   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 choose
      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.

   To minimize the probability of nickname collisions, each RBridge,
   when choosing its nickname, 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 choosing nicknames.

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




















R. Perlman, et al                                              [Page 24]


INTERNET-DRAFT                                          RBridge Protocol


4. Other RBridge Design Details

   Section 3 above specifies TRILL Headers, while this Section specified
   other RBridge design details.



4.1 Ethernet Data Encapsulation

   TRILL 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 without
   requiring any modification. To enable RBridges to distinguish 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 the Ethernet cloud of Figure 2.1 from RB1 to RB2. This
   encapsulation 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,
      respectively, over a shared Ethernet cloud.

   o  TRILL Header: Egress Nickname and Ingress Nickname. These specify
      the nickname values of the egress and ingress RBridges,
      respectively, for TRILL data frames unless they are 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.









R. Perlman, et al                                              [Page 25]


INTERNET-DRAFT                                          RBridge Protocol


   Outer Ethernet Header:
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             Outer Destination MAC Address  (RB2)              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Outer Destination MAC Address | Outer Source MAC Address      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                Outer Source MAC Address  (RB1)                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | 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                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Inner Destination MAC Address | Inner Source MAC Address      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Inner Source MAC Address                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Ethertype = C-Tag [802.1Q]    | Inner.VLAN Tag Information    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Payload:
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Ethertype of Original Payload |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
      |                                  Original Ethernet Payload    |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Frame CheckSum:
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  New FCS (Frame CheckSum)                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 4.1: TRILL Data Encapsulation over Ethernet



4.1.1 VLAN Tag Information

   A "VLAN Tag", also known as a "C-tag" (formerly Q-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 a tag is properly called a "priority tag"
   rather than a "VLAN Tag" [802.1Q].



R. Perlman, et al                                              [Page 26]


INTERNET-DRAFT                                          RBridge Protocol


   Use of 802.1Q S-tags (service tag) is beyond the scope of this
   document.

     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
     | 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. RBridges MAY also implement such configuration 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 and 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 cause frame re-ordering.

   The C-Tag Ethertype 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 when it was


R. Perlman, et al                                              [Page 27]


INTERNET-DRAFT                                          RBridge Protocol


   ingressed or the VLAN tag information associated with an ESADI (end
   station address distribution instance) TRILL IS-IS message when that
   message is created.  When a TRILL frame passes through a transit
   RBridge, the Inner.VLAN MUST NOT be changed.

   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 RBridges, 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.

   Transit RBridges forward TRILL frames with 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
   it is ingressed (see Appendix D).  TRILL IS-IS frames are 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).



4.1.4 Frame CheckSum (FCS)

   Each Ethernet frame has a single Frame CheckSum (FCS) that is
   computed to cover the entire frame, for detecting errors due to
   communications failures. It is typically calculated shortly before
   transmission and checked on receipt. Any 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


R. Perlman, et al                                              [Page 28]


INTERNET-DRAFT                                          RBridge Protocol


   the outer destination and source addresses, the decrementing of the
   hop count, etc.

   Although, as stated above, 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 a class
   of internal RBridge failures such as memory errors.



4.2 Link State Protocol (IS-IS)

   TRILL uses IS-IS [ISO10589] as the routing protocol, since it has the
   following advantages:

   o  it runs directly over Layer 2, so therefore 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
   addresses.

   A pseudonode is assigned a 7-byte ID by the DRB which created it,
   usually by taking a 6-byte ID owned by the DRB, and appending another
   byte. The only constraint is that the 7-byte ID be unique within the
   campus, and that the 7th byte be nonzero. An RBridge has a 7 byte ID
   consisting of its 6 byte system ID concatenated with a byte of 0.

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



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 a campus and are TRILL


R. Perlman, et al                                              [Page 29]


INTERNET-DRAFT                                          RBridge Protocol


   encapsulated.

   Layer 3 IS-IS native frames always have a multicast destination
   address of either AllL1ISs or AllL2ISs. When they are TRILL
   encapsulated, these multicast addresses appear as the Inner.MacDA.
   TRILL IS-IS messages are always encapsulated and always have a
   different Inner.MacDA, namely All-IS-IS-Rbridges.

   Within TRILL, there is a mandatory core IS-IS instance across all
   Rbridges in the campus as described in Section 4.2.3.  In addition,
   there can be optional end station address distribution instances
   (ESADIs) between the RBridges on each supported VLAN as described in
   Section 4.2.4. They are distinguished by the presence of an inner
   VLAN tag in the ESADI TRILL IS-IS frames and the absence of such a
   tag in the core instances frames.



4.2.3 Core TRILL IS-IS

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



4.2.3.1 Core IS-IS Link Protocol

   RBridges send core TRILL IS-IS Hellos on a link in order to discover
   RBridge neighbors. As with layer 3 IS-IS, one RBridge is elected DRB
   (Designated RBridge), based on configured priority (most significant
   field), and system ID.  The DRB, as described in Section 4.2.3.2,
   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 (see Section 4.2.3.3) for VLANs on the link.

4.2.3.1.1 Core Hello VLAN Tagging

   By default, RBridges tag TRILL IS-IS Hellos 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 cannot use
   VLAN 1 on a port, Hellos may need to be sent out a port with
   additional and/or other VLANs for 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).  In addition, RBn
   maintains the following TRILL specific VLAN parameters per port:



R. Perlman, et al                                              [Page 30]


INTERNET-DRAFT                                          RBridge Protocol


      a) Desired Designated VLAN: the VLAN that RBn, if it is DRB, will
         specify be used by all RBridges on the link to communicate all
         TRILL frames, except some Hellos.  This MUST be an enabled
         VLAN. 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 set from RBn's Desired
         Designated VLAN if RBn believes it is DRB or from the
         Designated VLAN in the DRB's Hellos if RBn is not DRB.

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

      d) Forwarding VLANs set: the set of VLANs for which an RBridge
         port is appointed VLAN forwarder. This, by definition, is the
         same as or a subset of the enabled VLANs for the port.

   On each of its ports an RBridge sends Hellos Outer.VLAN tagged with a
   set of VLANs. For each port, this set depends on the RBridge's DRB
   status and the above VLAN parameters. All RBridges send Hellos
   Outer.VLAN tagged with the Designated VLAN unless that VLAN is not
   enabled.  In addition, the DRB sends Hellos Outer.VLAN tagged with
   each enabled VLAN in its Announcing VLANs set. All non-DRB RBridges
   send 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, 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
   of Hellos. In that case, 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
   Hellos tagged with all its Enabled VLAN tags and any non-DRB RBridge
   RBn will send 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 Hellos. In particular, non-DRB RBridges
   could send Hellos on enabled VLANs for which they are not an
   appointed forwarder and which are not the Designated VLAN. While this


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   would not cause harm, other than a further communications and
   processing burden, there is no significant advantage is doing so.)

   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 on that
   port it receives no Hellos 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. Note that an RBridge RBn does not defer to the
   DRB listed in a Hello, even if that claimed DRB is higher priority,
   if the Hello was sent by an RBridge with lower priority than RBn.

4.2.3.1.2 Core Hello Contents

   A core TRILL IS-IS Hello includes the following information, in
   addition to the standard IS-IS Hello header information. 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.

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

   2. A copy of the Outer VLAN ID with which the Hello was tagged to
      detect VLAN mapping (see Section 4.2.3.1.3).

   3. The set of VLANs for which end station service is enabled on the
      port. If this is missing or null, it implies that the port is
      configured as a trunk port (see Section 4.7).

   4. 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.

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

   RBridge adjacencies over which TRILL data and TRILL IS-IS frames will
   be sent should only be formed on the Designated VLAN. Forming such
   adjacencies requires exchange of Hellos with the IS Neighbor TLV, so
   that TLV MUST NOT be included in Hellos sent on VLANs other than the
   Designated VLAN.

   It is anticipated that many links between RBridges will be point-to-
   point in which case a pseudonode merely adds to the complexity. If
   the DRB specifies the pseudonode ID as all zeros, this indicates that
   the RBridges on the link are just to 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 generated.


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   For example, if RB1 and RB2 are the only RBridges on the link and RB1
   is DRB, then if RB1 creates a pseudonode 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 declares no pseudonode, then
   there will be only 2 LSPs: RB1 and RB2 each reporting connectivity to
   each other.

   A DRB SHOULD NOT create a pseudonode for its link unless the port is
   an access port (see Section 4.7) or it has seen at least two
   simultaneous adjacencies on the link at some point since it last re-
   booted.

4.2.3.1.3  VLAN Mapping

   Although customer IEEE [802.1Q] bridges are not permitted to change
   the C-tag VLAN ID for a tagged frame they receive, that is, map
   VLANs, bridged LANs can nevertheless display this behavior. For
   example, one bridge can have a port configured to strip certain VLAN
   tags on output and send the resulting untagged frames onto a link
   leading to a second bridge whose port tags these frames with a
   different VLAN.  Although each of these bridges conforms to [802.1Q],
   in the aggregate they perform manipulations not permitted for a
   single customer 802.1Q bridge.  Since RBridge ports have essentially
   the same VLAN capabilities as customer 802.1Q bridges, this can occur
   even in the absence of bridges.

   RBridges include the Outer.VLAN information inside a TLV within each
   Hello message. When a Hello is received, they 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 is assumed that VLAN ID X is being mapped into
   VLAN ID Y.

   When a VLAN mapping from X to Y is detected, the RBridge detecting it
   disables its appointed forwarder status for both X and Y. The idea is
   to produce a clean failure so the network operator will correct the
   bridged LAN configuration.



4.2.3.2 Designated RBridge

   TRILL IS-IS elects one RBridge for each link to be the Designated
   RBridge (DRB), that is, to have special duties. The Designated
   RBridge:

   o  Chooses, for the link, and announces in its Hellos, the Designated
      VLAN ID to be used for inter-Bridge communication. This VLAN is
      used for all TRILL data frames and all non-Hello TRILL IS-IS


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      frames.  TRILL IS-IS Hellos are sent on this VLAN but may also be
      sent on others (see Section 4.2.3.1.1).

   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
      VLANs).

   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 Hellos on all its enabled VLANs that have
      been configured in the Announcing VLANs set of the DRB, which
      defaults to all enabled VLANs.



4.2.3.3 Appointed VLAN-x Forwarder

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

   o  Forwarding VLAN-x data traffic to and from the link.

   o  Learning the MAC address of local VLAN-x nodes based on 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.

   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 frames being
      decapsulated onto the link.

   o  Optionally listening to the messages in the end station address
      distribution instance (ESADI) TRILL IS-IS 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 an ESADI TRILL IS-IS.




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   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
      this RBridge is appointed forwarder for VLAN-x.

   o  Loop avoidance:

      -  Discarding all native frames that would have been sent onto or
         received the link 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 4.7.1.1).

      -  On a per VLAN basis, discarding all native frames that have
         been received from or would have been sent onto the link if,
         within the past five Hello times, it has reeived a Hello on
         that VLAN in which the sender asserts that it is appointed
         forwarder for the VLAN.

      -  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 the same VLAN. This is known as the "decapsulation
         check" or "root bridge collision check".

   o  Including a "port number" in its Hellos, and if it sees its own
      Hello on port p, where the port number in the received Hello is
      "q", then if q>p, not forwarding traffic to/from port p, as
      already provided in IS-IS.



4.2.3.4 Core TRILL IS-IS 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.

   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 neighbors.

   2. Optionally, 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 priority of RBn for becoming a distribution tree root and the
      number of additional distribution trees it wants computed for the
      campus (each 16 bits, see Section 4.3).


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INTERNET-DRAFT                                          RBridge Protocol


   5. The list of RBridge nicknames that RBn might select for a
      distribution tree root when RBn injects a multi-destination frame
      into the campus.  With this field RBridges can efficiently build
      receipt filters to avoid multicast loops (see Section 4.3.1).  If
      the list is empty or not provided, RBn can only select the highest
      priority distribution tree root (see Section 4.3).

   6. 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.) In addition, the LSP contains the following
      information on a per-VLAN basis:

      6.1 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 which 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
          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 mulicast addresses that MUST be treated as
          broadcast.

      6.2 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.

      6.3 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 4.2.3.3.)

      6.4 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).



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      6.5 Per VLAN end station address distribution instance (ESADI)
          TRILL IS-IS participation flag, priority, and holding time.
          If this flag is one, it indicates that the RBridge wishes to
          receive such ESADI frames (see Section 4.2.4.1).

      6.6 Per VLAN appointed forwarder status lost counter (see Section
          4.6.2).

   7. 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.



4.2.4 End Station Address Distribution Instance (ESADI) IS-IS

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

   Because of this forwarding, it appears to an ESADI IS-IS at an
   RBridge that it is directly connected by a shared virtual link to all
   other RBridges in the campus running that instance.  RBridges that do
   not implement that ESADI or are not appointed forwarder for that VLAN
   do not decapsulate or locally process any ESADI IS-IS 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 IS-IS processing is
   invoked due to such forwarding.



4.2.4.1 ESADI Protocol

   An RBridge participating in an end station address distribution
   instance (ESADI) does not send any additional Hellos.  The
   information available in the core TRILL IS-IS link state database is
   sufficient to determine the 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 instances, its holding time, and its IS-IS system ID
   for breaking ties in priority.



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   The DRB, for an ESADI, sends TRILL IS-IS CSNP messages on the ESADI
   virtual link. A participating RBridge that determines that some other
   RBridge should be 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 ESADI LSPs or CSNPs on the virtual link.



4.2.4.2 ESADI Information

   The information in the LSP for a optional TRILL IS-IS 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).



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
   TRILL WG decided that the computation of additional distribution
   trees was warranted. Having additional trees increases the
   computation load on every RBridge in the campus. However, 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.

   Each RBridge RBn may advertise in the core instance link state
   database its priority to be chosen as a tree root and the number of
   additional distribution trees it specifies that every RBridge in the
   campus must compute if RBn is the highest priority tree root. The
   priority is a 16-bit unsigned integer that defaults, for a zero
   configuration RBridge or if the RBridge does not advertise any
   priority in its LSP, to 0x8000.  The number of distribution trees to
   be computed is a 16-bit unsigned integer giving the number of trees
   to be computed in addition to the one rooted at the highest priority
   root. The number of additional tree defaults, for a zero
   configuration RBridge or if the RBridge does not advertise any number
   in its LSP, to one.

   The highest priority RBridge to be tree root is determined by the
   numerically lowest priority field or, if priority fields are equal,
   by the numerically lowest system ID. A tree is always calculated


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INTERNET-DRAFT                                          RBridge Protocol


   rooted at this highest priority RBridge and that RBridge specifies to
   all RBridges in the campus the total number of distribution trees to
   be calculated. If it indicates that k trees are to be calculated,
   then they are rooted at the k highest priority RBridges. Thus every
   RBridge calculates the same set of k distribution trees.

   Every RBridge RBn defaults, in the zero configuration case, to using
   a single distribution tree for multi-destination frames. For this
   purpose, it orders the trees being computed for the campus by
   increasing cost from RBn to the root RBridge of that tree and, if
   cost is equal, by decreasing priority to be a tree root and selects
   the first tree in that ordering.

   If RBn is to multi-path multi-destination frames, it can be
   configured with the number of different trees it would like to use,
   say j.  RBn selects the first trees in its priority-of-use ordering,
   up to the minimum of j and k number of trees.  However, RBn MUST
   announce, in its LSP, an intention to use any particular trees by
   listing the 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-byte
   IS-IS ID.

   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


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INTERNET-DRAFT                                          RBridge Protocol


   Forwarding 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 on that adjacency. The only ingress
   RBridges that appear on any of the adjacencies are RBridges that have
   explicitly stated, in their LSP, that they may select RBi as a
   distribution tree root or ingress RBridges that list no roots on
   adjacencies for the distribution tree with the highest priority root.
   If a multi-destination frame is received on a particular adjacency,
   marked as 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 that 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 which 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 which 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.

   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,
      and

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



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4.3.3 Tree Distribution Optimization

   RBridges MUST determine the VLAN associated with all native frames
   and properly enforce VLAN rules on the emission of native frames at
   egress RBridge ports according to how those ports are configured.
   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 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
   requesting will be sent to such end stations under these
   circumstances.)

   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. They silently discard such frames.

   See also "Considerations for Internet Group Management Protocol
   (IGMP) and Multicast Listener Discovery (MLD) Snooping Switches"
   [RFC4541].








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INTERNET-DRAFT                                          RBridge Protocol


4.3.4 Forwarding Using a Distribution Tree

   With maximum 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 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 not the range of IP multicast addresses
      which must be treated as broadcast, then 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
      links.

   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, or process the frame.

   ESADI (end station address distribution instance) TRILL IS-IS frames
   will be delivered only to RBridges that are appointed forwarders for
   their VLAN. Such frames look, to transit RBridges, like any multicast
   data frame tagged with an inner VLAN tag. 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 ESADI message, and pruned according to the inner VLAN
   tag. Thus all the RBridges that are appointed forwarders for a link
   in that VLAN receive them unless the RBridge has cleared its Other


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   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.

   Frames with a bad FCS are discarded on receipt.  Source address
   information ( { VLAN, Outer.MacSA, port } ) is learned from any non-
   control frame with a unicast sources address (see Section 4.6).



4.4.1 Receipt of a Native Frame

   If end station service is disabled on the port, 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). 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, then
   it is forwarded according to 4.4.1.1 if the frame is unicast and
   according to 4.4.1.2 if it is multicast or broadcast, unless the
   appointed forwarder is required to discard it due to a recenlty
   observed Hello inidcating a conflicting appointed forwarder or root
   bridge change (see Section 4.2.3.3).



4.4.1.1 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
   local egress port or discover that the destination is unknown.

   If the destination address is known, 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


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      frame, since the destination should already have received it.

   2. If the destination is known to be on a different local link for
      which the RBridge is also appointed forwarder for the frame's
      VLAN, the RBridge forwards the frame in native form to that link
      unless inhibited for this different local link as described in
      Section 4.2.3.3.

   3. If the destination is known to be on a link for which a different
      RBridge is appointed forwarder for the frame's VLAN, say RBridge
      RBm, then RB1 converts the native frame to a TRILL data frame with
      an Outer.MacSA of RB1 and 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 a unicast destination address is unknown, RB1 handles the frame as
   described in Section 4.4.1.2 for a broadcast frame except that the
   Inner.MacDA is the original native frame unicast destination address.



4.4.1.2 Native Multicast and Broadcast Frames

   If the destination address of a native frame is the broadcast address
   or a multicast address other than All-Rbridges or All-IS-IS-Rbridges,
   the frame is processed as described below. A native (non-TRILL) frame
   sent to the All-Rbridges or All-IS-IS-Rbridges address is erroneous
   and is silently discarded.

   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
   4.5).

   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 as described
   in Section 4.2.3.3. 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 wants to use. It
   then forwards the frame on the pruned distribution tree (see Section
   4.3).

   The default is for RB1 to write the nickname for the distribution
   tree whose root is least cost from RB1 into the egress nickname
   field. However, RB1 MAY choose a different distribution tree if RB1
   has been configured to path-split multicast.  In that case RB1 MUST


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   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 may choose
   as a distribution tree or the tree with the highest priority root if
   none is announced.

   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
   be flooded if a multicast destination is used.



4.4.2 Receipt of a TRILL Frame

   A TRILL outer Ethertype indicates a TRILL frame. Such frames will be
   received with an Outer.MacDA that is unicast or that is the All-
   RBridges multicast address. TRILL frames with any other Outer.MacDA
   are erroneous and are discarded except that a TRILL frame with the
   broadcast Outer.MacDA MAY be treated as if the Outer.MacDA was the
   All-Rbridges multicast address. TRILL frames received by an RBridge
   on a port are never blocked because of that RBridge's appointed
   forwarder or Designated RBridge status for that port.

   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-IS-IS-
   Rbridges multicast address, processing proceeds as in Section 4.4.2.1
   below. If it is any other address, processing proceeds as in Section
   4.4.2.2.



4.4.2.1 TRILL IS-IS Frames

   If there is no Inner VLAN tag, the frame is a core instance TRILL IS-
   IS frame. In that case, if M == 1, the frame is silently discarded.
   Otherwise, it is processed by the core IS-IS instance on RBn and is
   not forwarded. Note that, in this instance, nicknames may not yet


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   have been established and the ingress and egress nickname fields are
   ignored.

   If there is an Inner VLAN tag, the frame is an ESADI (end station
   address distribution instance) TRILL IS-IS frame. If M == 0, the
   frame is silently discarded.  The egress nickname designates the
   distribution tree. In this case, the frame is forwarded as described
   in Section 4.4.2.2.2. In addition, if the forwarding Rbridge is an
   appointed forwarder for a link in the specified VLAN and implements
   an ESADI of TRILL IS-IS for that VLAN and that instance is enabled,
   the inner frame is decapsulated and provided to that local IS-IS
   instance.



4.4.2.2 TRILL Data Frames

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

   The M flag is then checked. If it is zero, processing continues as
   described in Section 4.4.2.2.1, if it is one, processing continues as
   described in Section 4.4.2.2.2.

4.4.2.2.1 Known Unicast TRILL Data Frames

   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 sent
   onto the link containing the destination or locally processed if the
   RBridge itself is the destination.

   A known unicast TRILL data frame can arrive at the egress Rbridge
   only to find that the MAC destination address is not actually known
   by that RBridge. One way this can happen is that the destination MAC
   address may have timed out in the egress RBridge cache. In this case,
   the egress RBridge decapsulates the frame to native form and sends it
   out on all links that are in the frame's VLAN for which the RBridge
   is appointed forwarder and has not been inhibited as described in
   Section 4.2.3.3, 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 is known to be a point-
   to-point link to another RBridge.

   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. If the egress RBridge is not
   known to the transit RBridge, the frame is silently discarded.


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4.4.2.2.2 Multi-Destination TRILL Data Frames

   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, inhibition as described in Section 4.2.3.3,
   and/or locally processed as appropriate.

   If the hop count in the frame is zero, it is then silently discarded.
   If non-zero, it is decreased (see Section 3.6) and the frame
   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 be flooded
   if a multicast destination is used.



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. If VRP frame handling is not implemented,
   VRP frames are discarded.

      Name   Section   Destination Address

      BPDU   4.7.1     01-80-C2-00-00-00
      VRP    4.7.2     01-80-C2-00-00-21








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4.5 IGMP, MLD, and MRD Learning

   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, encapculated 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
   link.

   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"
   [RFC4541].



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


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         link, so the RBridge need do nothing, or

      -  known to be destined for another end station on another local
         link where the RBridge is also appointed forwarder so it can be
         directly forwarded in native form or

      -  neither of the above, so the frame 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 ESADIs (end stations address distribution
      instances) of IS-IS which receives remote information and
      transmits local 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.

   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.

   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


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   observation of data has a confidence of 1 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 an IS-IS 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 { confidence, VLAN, MAC
   address, local port } for addresses learned from local native frames
   and { confidence, VLAN, MAC address, egress RBridge nickname } for
   addresses learned from remote encapsulated frames, plus 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.

   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.

   In case B and in case C where the new information is learned as
   above, timer information is reset.



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


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   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 an IS-IS ESADI. It is up to the originating RBridge to
   decide when to remove such information from the ESADI LSP (or up to
   IS-IS 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 frames 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 an IS-IS 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 which 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.



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:



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   o  The RBridge no longer has the information needed to participate in
      an IS-IS ESADI for the VLANs whose 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 is adjusted to be no
      longer than the RBridge's "Forward Delay".

   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 specification 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 Port Structure

   An RBridge port can be modeled as having a structure similar to that
   of an [802.1Q] bridge port as shown in Figure 4.3. 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 which provides the correct behavior.

   There are three per port configuration bits as follows:

   o  Disable port bit. With the possible exception of some low level
      control frames, all frames received are discarded and no frames
      are sent

   o  End station service disable (trunk port) bit. All native frames
      received on the port and all native frames that would have been


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      sent on the port are discarded. (See Appendix B.)

      A port with end station service disabled reports, in the Hellos it
      sends out that port, that it has no VLANs for which it is 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 which 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 Transit data disable (access port) bit. The idea is to avoid
      sending RBridge transit traffic (TRILL data frames) on the port
      since it is intended for end station traffic (see Appendix B). If
      there are no TRILL IS-IS adjacencies on the port, no special
      action need be taken. If there adjacencies, and the RBridge is
      DRB, it creates a pseudo-node for the link with the IS-IS overflow
      bit on. This will cause IS-IS routing to avoid sending transit
      data on the link if any other path is available.






























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                                         +-------------------------+
              +--------------------------+                         |
              |                          |         RBridge         |
       +------+-----+  +- - - - - - +    |                         |
       |            |  |            |    |    Forwarding Engine,   |
       | Port BPDU  |  | Port VRP   +- - +       IS-IS, Etc.       |
       | Processing |  | Processing |    +--+--++------------------+
       |            |  |            |       |  ||
       +-----++-----+  + - - ++ - - +       |  ||
             ||               |        +----+  ||
             ||               |        |       ||
             ||              |         |  +----++------+ <- EISS
             ||               |        |  |            |
             ||              |         |  | Port VLAN  |
             ||               |        |  | Processing |
             ||              |         |  |            |
             ||               |        |  +----++------+
             ||              |         |       ||
       +-----++--------------++--------+-------++-----+ <-- ISS
       |                                              |
       |     Lower Level Port/Link Control Logic      |
       |                                              |
       +-------------++-------------------------------+
                     ||
                     ||   +------------+
                     ||   |            |
                     ||   |    PHY     |
                     |+---+ (Physical  +------- Link
                     +----+ Interface) +-------
                          |            |
                          +------------+

                      Figure 4.3: 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, 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 interface to this lower level port control logic
   corresponds to the Internal Sublayer Service (ISS) in 802.1Q.

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

   High level control frames (BPDUs and, if supported, VRP frames) are


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   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, if VRP is implemented, VLAN tagged VRP frame, is
   unspecified. If VRP is not implemented, then VRP frames are
   discarded.

   Handling of BPDUs is described in Section 4.7.1 below. Handling of
   VRP frames is described in Section 4.7.2 below.



4.7.1 BPDU Handling

   If campus topology were static, RBridges would simply be end
   stations, thus 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, these are 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.



4.7.1.1 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
   receives.

   RBridges silently discard any topology change BPDUs they receive.



4.7.1.2 Root Bridge Changes

   A change in the root bridge seen out a port may indicate a change in
   bridged LAN topology including the possibility of the merger of two
   bridged LANs or the like. 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 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.


R. Perlman, et al                                              [Page 55]


INTERNET-DRAFT                                          RBridge Protocol


   For example, consider two bridged LANs carrying multiple VLANs, each
   with various appointed forwarders. Should they become merged, due to
   a repeater coming up 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 cease
   forwarding native frames to or from the link for the time period
   while things are sorted out by BPDUs within the merged bridged LAN
   and TRILL IS-IS Hellos between all the RBridges involved.



4.7.1.3 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
   BPDU.

   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.  Except for this optional capability,
   RBridges MUST NOT send spanning tree BPDUs.



4.7.2 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 VLANs whose enablement is dynamic. If an
   RBridge supports a VRP, the actual enablement of dynamic VLANs is
   determined by GVRP/MVRP frames received as it would be for an
   [802.1Q] bridge.

   An RBridge that supports a VRP sends GVRP/MVRP frames as an [802.1Q]
   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 traffic in the
   Designated VLAN, unless the Designated VLAN is disabled on the port,
   and to not request traffic in any other VLAN.


R. Perlman, et al                                              [Page 56]


INTERNET-DRAFT                                          RBridge Protocol


5. RBridge Addresses, 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
   frame.

   Two Layer 2 multicast addresses must be assigned.

   o  All-RBridges for use as Outer.MacDA in multi-destination frames.

   o  All-IS-IS-RBridges as the Inner.MacDA for TRILL IS-IS frames.

   RBridges may be configured with a nickname and nickname selection
   priority.

   RBridges may be configured to have ESADIs (end station address
   distribution instances) of TRILL IS-IS 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 configured.

   The per RBridge parameters of (1) priority to be a distribution tree
   root and (2) desired number of distribution trees for the campus, as
   discussed in Section 4.3, may be configured.

   The per RBridges parameters Aging Timer and Forward Delay, as
   described in Section 4.6, may be configured.

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

   o  Three per-port configuration bits: disable port, disable end
      station service, and access port (see Section 4.7).

   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 port.




R. Perlman, et al                                              [Page 57]


INTERNET-DRAFT                                          RBridge Protocol


6. Security Considerations

   Layer 2 bridging in not inherently secure.  It is, for example,
   subject to forgery 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
   instances to use IS-IS security [RFC3567] and ignore unauthenticated
   TRILL IS-IS messages 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
   proper (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 the
   optional IS-IS ESADIs (see Section 4.6).

   TRILL encapsulates native frames inside the TRILL Ethertype while
   they are in transit between that frame's 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 examine the interior of such frames for security
   checking purposes. This may render them ineffective.  Routers and
   hosts appear to RBridges to be end stations and such 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.

   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
   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.

   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.








R. Perlman, et al                                              [Page 58]


INTERNET-DRAFT                                          RBridge Protocol


7. Assignment Considerations

   This section discuses IANA and IEEE 802 assignment considerations.



7.1 IANA Considerations

   A new IANA registry is created for TRILL versions and TRILL Header
   Reserved bits. The initial contents of this registry is TRILL Version
   number 0, which is specified in this document.

   New TRILL Header Version numbers and uses of TRILL Header Reserved
   bits are assigned by an IETF Standards Action [RFC5226] as modified
   by [RFC4020].



7.2 IEEE 802 Assignment Considerations

   The Ethertype <tbd> is assigned by IEEE 802 to indicate a TRILL
   encapsulated frame.

   The Layer 2 multicast addresses <tbd1> and <tbd2> are assigned by
   IEEE 802 for "All-Rbridges" and "All-IS-IS-Rbridges" respectively.



























R. Perlman, et al                                              [Page 59]


INTERNET-DRAFT                                          RBridge Protocol


8. Normative References

   [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.

   [RFC4020] Kompella, K. and A. Zinin, "Early IANA Allocation of
      Standards Track Code Points", BCP 100, RFC 4020, February 2005.

   [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 60]


INTERNET-DRAFT                                          RBridge Protocol


9. Informative References

   [802.1AB] "IEEE Standard for Local and metropolitan area networks /
      Station and Media Access Control Connectivity Discovery",
      802.1AB-2005, 6 May 2005.

   [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.

   [Arch] Gray, E., "The Architecture of an RBridge Solution to TRILL",
      draft-ietf-trill-rbridge-arch-05.txt, February 2008, work in
      progress.

   [PAS] Touch, J., & R. Perlman, "Transparent Interconnection of Lots
      of Links (TRILL) / Problem and Applicability Statement", draft-
      ietf-trill-prob-04.txt, June 2008, 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.

   [RFC3410] Case, J., Mundy, R., Partain, D., and B.  Stewart,
      "Introduction and Applicability Statements for Internet-Standard
      Management Framework", RFC 3410, December 2002.

   [RFC3567] Li, T. and R. Atkinson, "Intermediate System to
      Intermediate System (IS-IS) Cryptographic Authentication", RFC
      3567, July 2003.

   [RFC4086] Eastlake, D., 3rd, Schiller, J., and S. Crocker,
      "Randomness Requirements for Security", BCP 106, RFC 4086, June
      2005.

   [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.

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



R. Perlman, et al                                              [Page 61]


INTERNET-DRAFT                                          RBridge Protocol


Appendix A: Incremental Deployment Considerations

   Partial RBridge deployment can cause some problems as described below
   for link cost determination (Section A.1) and 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
   faster.

            +-----+        +----+        +----+        +-----+
            |     |  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 end station 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


R. Perlman, et al                                              [Page 62]


INTERNET-DRAFT                                          RBridge Protocol


      occur 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 singly connected to a core bridge, 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 address will
      typically be flooded throughout a bridged LAN link, 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 sending the TRILL
      traffic as a unicast frame to that recipient rather than
      multicasting it. (Since one purpose of TRILL IS-IS Hello frames is
      to find previously unknown Rbridges, they must always be
      multicast.)

   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
      4.2.3.2). 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 63]


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
      partitions.

   Conversely to item 3 above, there may be bridged LANs which use
   VLANs, or use more VLANs than would otherwise be necessary, to evade
   the congestion that can be caused by the spanning tree protocol.
   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 detailed 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 respectively), 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 the other RBridge for the remaining VLANs. Should
   either of the RBs 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 arbitrarily 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


R. Perlman, et al                                              [Page 65]


INTERNET-DRAFT                                          RBridge Protocol


   logically partition the bridged LAN by blocking the B1-B2 link at one
   end or the other as desired (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 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 R1 transmits
   and "RB2" when R2 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 tie-breaker 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
   none.

   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 link and the RB2 link.


R. Perlman, et al                                              [Page 66]


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 point-
   to-point link to another RBridge, 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 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 provide some other specific VLAN or the like. Such
   configuration minimizes wasted Hellos and eliminates useless
   decapsulation and transmission of multi-destination traffic in native
   form onto the link. (see Sections 4.2.3.1 and 4.7)

   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 transit
   traffic on such as link as it is intended for end station traffic.
   This can be accomplished by turning on the access port configuration
   bit for the RBridge port or ports connected to the link (see Section
   4.7).

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




















R. Perlman, et al                                              [Page 67]


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 and some of which use a single
   root for all such frames.

                              +---+
                              |RBy|---------------+
                              +---+               |
                             /  |  \              |
                           /    |    \            |
                         /      |      \          |
                      +---+   +---+   +---+       |
                      |RB1|---|RB2|---|RB3|       |
                      +---+   +---+   +---+\      |
                        |       |       |    \    |
                      +---+   +---+   +---+    \+---+
                      |RB4|---|RB5|---|RB6|-----|RBz|
                      +---+   +---+   +---+    /+---+
                        |       |       |    /
                      +---+   +---+   +---+/
                      |RB7|---|RB8|---|RB9|
                      +---+   +---+   +---+

                  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
   not.

   As an alternative to multipathing, it might be possible to combine
   the three paths between RB1 and RB2 into one logical link through the
   "channel aggregation" feature of 802.3 (see Clause 43 of [802.3]).
   Rbridges MAY implement channel aggregation. However, channel
   aggregation requires multiple single hop equal bandwidth links (no
   intervening bridges). Equal cost multipathing is somewhat 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 such the 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 is
   determined for incoming native frames, omitting some details, is
   given in the bulleted items below:

   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 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.

   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 before publication.



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
       coverage.

    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.

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


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

   15. Explain about multiple Designated RBridges because of multiple
       VLANS.

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

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

   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
       messages.

    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.







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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
       description.

    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 psuedo code.

    8. Change to one DRB per physical link which 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.]


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

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

    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.


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



Changes from -07 to -08

    1. Add "low" and "high" level control frame definitions to Section
       1.2 and note concerning frames which would qualify as both


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       "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.

    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
       forwarding.

    9. Add requirement for configurable (default 30 second) prohibition
       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
       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


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       the DRB or even from an RBridge in direct communication with the
       DRB.

   13. Add provisions for pseudonode minimization.

   14. Update refence to RFC 2434 to RC 5226.

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











































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Disclaimer

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   The IETF invites any interested party to bring to its attention any
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   This document is subject to the rights, licenses and restrictions
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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
   Eastlake Enterprises
   155 Beaver Street
   Milford, MA 01757 USA

   Phone: +1-508-634-2066
   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
   1745 Technology Drive
   San Jose, CA 95110 USA

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







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Expiration and File Name

   This draft expires in January 12, 2009.

   Its file name is draft-ietf-trill-rbridge-08.txt.















































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