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TRILL Working Group                                        Radia Perlman
INTERNET-DRAFT                                                Intel Labs
Intended status: Informational                           Donald Eastlake
                                                          Anoop Ghanwani
                                                            Hongjun Zhai
Expires: December 31, 2013                                  July 1, 2013

                       Flexible Multilevel TRILL
             (Transparent Interconnection of Lots of Links)


   Extending TRILL to multiple levels has one challenge that is not
   addressed by the already-existing capability of IS-IS to have
   multiple levels. The issue is with RBridge nicknames. There have been
   two proposed approaches.  One approach, which we refer to as the
   "unique nickname" approach, gives unique nicknames to all the
   RBridges in the multilevel campus, either by having the
   level-1/level-2 border RBridges advertise which nicknames are not
   available for assignment in the area, or by partitioning the 16-bit
   nickname into an "area" field and a "nickname inside the area" field.
   The other approach, which we refer to as the "aggregated nickname"
   approach, involves assigning nicknames to the areas, and allowing
   nicknames to be reused in different areas, by having the border
   RBridges rewrite the nickname fields when entering or leaving an
   area. Each of those approaches has advantages and disadvantages. The
   design in this document allows a choice of approach in each area,
   allowing the simplicity of the unique nickname approach in
   installations in which there is no danger of running out of
   nicknames, and allowing nickname rewriting to be phased into larger
   installations on a per-area basis.

Status of This Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and 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-

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   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/1id-abstracts.html. The list of Internet-Draft
   Shadow Directories can be accessed at


   The helpful comments of the following are hereby acknowledged: David
   Michael Bond and Dino Farinacci.

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Table of Contents

      1. Introduction............................................4
      1.1 TRILL Scalability Issues...............................4
      1.2 Improvements Due to Multilevel.........................5
      1.3 Unique and Aggregated Nickanmes........................6
      1.3 More on Areas..........................................6
      1.4 Terminology and Acronyms...............................7

      2. Multilevel TRILL Issues.................................8
      2.1 Non-zero Area Addresses................................9
      2.2 Aggregated versus Unique Nicknames.....................9
      2.2.1 More Details on Unique Nicknames....................10
      2.2.2 More Details on Aggregated Nicknames................11 Border Learning Aggregated Nicknames..............11 Swap Nickname Field Aggregated Nicknames..........13 Comparison........................................14
      2.3 Building Multi-Area Trees.............................14
      2.4 The RPF Check for Trees...............................15
      2.5 Area Nickname Acquisition.............................15
      2.6 Link State Representation of Areas....................16

      3. Area Partition.........................................17

      4. Multi-Destination Scope................................18
      4.1 Unicast to Multi-destination Conversions..............18
      4.1.1 New Tree Encoding...................................19
      4.2 Selective Broadcast Domain Reduction..................19

      5. Co-Existence with Old RBridges.........................21
      6. Multi-Access Links with End Stations...................22

      7. Summary................................................23

      8. Security Considerations................................23
      9. IANA Considerations....................................23
      10. Normative References..................................24
      11. Informative References................................24
      Authors' Addresses........................................26

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1. Introduction

   The IETF TRILL (Transparent Interconnection of Lot of Links) protocol
   [RFC6325] provides optimal pair-wise data forwarding without
   configuration, safe forwarding even during periods of temporary
   loops, and support for multipathing of both unicast and multicast
   traffic in networks with arbitrary topology and link technology,
   including multi-access links.  TRILL accomplishes this by using IS-IS
   (Intermediate System to Intermediate System [IS-IS] [RFC1195]
   [RFC6326]) link state routing using a header that includes a hop
   count. The design supports data labels (VLANs and Fine Grained Labels
   [RFCfgl]) and optimization of the distribution of multi-destination
   frames based on VLANs and multicast groups. Devices that implement
   TRILL are called RBridges or TRILL Switches.

   Familiarity with [RFC6325] is assumed in this document.

1.1 TRILL Scalability Issues

   There are multiple issues that might limit the scalability of a
   TRILL-based network:

   1. the routing computation load,
   2. the volatility of the LSP (Link State PDU) database creating too
      much control traffic,
   3. the volatility of the LSP database causing the TRILL network to be
      in an unconverged state too much of the time,
   4. the size of the LSP database,
   5. the limit of the number of RBridges, due to the 16-bit nickname
   6. the traffic due to upper layer protocols use of broadcast and
      multicast, and
   7. the size of the end node learning table (the table that remembers
      (egress RBridge, label/MAC) pairs).

   Extending TRILL IS-IS to be multilevel (hierarchical) helps with all
   but the last of these issues.

   IS-IS was designed to be multilevel [IS-IS] [RFC1195].  A network can
   be partitioned into "areas".  Routing within an area is known as
   "Level 1 routing".  Routing between areas is known as "Level 2
   routing".  The Level 2 IS-IS network consists of Level 2 routers and
   links between the Level 2 routers.  Level 2 routers may participate
   in one or more areas, in addition to their role as Level 2 routers.

   Each area is connected to Level 2 through one or more "border
   routers", which participate both as a router inside the area, and as
   a router inside the Level 2 "area".  Care must be taken that it is

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   clear, when transitioning multidestination packets between Level 2
   and a Level 1 area in either direction, which (single) border RBridge
   will transition a particular data packet between the levels or else
   duplication of traffic can occur.

1.2 Improvements Due to Multilevel

   Partitioning the network into areas solves the first four scalability
   issues described above, namely,

   1. the routing computation load,

   2. the volatility of the LSP (Link State PDU) database creating too
      much control traffic,

   3. the volatility of the LSP database causing the TRILL network to be
      in an unconverged state too much of the time,

   4. the size of the LSP database.

   Problem #6, namely, the traffic due to upper layer protocols use of
   broadcast and multicast, can be addressed by introducing a locally-
   scoped multidestination delivery, scoped to an area or a single link.
   See further discussion in Section 4.2.

   Problem #5, namely, the limit of the number of RBridges, due to the
   16-bit nickname space, will only be addressed with the aggregated
   nickname approach. Since the aggregated nickname approach requires
   some complexity in the border RBridge (for rewriting the nicknames in
   the TRILL header), the design in this document allows a campus with a
   mixture of unique-nickname areas, and aggregated-nickname areas.
   Nicknames must be unique across all unique-nickname areas, whereas
   nicknames inside an aggregated-nickname area are visible only inside
   the area.  Nicknames inside an aggregated-nickname area must not
   conflict with nicknames assigned to RBridges in any unique-nickname
   areas, and must not conflict with the aggregated nickname given to
   aggregated-nickname areas, but the nicknames inside an aggregated-
   nickname area may be the same as nicknames used within other
   aggregated-nickname areas.

   RBridges within an area need not be aware of whether they are in an
   aggregated nickname area or a unique nickname area.  The border
   RBridges in area A1 will claim, in their LSP inside area A1, which
   nicknames (or nickname ranges) are not available for choosing as
   nicknames by area A1 RBridges.

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1.3 Unique and Aggregated Nickanmes

   We describe two alternatives for hierarchical or multilevel TRILL.
   One we call the "unique nickname" alternative.  The other we call the
   "aggregated nickname" alternative. In the aggregated nickname
   alternative, border RBridges replace either the ingress or egress
   nickname field in the TRILL header of unicast frames with an
   aggregated nickname representing an entire area.

   The unique nickname alternative has the advantage that border
   RBridges are simpler and do not need to do TRILL Header nickname
   modification.  It also simplifies testing and maintenance operations
   that originate in one area and terminate in a different area.

   The aggregated nickname alternative has the following advantages:

      o it solves problem 5 above, the 16-bit RBridge nickname limit, in
         a simple way,
      o  it lessens the amount of inter-area routing information that
         must be passed in IS-IS, and
      o  it greatly reduces the RPF (Reverse Path Forwarding) Check
         information (since only the area nickname needs to appear,
         rather than all the ingress RBridges in that area).

   In both cases, it is possible and advantageous to compute multi-
   destination frame distribution trees such that the portion computed
   within a given area is rooted within that area.

1.3 More on Areas

   Each area is configured with an "area address", which is advertised
   in IS-IS messages, so as to avoid accidentally interconnecting areas.
   Note that, although the area address had other purposes in CLNP (IS-
   IS was originally designed for CLNP/DECnet), for TRILL the only
   purpose of the area address would be to avoid accidentally
   interconnecting areas.

   Currently, the TRILL specification says that the area address must be
   zero. If we change the specification so that the area address value
   of zero is just a default, then most of IS-IS multilevel machinery
   works as originally designed.  However, there are TRILL-specific
   issues, which we address below in this document.

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1.4 Terminology and Acronyms

   This document generally uses the acronyms defined in [RFC6325] plus
   the additional acronym DBRB. However, for ease of reference, most
   acronyms used are listed here:

      CLNP - ConnectionLess Network Protocol

      DECnet - a proprietary routing protocol that was used by Digital
      Equipment Corporation

      DBRB - Designated Border RBridge

      IS-IS - Intermediate System to Intermediate System

      LSP - Link State PDU

      PDU - Protocol Data Unit

      RBridge - Routing Bridge

      RPF - Reverse Path Forwarding

      TRILL - TRansparent Interconnection of Lots of Links

      TRILL switch - an alternative name for an RBridge

      VLAN - Virtual Local Area Network

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2. Multilevel TRILL Issues

   The TRILL-specific issues introduced by multilevel include the

   a. Configuration of non-zero area addresses, encoding them in IS-IS
      PDUs, and possibly interworking with old TRILL switches that do
      not understand nonzero area addresses.

         See Section 2.1.

   b. Nickname management.

         See Sections 2.5 and 2.2.

   c. Advertisement of pruning information (VLAN reachability, IP
      multicast addresses) across areas.

         Distribution tree pruning information is only an optimization,
         as long as multi-destination packets are not prematurely
         pruned.  For instance, border RBridges could advertise they can
         reach all possible VLANs, and have an IP multicast router
         attached.  This would cause all multi-destination traffic to be
         transmitted to border RBridges, and possibly pruned there, when
         the traffic could have been pruned earlier based on VLAN or
         multicast group if border RBridges advertised more detailed
         VLAN and/or multicast listener and multicast router attachment

   d. Computation of distribution trees across areas for multi-
      destination frames.

         See Section 2.3.

   e. Computation of RPF information for those distribution trees.

         See Section 2.4.

   f. Computation of pruning information across areas.

         See Sections 2.3 and 2.6.

   g. Compatibility, as much as practical, with existing, unmodified

         The most important form of compatibility is with existing TRILL
         fast path hardware. Changes that require upgrade to the slow
         path firmware/software are more tolerable. Compatibility for
         the relatively small number of border RBridges is less
         important than compatibility for non-border RBridges.

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         See Section 5.

2.1 Non-zero Area Addresses

   The current TRILL base protocol specification [RFC6325] [RFC6326]
   [RFC6327] says that the area address in IS-IS must be zero.  The
   purpose of the area address is to ensure that different areas are not
   accidentally hooked together.  Furthermore, zero is an invalid area
   address for layer 3 IS-IS, so it was chosen as an additional safety
   mechanism to ensure that layer 3 IS-IS would not be confused with
   TRILL IS-IS.  However, TRILL uses a different multicast address and
   an Ethertype to avoid such confusion, so it is not necessary to worry
   about this.

   Since current TRILL RBridges will reject any IS-IS messages with
   nonzero area addresses, the choices are as follows:

   a.1 upgrade all RBridges that are to interoperate in a potentially
       multilevel environment to understand non-zero area addresses,
   a.2 neighbors of old RBridges must remove the area address from IS-IS
       messages when talking to an old RBridge (which might break IS-IS
       security and/or cause inadvertent merging of areas),
   a.3 ignore the problem of accidentally merging areas entirely, or
   a.4 keep the fixed "area address" field as 0 in TRILL, and add a new,
       optional TLV for "area name" that, if present, could be compared,
       by new RBridges, to prevent accidental area merging.

   In principal, different solutions could be used in different areas
   but it would be much simpler to adopt one of these choices uniformly.

2.2 Aggregated versus Unique Nicknames

   In the unique nickname alternative, all nicknames across the campus
   must be unique.  In the aggregated nickname alternative, RBridge
   nicknames within an aggregated area are only of local significance,
   and the only nickname externally (outside that area) visible is the
   "area nickname" (or nicknames), which aggregates all the internal

   The unique nickname approach simplifies border RBridges.

   The aggregated nickname approach eliminates the potential problem of
   nickname exhaustion, minimizes the amount of nickname information
   that would need to be forwarded between areas, minimizes the size of
   the forwarding table, and simplifies RPF calculation and RPF

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2.2.1 More Details on Unique Nicknames

   With unique cross-area nicknames, it would be intractable to have a
   flat nickname space with RBridges in different areas contending for
   the same nicknames.  Instead, each area would need to be configured
   with a block of nicknames.  Either some RBridges would need to
   announce that all the nicknames other than that block are taken (to
   prevent the RBridges inside the area from choosing nicknames outside
   the area's nickname block), or a new TLV would be needed to announce
   the allowable nicknames, and all RBridges in the area would need to
   understand that new TLV. An example of the second approach is given
   in [NickFlags].

   Currently the encoding of nickname information in TLVs is by listing
   of individual nicknames; this would make it painful for a border
   RBridge to announce into an area that it is holding all other
   nicknames to limit the nicknames available within that area.  The
   information could be encoded as ranges of nicknames to make this
   somewhat manageable; however, a new TLV for announcing nickname
   ranges would not be intelligible to old RBridges.

   There is also an issue with the unique nicknames approach in building
   distribution trees, as follows:

      With unique nicknames in the TRILL campus and TRILL header
      nicknames not rewritten by the border RBridges, there would have
      to be globally known nicknames for the trees.  Suppose there are k
      trees.  For all of the trees with nicknames located outside an
      area, the local trees would be rooted at a border RBridge or
      RBridges.  Therefore, there would be either no splitting of multi-
      destination traffic with the area or restricted splitting of
      multi-destination traffic between trees rooted at a highly
      restricted set of RBridges.

      As an alternative, just the "egress nickname" field of multi-
      destination TRILL Data frames could be mapped at the border,
      leaving known unicast frames un-mapped. However, this surrenders
      much of the unique nickname advantage of simpler border RBridges.

   Scaling to a very large campus with unique nicknames might exhaust
   the 16-bit TRILL nicknames space. One method of expanding that to a
   24-bit space is given in [MoreNicks]; however, that technique would
   require all RBridges in the campus to understand larger nicknames.

   For an example of a more specific multilevel proposal using unique
   nicknames, see [DraftUnique].

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2.2.2 More Details on Aggregated Nicknames

   The aggregated nickname approach enables passing far less nickname
   information. It works as follows, assuming both the source and
   destination areas are using aggregated nicknames:

      Each area would be assigned a 16-bit nickname. This would not be
      the nickname of any actual RBridge. Instead, it would be the
      nickname of the area itself.  Border RBridges would know the area
      nickname for their own area(s).

   The TRILL Header nickname fields in TRILL Data packets being
   transported through a multilevel RBridge campus with aggregated
   nicknames are as follows:

     - When both the ingress and egress RBridges are in the same area,
        there need be no change from the existing base TRILL protocol
        standard in the TRILL Header nickname fields.

     - When being transported in Level 2, the ingress nickname is the
        nickname of the ingress RBridge's area while the egress nickname
        is either the nickname of the egress RBridge's area or a tree

     - When being transported in Level 1 to Level 2, the ingress
        nickname is the nickname of the ingress RBridge itself while the
        egress nickname is either the nickname of the area of the egress
        RBridge or a tree nickname.

     - When being transported from Level 2 to Level 1, the ingress
        nickname is the nickname of the ingress RBridge's area while the
        egress nickname is either the nickname of the egress RBridge
        itself or a tree nickname.

   There are two variations of the aggregated nickname approach. The
   first is the Border Learning approach, which is described in Section The second is the Swap Nickname Field approach, which is
   described in Section Section compares the advantages
   and disadvantages of these two variations. Border Learning Aggregated Nicknames

   This section provides an illustrative example and description of the
   border learning variation of aggregated nicknames.

   In the following picture, RB2 and RB3 are area border RBridges.  A
   source S is attached to RB1.  The two areas have nicknames 15961 and
   15918, respectively.  RB1 has a nickname, say 27, and RB4 has a

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   nickname, say 44 (and in fact, they could even have the same
   nickname, since the RBridge nickname will not be visible outside
   these aggreated areas).

            Area 15961              level 2             Area 15918
    +-------------------+     +-----------------+     +--------------+
    |                   |     |                 |     |              |
    |  S--RB1---Rx--Rz----RB2---Rb---Rc--Rd---Re--RB3---Rk--RB4---D  |
    |     27            |     |                 |     |     44       |
    |                   |     |                 |     |              |
    +-------------------+     +-----------------+     +--------------+

   Let's say that S transmits a frame to destination D, which is
   connected to RB4, and let's say that D's location is learned by the
   relevant RBridges already.  The relevant RBridges have learned the

   1) RB1 has learned that D is connected to nickname 15918
   2) RB3 has learned that D is attached to nickname 44.

   The following sequence of events will occur:

   -  S transmits an Ethernet frame with source MAC = S and destination
      MAC = D.

   -  RB1 encapsulates with a TRILL header with ingress RBridge = 27,
      and egress = 15918 producing a TRILL Data packet.

   -  RB2 has announced in the Level 1 IS-IS instance in area 15961,
      that it is attached to all the area nicknames, including 15918.
      Therefore, IS-IS routes the packet to RB2. (Alternatively, if a
      distinguished range of nicknames is used for Level 2, Level 1
      RBridges seeing such an egress nickname will know to route to the
      nearest border router, which can be indicated by the IS-IS
      attached bit.)

   -  RB2, when transitioning the packet from Level 1 to Level 2,
      replaces the ingress RBridge nickname with the area nickname, so
      replaces 27 with 15961. Within Level 2, the ingress RBridge field
      in the TRILL header will therefore be 15961, and the egress
      RBridge field will be 15918. Also RB2 learns that S is attached to
      nickname 27 in area 15961 to accommodate return traffic.

   -  The packet is forwarded through Level 2, to RB3, which has
      advertised, in Level 2, reachability to the nickname 15918.

   -  RB3, when forwarding into area 15918, replaces the egress nickname
      in the TRILL header with RB4's nickname (44).  So, within the
      destination area, the ingress nickname will be 15961 and the
      egress nickname will be 44.

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   -  RB4, when decapsulating, learns that S is attached to nickname
      15961, which is the area nickname of the ingress.

   Now suppose that D's location has not been learned by RB1 and/or RB3.
   What will happen, as it would in TRILL today, is that RB1 will
   forward the packet as a multi-destination frame, choosing a tree.  As
   the multi-destination frame transitions into Level 2, RB2 replaces
   the ingress nickname with the area nickname. If RB1 does not know the
   location of D, the packet must be flooded, subject to possible
   pruning, in Level 2 and, subject to possible pruning, from Level 2
   into every Level 1 area that it reaches on the Level 2 distribution

   Now suppose that RB1 has learned the location of D (attached to
   nickname 15918), but RB3 does not know where D is.  In that case, RB3
   must turn the frame into a multi-destination packet within area
   15918.  In this case, care must be taken so that, in case RB3 is not
   the Designated transitioner between Level 2 and its area for that
   multi-destination packet, but was on the unicast path, that another
   border RBridge in that area not forward the now multi-destination
   frame back into Level 2.  Therefore, it would be desirable to have a
   marking, somehow, that indicates the scope of this packet's
   distribution to be "only this area" (see also Section 4).

   In cases where there are multiple transitioners for unicast packets,
   the border learning mode of operation requires that the address
   learning between them be shared by some protocol such as running
   ESADI [RFCesadi] for all label (VLANs and/or FGLs) of interest to
   avoid excessive unknown unicast flooding.

   The potential issue described at the end of Section 2.2.1 with trees
   in the unique nickname alternative is eliminated with aggregated
   nicknames.  With aggregated nicknames, each border RBridge that will
   transition multi-destination packets can have a mapping between Level
   2 tree nicknames and Level 1 tree nicknames.  There need not even be
   agreement about the total number of trees; just that the border
   RBridge have some mapping, and replace the egress RBridge nickname
   (the tree name) when transitioning levels. Swap Nickname Field Aggregated Nicknames

   As a variant, two additional fields could exist in TRILL Data frames
   we call the "ingress swap nickname field" and the "egress swap
   nickname field". The changes in the example above would be as

   -  RB1 will have learned the area nickname of D and the RBridge
      nickname of RB4 to which D is attached. In encapsulating a frame

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      to D, it puts the area nickname of D (15918) in the egress
      nickname field of the TRILL Header and puts the nickname of RB3
      (44) in a egress swap nickname field.

   -  RB2 moves the ingress nickname to the ingress swap nickname field
      and inserts 15961, the area nickname for S, into the ingress
      nickname field.

   -  RB3 swaps the egress nickname and the egress swap nickname fields,
      which sets the egress nickname to 44.

   -  RB4 learns the correspondence between the source MAC/VLAN of S and
      the { ingress nickname, ingress swap nickname field } pair as it
      decapsulates and egresses the frame.

   See [DraftAggregated] for a multilevel proposal using aggregated swap
   nicknames. Comparison

   The Border Learning variant described in Section above
   minimizes the change in non-border RBridges but imposes the burden on
   border RBridges of learning and doing lookups in all the end station
   MAC addresses within their area(s) that are used for communication
   outside the area. The burden could be reduced by decreasing the area
   size and increasing the number of areas.

   The Swap Nickname Field variant described in Section
   eliminates the extra address learning burden on border RBridges but
   requires more extensive changes to non-border RBridges. In particular
   they must learn to associate both an RBridge nickname and an area
   nickname with end station MAC/label pairs (except for addresses that
   are local to their area).

   The Swap Nickname Field alternative is more scalable but less
   backward compatible for non-border RBridges. It would be possible for
   border and other level 2 RBridges to support both Border Learning,
   for support of legacy Level 1 RBridges, and Swap Nickname, to support
   Level 1 RBridges that understood the Swap Nickname method.

2.3 Building Multi-Area Trees

   It is easy to build a multi-area tree by building a tree in each area
   separately, (including the Level 2 "area"), and then having only a
   single border RBridge, say RB1, in each area, attach to the Level 2
   area.  RB1 would forward all multi-destination frames between that

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   area and Level 2.

   People might find this unacceptable, however, because of the desire
   to path split (not always sending all multi-destination traffic
   through the same border RBridge).

   This is the same issue as with multiple ingress RBridges injecting
   traffic from a pseudonode, and can be solved with the mechanism that
   was adopted for that purpose: the affinity TLV [DraftCMT].  For each
   tree in the area, at most one border RB announces itself in an
   affinity TLV with that tree name.

2.4 The RPF Check for Trees

   For multi-destination frames originating locally in RB1's area,
   computation of the RPF check is done as today.  For multi-destination
   frames originating outside RB1's area, computation of the RPF check
   must be done based on which one of the border RBridges (say RB1, RB2,
   or RB3) injected the frame into the area.

   An RBridge, say RB4, located inside an area, must be able to know
   which of RB1, RB2, or RB3 transitioned the frame into the area from
   Level 2.  (or into Level 2 from an area).

   This could be done based on having the DBRB announce the transitioner
   assignments to all the RBridges in the area, or the Affinity TLV
   mechanism given in [DraftCMT], or the New Tree Encoding mechanism
   discussed in Section 4.1.1.

2.5 Area Nickname Acquisition

   In the aggregated nickname alternative, each area must acquire a
   unique area nickname.  It is probably simpler to allocate a block of
   nicknames (say, the top 4000) to be area addresses, and not used by
   any RBridges.

   The area nicknames need to be advertised and acquired through Level

   Within an area, all the border RBridges must discover each other
   through the Level 1 link state database, by advertising, in their LSP
   "I am a border RBridge".

   Of the border RBridges, one will have highest priority (say RB7). RB7
   can dynamically participates, in Level 2, to acquire a nickname for
   the area.  RB7 could give the area a pseudonode IS-IS ID, such as

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   RB7.5, within Level 2.  So an area would appear, in Level 2, as a
   pseudonode and the pseudonode can participate, in Level 2, to acquire
   a nickname for the area.

   Within Level 2, all the border RBridges [for an area] can advertise
   reachability to the pseudonode, which would mean connectivity to the
   area nickname.

2.6 Link State Representation of Areas

   Within an area, say area A1, there is an election for the DBRB,
   (Designated Border RBridge), say RB1.  This can be done through LSPs
   within area A1.  The border RBridges announce themselves, together
   with their DBRB priority. (Note that the election of the DBRB cannot
   be done based on Hello messages, because the border RBridges are not
   necessarily physical neighbors of each other.  They can, however,
   reach each other through connectivity within the area, which is why
   it will work to find each other through Level 1 LSPs.)

   RB1 acquires the area nickname (in the aggregated nickname approach),
   gives the area a pseudonode IS-IS ID (just like the DRB would give a
   pseudonode IS-IS ID to a link).  RB1 advertises, in area A1, what the
   pseudonode IS-IS ID for the area is (and the area nickname that RB1
   has acquired).

   The pseudonode LSP initiated by RB1 for the area includes any
   information extraneous to area A1 that should be input into area A1
   (such as area nicknames of external areas, or perhaps (in the unique
   nickname variant) all the nicknames of external RBridges in the TRILL
   campus and pruning information such as multicast listeners and
   labels).  All the other border RBridges for the area announce (in
   their LSP) attachment to that pseudonode.

   Within Level 2, RB1 generates a Level 2 LSP on behalf of the area,
   also represented as a pseudonode.  The same pseudonode ID could be
   used within Level 1 and Level 2, for the area.  (There does not seem
   any reason why it would be useful for it to be different, but there's
   also no reason why it would need to be the same).  Likewise, all the
   area A1 border RBridges would announce, in their Level 2 LSPs,
   connection to the pseudonode.

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3. Area Partition

   It is possible for an area to become partitioned, so that there is
   still a path from one section of the area to the other, but that path
   is via the Level 2 area.

   With multilevel TRILL, an area will naturally break into two areas in
   this case.

   An area address might be configured to ensure two areas are not
   inadvertently connected.  That area address appears in Hellos and
   LSPs within the area.  If two chunks, connected only via Level 2,
   were configured with the same area address, this would not cause any
   problems. (They would just operate as separate Level 1 areas.)

   A more serious problem occurs if the Level 2 area is partitioned in
   such a way that it could be healed by using a path through a Level 1
   area. TRILL will not attempt to solve this problem. Within the Level
   1 area, a single border RBridge will be the DBRB, and will be in
   charge of deciding which (single) RBridge will transition any
   particular multi-destination packets between that area and Level 2.
   If the Level 2 area is partitioned, this will result in multi-
   destination frames only reaching the portion of the TRILL campus
   reachable through the partition attached to the RBridge that
   transitions that frame.  It will not cause a loop.

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4. Multi-Destination Scope

   There are at least two reasons it would be desirable to be able to
   mark a multi-destination frame with a scope that indicates the frame
   should not exit the area, as follows:

   1. To address an issue in the border learning variant of the
      aggregated nickname alternative, when a unicast packet turns into
      a multi-destination packet when transitioning from Level 2 to
      Level 1, as discussed in Section 4.1.

   2. To constrain the broadcast domain for certain discovery,
      directory, or service protocols as discussed in Section 4.2.

   Multi-destination frame distribution scope restriction could be done
   in a number of ways. For example, there could be a flag in the packet
   that means "for this area only". However, the technique that might
   require the least change to RBridge fast path logic would be to
   indicate this in the egress nickname that designates the distribution
   tree being used. There could be two general tree nicknames for each
   tree, one being for distribution restricted to the area and the other
   being for multi-area trees. Or, alternatively, there would be a set
   of N (perhaps 16) special currently reserved nicknames used to
   specify the N highest priority trees but with the variation that if
   the special nickname is used for the tree, the frame is not
   transitioned between areas.

4.1 Unicast to Multi-destination Conversions

   In the border learning variant of the aggregated nickname
   alternative, a unicast packet might be known at the Level 1 to Level
   2 transition, be forwarded as a unicast packet to the least cost
   border RBridge advertising connectivity to the destination area, but
   turn out to have an unknown destination MAC/VLAN pair when it arrives
   at that border RBridge.

   In this case, the packet must be converted into a multi-destination
   packet and flooded in the destination area.  However, if the border
   RBridge doing the conversion is not the border RBridge designated to
   transition the resulting multi-destination packet, there is the
   danger that the designated transitioner may pick up the packet and
   flood it back into Level 2 from which it may be flooded into multiple
   areas.  This danger can be avoided by restricting any multi-
   destination packet that results from such a conversion to the
   destination area through a flag in the packet or though distributing
   it on a tree that is restricted to the area.

   Alternatively, a multi-destination packet intended only for the area

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   could be tunneled (within the area) to the RBridge Rx, that is the
   appointed transitioner for that form of packet (say, based on VLAN or
   FGL), with instructions that Rx only transmit the packet within the
   area, and Rx could initiate the multi-destination packet within the
   area.  Since Rx introduced the packet, and is the only one allowed to
   transition that packet to Level 2, this would accomplish scoping of
   the packet to within the area.  Since this case only occurs in the
   unusual case when unicast packets need to be turned into multi-
   destination as described above, the suboptimality of tunneling
   between the border RBridge that receives the unicast packet and the
   appointed level transitioner for that frame, would not be an issue.

4.1.1 New Tree Encoding

   The current encoding, in a TRILL header, of a tree, is of the
   nickname of the tree root. This requires all 16 bits of the egress
   nickname field. TRILL could instead, for example, use the bottom 6
   bits to encode the tree number (allowing 64 trees), leavinig 10 bits
   to encode information such as:

   o  scope: a flag indicating whether it should be single area only, or
      entire campus
   o  border injector: an indicator of which of the k border RBridges
      injected this packet

   If TRILL were to adopt this new encoding, it would also avoid the
   limitations of the Affinity sub-TLV [DraftCMT] in the single area
   case; any of the RBridges attached to a pseudonode could inject a
   multi-destination packet. This would require all RBridges to be
   changed to understand the new encoding for a tree, and it would
   require a TLV in the LSP to indicate which number each of the
   RBridges attached to the pseudonode would be.

4.2 Selective Broadcast Domain Reduction

   There are a number of service, discovery, and directory protocols
   that, for convenience, are accessed via multicast or broadcast
   frames. Examples are DHCP, the NetBIOS Service Location Protocol, and
   multicast DNS.

   Some such protocols provide means to restrict distribution to an IP
   subnet or equivalent to reduce size of the broadcast domain they are
   using and then provide a proxy that can be placed in that subnet to
   use unicast to access a service elsewhere. In cases where a proxy
   mechanism is not currently defined, it may be possible to create one
   that references a central server or cache. With multilevel TRILL, it

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   is possible to construct very large IP subnets that could become
   saturated with multi-destination traffic of this type unless packets
   can be further restricted in their distribution. Such restricted
   distribution can be accomplished for some protocols, say protocol P,
   as follows:

   -  Either (1) at all ingress RBridges in an area place all protocol P
      multi-destination packets on a distribution tree restricted to the
      area or (2) at all border RBridges between that area and Level 2,
      detect protocol P multi-destination packets and do not transition

   - Then place one protocol P proxy (or more for redundancy) inside
      each area. These proxies unicast protocol P requests or other
      messages to the actual campus server(s) for P. They also receive
      unicast responses or other messages from those servers and deliver
      them within the area via unicast, multicast, or broadcast as
      appropriate. Such proxies would not be needed if it was acceptable
      for all protocol P traffic to be restricted to an area.

   While it might seem logical to connect the campus servers to RBridges
   in Level 2, they could be placed within one or more areas so that, in
   some cases, those areas might not require a local proxy server.

R. Perlman, et al                                              [Page 20]

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5. Co-Existence with Old RBridges

   RBridges that are not multilevel aware may have a problem with
   calculating RPF Check and filtering information, since they would not
   be aware of assignment of border RBridge transitioning.

   A possible solution, as long as any old RBridges exist within an
   area, is to have the border RBridges elect a single DBRB (Designated
   Border RBridge), and have all inter-area traffic go through the DBRB
   (unicast as well as multi-destination).  If that DBRB goes down, a
   new one will be elected, but at any one time, all inter-area traffic
   (unicast as well as multi-destination) would go through that one
   DRBR. However this eliminates load splitting at level transition.

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6. Multi-Access Links with End Stations

   Care must be taken, in the case where there are multiple RBridges on
   a link with end stations, that only one RBridge ingress/egress any
   given data packet from/to the end nodes. With existing, single level
   TRILL, this is done by electing a single Designated RBridge per link,
   which appoints a single Appointed Forwarder per VLAN [RFC6327]
   [RFC6439].  But suppose there are two (or more) RBridges on a link;
   R1 in area 1000, and R2, in area 2000, and that the link contains end
   nodes.  If R1 and R2 ignore each other's Hellos then they will both
   ingress/egress end node traffic from the link.

   A simple rule is to use the RBridge(s) having the lowest numbered
   area, comparing area numbers as unsigned integers, to handle native
   traffic. This would automatically give multilevel-ignorant legacy
   RBridges, that would be using area number zero, highest priority for
   handling end stations, which they would try to do anyway.

   Other methods are possible. For example, including doing the
   selection of Appointed Forwarders and of the RBridge in charge of
   that selection across all RBridges on the link regardless of area.
   However, a special case would then have to be made in any case for
   legacy RBridges using area number zero.

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7. Summary

   This draft discusses issues and possible approaches to multilevel
   TRILL.  The alternative using area nicknames for aggregation has
   significant advantages in terms of scalability over using campus wide
   unique nicknames, not just of avoiding nickname exhaustion, but by
   allowing RPF Checks to be aggregated based on an entire area;
   however, the alternative using unique nicknames is simpler and avoids
   the changes in border RBridges required to support aggregated
   nicknames. It is possible to support both. For example, a TRILL
   campus could use simpler unique nicknames until scaling begins to
   cause problems and then start to introduce areas with aggregated

   Some issues are not difficult, such as dealing with partitioned
   areas.  Some issues are more difficult, especially dealing with old

8. Security Considerations

   This document explores alternatives for the use of multilevel IS-IS
   in TRILL. It does not consider security issues. For general TRILL
   Security Considerations, see [RFC6325].

9. IANA Considerations

   This document requires no IANA actions. RFC Editor: Please delete
   this section before publication.

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10. Normative References

   As an Informational document, this draft has no normative references.

11. Informative References

   [IS-IS] - ISO/IEC 10589:2002, Second Edition, "Intermediate System to
         Intermediate System Intra-Domain Routing Exchange Protocol for
         use in Conjunction with the Protocol for Providing the
         Connectionless-mode Network Service (ISO 8473)", 2002.

   [RFC1195] - Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
         dual environments", RFC 1195, December 1990.

   [RFC6325] - Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A.
         Ghanwani, "Routing Bridges (RBridges): Base Protocol
         Specification", RFC 6325, July 2011.

   [RFC6326] - Eastlake, D., Banerjee, A., Dutt, D., Perlman, R., and A.
         Ghanwani, "Transparent Interconnection of Lots of Links (TRILL)
         Use of IS-IS", RFC 6326, July 2011.

   [RFC6327] - Eastlake 3rd, D., Perlman, R., Ghanwani, A., Dutt, D.,
         and V. Manral, "Routing Bridges (RBridges): Adjacency", RFC
         6327, July 2011.

   [RFC6439] - Perlman, R., Eastlake, D., Li, Y., Banerjee, A., and F.
         Hu, "Routing Bridges (RBridges): Appointed Forwarders", RFC
         6439, November 2011.

   [RFCfgl] - Donald Eastlake, Mingui Zhang, Puneet Agarwal, Radia
         Perlman, Dinesh Dutt, draft-ietf-trill-fine-labeling, in RFC
         Editor's queue.

   [RFCesadi] - Hongjun Zhai, Fangwei Hu, Radia Perlman, Donald
         Eastlake, Olen Stokes, draft-ietf-trill-esadi, work in

   [DraftAggregated] - Bhargav Bhikkaji, Balaji Venkat Venkataswami,
         Narayana Perumal Swamy, "Connecting Disparate Data
         Center/PBB/Campus TRILL sites using BGP", draft-balaji-trill-
         over-ip-multi-level, Work In Progress.

   [DraftCMT] - Tissa Senevirathne, Janardhanan Pathang, Jon Hudson,
         "Coordinated Multicast Trees (CMT) for TRILL", draft-tissa-
         trill-cmt, Work in Progress.

   [DraftUnique] - Tissa Senevirathne, Les Ginsberg, Janardhanan

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         Pathangi, Jon Hudson, Sam Aldrin, Ayan Banerjee, Sameer
         Merchant, "Default Nickname Based Approach for Multilevel
         TRILL", draft-tissa-trill-multilevel, Work In Progress.

   [MoreNicks] - draft-tissa-trill-mt-encode, Work In Progress.

   [NickFlags] - Eastlake, D., W. Hao, draft-eastlake-trill-nick-label-
         prop, Work In Progress.

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Authors' Addresses

   Radia Perlman
   Intel Labs
   2200 Mission College Blvd.
   Santa Clara, CA 95054-1549 USA

   Phone: +1-408-765-8080
   Email: Radia@alum.mit.edu

   Donald Eastlake
   Huawei Technologies
   155 Beaver Street
   Milford, MA 01757 USA

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

   Anoop Ghanwani
   350 Holger Way
   San Jose, CA 95134 USA

   Phone: +1-408-571-3500
   Email: anoop@alumni.duke.edu

   Hongjun Zhai
   68 Zijinghua Road, Yuhuatai District
   Nanjing, Jiangsu 210012 China

   Phone: +86 25 52877345
   Email: zhai.hongjun@zte.com.cn

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R. Perlman, et al                                              [Page 27]

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