TRILL Working Group                                        Radia Perlman
INTERNET-DRAFT                                                       EMC
Intended status: Informational                           Donald Eastlake
                                                            Mingui Zhang
                                                          Anoop Ghanwani
                                                            Hongjun Zhai
Expires: January 3, 2018                                    July 3, 2017

                   Alternatives for Multilevel TRILL
             (Transparent Interconnection of Lots of Links)


   Although TRILL is based on IS-IS, which supports multilevel unicast
   routing, extending TRILL to multiple levels has challenges that are
   not addressed by the already-existing capabilities of IS-IS.  One
   issue is with the handling of multi-destination packet distribution
   trees. Other issues are with TRILL switch nicknames. How are such
   nicknames allocated across a multilevel TRILL network? Do nicknames
   need to be unique across an entire multilevel TRILL network or can
   they merely be unique within each multilevel area?

   This informational document enumerates and examines alternatives
   based on a number of factors including backward compatibility,
   simplicity, and scalability and makes recommendations in some cases.

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 <>.

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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   The list of current Internet-Drafts can be accessed at The list of Internet-Draft
   Shadow Directories can be accessed at

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Table of Contents

      1. Introduction............................................4
      1.1 The Motivation for Multilevel..........................4
      1.2 Improvements Due to Multilevel.........................5
      1.2.1. The Routing Computation Load........................5
      1.2.2. LSDB Volatility Creating Too Much Control Traffic...5
      1.2.3. LSDB Volatility Causing To Much Time Unconverged....6
      1.2.4. The Size Of The LSDB................................6
      1.2.5 Nickname Limit.......................................6
      1.2.6 Multi-Destination Traffic............................7
      1.3 Unique and Aggregated Nicknames........................7
      1.4 More on Areas..........................................8
      1.5 Terminology and Acronyms...............................8

      2. Multilevel TRILL Issues................................10
      2.1 Non-zero Area Addresses...............................11
      2.2 Aggregated versus Unique Nicknames....................11
      2.2.1 More Details on Unique Nicknames....................12
      2.2.2 More Details on Aggregated Nicknames................13 Border Learning Aggregated Nicknames..............14 Swap Nickname Field Aggregated Nicknames..........16 Comparison........................................17
      2.3 Building Multi-Area Trees.............................17
      2.4 The RPF Check for Trees...............................18
      2.5 Area Nickname Acquisition.............................18
      2.6 Link State Representation of Areas....................19

      3. Area Partition.........................................20

      4. Multi-Destination Scope................................21
      4.1 Unicast to Multi-destination Conversions..............21
      4.1.1 New Tree Encoding...................................22
      4.2 Selective Broadcast Domain Reduction..................22

      5. Co-Existence with Old TRILL switches...................24
      6. Multi-Access Links with End Stations...................25

      7. Summary................................................27

      8. Security Considerations................................28
      9. IANA Considerations....................................28

      Normative References......................................29
      Informative References....................................29
      Authors' Addresses........................................32

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1. Introduction

   The IETF TRILL (Transparent Interconnection of Lot of Links) protocol
   [RFC6325] [RFC7177] [RFC7780] provides optimal pair-wise data routing
   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]
   [RFC7176]) link state routing in conjunction with a header that
   includes a hop count. The design supports data labels (VLANs and Fine
   Grained Labels [RFC7172]) and optimization of the distribution of
   multi-destination data based on data label and multicast group.
   Devices that implement TRILL are called TRILL Switches or RBridges.

   Familiarity with [IS-IS], [RFC6325], and [RFC7780] is assumed in this

1.1 The Motivation for Multilevel

   The primary motivation for multilevel TRILL is to improve
   scalability.  The following issues might limit the scalability of a
   TRILL-based network:

   1. The routing computation load
   2. The volatility of the link state database (LSDB) creating too much
      control traffic
   3. The volatility of the LSDB causing the TRILL network to be in an
      unconverged state too much of the time
   4. The size of the LSDB
   5. The limit of the number of TRILL switches, due to the 16-bit
      nickname space (for further information on why this might be a
      problem, see Section 1.2.5)
   6. The traffic due to upper layer protocols use of broadcast and
   7. The size of the end node learning table (the table that remembers
      (egress TRILL switch, label/MAC) pairs)

   As discussed below, extending TRILL IS-IS to be multilevel
   (hierarchical) can help with all of these issues except issue 7.

   IS-IS was designed to be multilevel [IS-IS].  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 Level 1 areas, in addition to their role as Level 2 routers.

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   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
   clear, when transitioning multi-destination packets between Level 2
   and a Level 1 area in either direction, that exactly one border TRILL
   switch will transition a particular data packet between the levels or
   else duplication or loss of traffic can occur.

1.2 Improvements Due to Multilevel

   Partitioning the network into areas directly solves the first four
   scalability issues listed above as described in Sections 1.2.1
   through 1.2.4. Multilevel also contributes to solving issues 5 and 6
   as discussed in Section 1.2.5 and 1.2.6 respectively.

   In the subsections below, N indicates the number of TRILL switches in
   a TRILL campus. As a simplifying assumption, it is assumed that each
   TRILL switch has k links to other TRILL switches. An "optimized"
   multilevel campus is assumed to have Level 1 areas containing sqrt(N)

1.2.1. The Routing Computation Load

   The Dijkstra algorithm uses computational effort on the order of the
   number of links in a network (N*k) times the log of the number of
   nodes to calculate least cost routes at a router (Section 12.3.3
   [InterCon]). Thus, in a single level TRILL campus, it is on the order
   of N*k*log(N). In an optimized multilevel campus, it is on the order
   of sqrt(N)*k*log(N). So, for example, assuming N is 3,000, the level
   of computational effort would be reduced by about a factor of 50.

1.2.2. LSDB Volatility Creating Too Much Control Traffic

   The rate of LSDB changes is assumed to be approximately proportional
   to the number of routers and links in the TRILL campus or N*(1+k) for
   a single level campus. With an optimized multilevel campus, each area
   would have about sqrt(N) routers and proportionately fewer links
   reducing the rate of LSDB changes by about a factor of sqrt(N).

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1.2.3. LSDB Volatility Causing To Much Time Unconverged

   With the simplifying assumption that routing converges after each
   topology change before the next such change, the fraction of time
   that routing is unconverged is proportional to the product of the
   rate of change occurrence and the convergence time. The rate of
   topology changes per some arbitrary unit of time will be roughly
   proportional to the number of router and links (Section 1.2.2). The
   convergence time is approximately proportional to the computation
   involved at each router (Section 1.2.1). Thus, based on these
   simplifying assumptions, the time spent unconverged in a single level
   network is proportional to (N*(1+k))*(N*k*log(N)) while that time for
   an optimized multilevel network would be proportional to
   (sqrt(N)*(1+k))*(sqrt(N)*k*log(N)).  Thus, in changing to multilevel,
   the time spent unconverged, using these simplifying assumptions, is
   improved by about a factor of N.

1.2.4. The Size Of The LSDB

   The size of the LSDB, which consists primarily of information about
   routers (TRILL switches) and links, is also approximately
   proportional to the number of routers and links. So, as with item 2
   in Section 1.2.2 above, it should improve by about a factor of
   sqrt(N) in going from single to multilevel.

1.2.5 Nickname Limit

   For many TRILL protocol purposes, RBridges are designated by 16-bit
   nicknames. While some values are reserved, this appears to provide
   enough nicknames to designated over 65,000 RBridges. However, this
   number is effectively reduced by the following two factors:

    - Nicknames are consumed when pseudo-nicknames are used for the
      active-active connection of end stations. Using the techniques in
      [RFC7781], for example, could double the nickname consumption if
      there are extensive active-active edge groups connected to
      different sets of edge TRILL switch ports.

    - There might be problems in multilevel campus wide contention for
      single nickname allocation of nicknames were allocated
      individually from a single pool for the entire campus. Thus it
      seems likely that a hierarchical method would be chosen where
      blocks of nicknames are allocated at Level 2 to Level 1 areas and
      contention for a nickname by an RBridge in such a Level 1 area
      would be only within that area. Such hierarchical allocation leads
      to further effective loss of nicknames similar to the situation

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      with IP addresses discussed in [RFC3194].

   Even without the above effective reductions in nickname space, a very
   large multilevel TRILL campus, say one with 200 areas each containing
   500 TRILL switches, could require 100,000 or more nicknames if all
   nicknames in the campus must be unique, which is clearly impossible
   with 16-bit nicknames.

   This scaling limit, namely, 16-bit nickname space, will only be
   addressed with the aggregated nickname approach. Since the aggregated
   nickname approach requires some complexity in the border TRILL
   switches (for rewriting the nicknames in the TRILL header), the
   suggested design in this document allows a campus with a mixture of
   unique-nickname areas, and aggregated-nickname areas. Thus a TRILL
   network could start using multilevel with the simpler unique nickname
   method and later add aggregated areas as a later stage of network

   With this design, nicknames must be unique across all Level 2 and
   unique-nickname area TRILL switches taken together, whereas nicknames
   inside an aggregated-nickname area are visible only inside that area.
   Nicknames inside an aggregated-nickname area must still not conflict
   with nicknames visible in Level 2 (which includes all nicknames
   inside unique nickname areas), but the nicknames inside an
   aggregated-nickname area may be the same as nicknames used within one
   or more other aggregated-nickname areas.

   With the design suggested in this document, TRILL switches within an
   area need not be aware of whether they are in an aggregated nickname
   area or a unique nickname area.  The border TRILL switches in area A1
   will indicate, in their LSP inside area A1, which nicknames (or
   nickname ranges) are available, or alternatively which nicknames are
   not available, for choosing as nicknames by area A1 TRILL switches.

1.2.6 Multi-Destination Traffic

   Scaling limits due to protocol use of broadcast and multicast, can be
   addressed in many cases in a multilevel campus by introducing
   locally-scoped multi-destination delivery, limited to an area or a
   single link. See further discussion of this issue in Section 4.2.

1.3 Unique and Aggregated Nicknames

   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

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   alternative, border TRILL switches replace either the ingress or
   egress nickname field in the TRILL header of unicast packets with an
   aggregated nickname representing an entire area.

   The unique nickname alternative has the advantage that border TRILL
   switches 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 scaling problem #5 above, the 16-bit 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 logically reduces the RPF (Reverse Path Forwarding) Check
         information (since only the area nickname needs to appear,
         rather than all the ingress TRILL switches in that area).

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

   For further discussion of the unique and aggregated nickname
   alternatives, see Section 2.2.

1.4 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.
   For TRILL the only purpose of the area address would be to avoid
   accidentally interconnecting areas although the area address had
   other purposes in CLNP (Connectionless Network Layer Protocol), IS-IS
   was originally designed for CLNP/DECnet.

   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 Section 2.1.

1.5 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:

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      CLNP - ConnectionLess Network Protocol

      DECnet - a proprietary routing protocol that was used by Digital
      Equipment Corporation. "DECnet Phase 5" was the origin of IS-IS.

      Data Label - VLAN or Fine Grained Label [RFC7172]

      DBRB - Designated Border RBridge

      ESADI - End Station Address Distribution Information

      IS-IS - Intermediate System to Intermediate System [IS-IS]

      LSDB - Link State Data Base

      LSP - Link State PDU

      PDU - Protocol Data Unit

      RBridge - Routing Bridge, an alternative name for a TRILL switch

      RPF - Reverse Path Forwarding

      TLV - Type Length Value

      TRILL - Transparent Interconnection of Lots of Links or Tunneled
      Routing in the Link Layer [RFC6325] [RFC7780]

      TRILL switch - a device that implements the TRILL protocol
      [RFC6325] [RFC7780], sometimes called 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 non-zero area addresses.

         See Section 2.1.

   b. Nickname management.

         See Sections 2.5 and 2.2.

   c. Advertisement of pruning information (Data Label 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 TRILL switches could advertise
         they can reach all possible Data Labels, and have an IP
         multicast router attached.  This would cause all multi-
         destination traffic to be transmitted to border TRILL switches,
         and possibly pruned there, when the traffic could have been
         pruned earlier based on Data Label or multicast group if border
         TRILL switches advertised more detailed Data Label and/or
         multicast listener and multicast router attachment information.

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

         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
      TRILL switches.

         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 TRILL switches is less
         important than compatibility for non-border TRILL switches.

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         See Section 5.

2.1 Non-zero Area Addresses

   The current TRILL base protocol specification [RFC6325] [RFC7177]
   [RFC7780] 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 merged.  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 packets would not be confused with TRILL
   IS-IS packets.  However, TRILL uses other techniques to avoid
   confusion on a link, such as different multicast addresses and
   Ethertypes on Ethernet [RFC6325], different PPP (Point-to-Point
   Protocol) code points on PPP [RFC6361], and the like. Thus, using an
   area address in TRILL that might be used in layer 3 IS-IS is not a

   Since current TRILL switches will reject any IS-IS messages with non-
   zero area addresses, the choices are as follows:

   a.1 upgrade all TRILL switches that are to interoperate in a
       potentially multilevel environment to understand non-zero area
   a.2 neighbors of old TRILL switches must remove the area address from
       IS-IS messages when talking to an old TRILL switch (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" to Hellos that, if present, could be
       compared, by new TRILL switches, to prevent accidental area

   In principal, different solutions could be used in different areas
   but it would be much simpler to adopt one of these choices uniformly.
   A simple solution would be a.1 above with each TRILL switch using a
   dominant area nickname as its area address. For the unique nickname
   alternative, the dominant nickname could be the lowest value nickname
   held by any border RBridge of the area. For the aggregated nickname
   alternative, it could be the lowest nickname held by a border RBridge
   of the area or a nickname representing the area.

2.2 Aggregated versus Unique Nicknames

   In the unique nickname alternative, all nicknames across the campus
   must be unique.  In the aggregated nickname alternative, TRILL switch
   nicknames within an aggregated area are only of local significance,

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   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 TRILL switches.

   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

2.2.1 More Details on Unique Nicknames

   With unique cross-area nicknames, it would be intractable to have a
   flat nickname space with TRILL switches in different areas contending
   for the same nicknames.  Instead, each area would need to be
   configured with or allocate one or more block of nicknames.  Either
   some TRILL switches would need to announce that all the nicknames
   other than that in blocks available to the area are taken (to prevent
   the TRILL switches inside the area from choosing nicknames outside
   the area's nickname block), or a new TLV would be needed to announce
   the allowable or the prohibited nicknames, and all TRILL switches in
   the area would need to understand that new TLV.

   Currently the encoding of nickname information in TLVs is by listing
   of individual nicknames; this would make it painful for a border
   TRILL switch to announce into an area that it is holding all other
   nicknames to limit the nicknames available within that area. Painful
   means tens of thousands of individual nickname entries in the Level 1
   LSDB. The information could be encoded as ranges of nicknames to make
   this manageable by specifying a new TLV similar to the Nickname Flags
   APPsub-TLV specified in [RFC7780] but providing flags for blocks of
   nicknames rather than single nicknames. Although this would require
   updating software, such a new TLV is the preferred method.

   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 TRILL switches, 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 TRILL switch
      or switches.  Therefore, there would be either no splitting of
      multi-destination traffic within the area or restricted splitting
      of multi-destination traffic between trees rooted at a highly
      restricted set of TRILL switches.

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

   Scaling to a very large campus with unique nicknames might exhaust
   the 16-bit TRILL nicknames space particularly if (1) additional
   nicknames are consumed to support active-active end station groups at
   the TRILL edge using the techniques standardized in [RFC7781] and (2)
   use of the nickname space is less efficient due to the allocation of,
   for example, power-of-two size blocks of nicknames to areas in the
   same way that use of the IP address space is made less efficient by
   hierarchical allocation (see [RFC3194]). One method to avoid nickname
   exhaustion might be to expand nicknames to 24 bits; however, that
   technique would require TRILL message format and fast path processing
   changes and that all TRILL switches in the campus understand larger

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:

   There are at least two ways areas could be identified.

      One method would be to assign each area a 16-bit nickname. This
      would not be the nickname of any actual TRILL switch. Instead, it
      would be the nickname of the area itself.  Border TRILL switches
      would know the area nickname for their own area(s).  For an
      example of a more specific multilevel proposal using unique
      nicknames, see [DraftUnique].

      Alternatively, areas could be identified by the set of nicknames
      that identify the border routers for that area. (See [SingleName]
      for a multilevel proposal using such a set of nicknames.)

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

     -  When both the ingress and egress TRILL switches 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 between different Level 1 areas in Level
        2, the ingress nickname is a nickname of the ingress TRILL

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        switch's area while the egress nickname is either a nickname of
        the egress TRILL switch's area or a tree nickname.

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

     -  When being transported from Level 2 to Level 1, the ingress
        nickname is a nickname for the ingress TRILL switch's area while
        the egress nickname is either the nickname of the egress TRILL
        switch 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 of the aggregated nickname
   approach. Border Learning Aggregated Nicknames

   This section provides an illustrative example and description of the
   border learning variation of aggregated nicknames where a single
   nickname is used to identify an area.

   In the following picture, RB2 and RB3 are area border TRILL switches
   (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 nickname, say 44 (and in fact, they could even have the
   same nickname, since the TRILL switch nickname will not be visible
   outside these aggregated 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 has already been
   learned by the relevant TRILL switches.  These relevant switches have
   learned the following:

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

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   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
      TRILL switches 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 TRILL switch 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.

   -  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 multi-destination, choosing a tree.  As the
   multi-destination packet 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 packet into a multi-destination packet within area
   15918.  In this case, care must be taken so that in the case in which
   RB3 is not the Designated transitioner between Level 2 and its area
   for that multi-destination packet, but was on the unicast path, that

R. Perlman, et al                                              [Page 15]

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   border TRILL switch in that area does not forward the now multi-
   destination packet 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

   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 [RFC7357] for all Data Labels 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 TRILL switch 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 TRILL switch have some mapping, and replace the egress TRILL
   switch nickname (the tree name) when transitioning levels. Swap Nickname Field Aggregated Nicknames

   There is a variant possibility where two additional fields could
   exist in TRILL Data packets that could be called the "ingress swap
   nickname field" and the "egress swap nickname field". This variant is
   described below for completeness but would require fast path hardware
   changes from the existing TRILL protocol. The changes in the example
   above would be as follows:

   -  RB1 will have learned the area nickname of D and the TRILL switch
      nickname of RB4 to which D is attached. In encapsulating a frame
      to D, it puts an area nickname of D (15918) in the egress nickname
      field of the TRILL Header and puts a nickname of RB3 (44) in a
      egress swap nickname field.

   -  RB2 moves the ingress nickname to the ingress swap nickname field
      and inserts 15961, an 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

R. Perlman, et al                                              [Page 16]

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   nicknames with a single nickname representing an area. Comparison

   The Border Learning variant described in Section above
   minimizes the change in non-border TRILL switches but imposes the
   burden on border TRILL switches of learning and doing lookups in all
   the end station MAC addresses within their area(s) that are used for
   communication outside the area. This 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 TRILL switches
   but requires changes to the TRILL data packet header and more
   extensive changes to non-border TRILL switches. In particular, with
   this alternative, non-border TRILL switches must learn to associate
   both a TRILL switch 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 TRILL switches. It would be
   possible for border and other level 2 TRILL switches to support both
   Border Learning, for support of legacy Level 1 TRILL switches, and
   Swap Nickname, to support Level 1 TRILL switches that understood the
   Swap Nickname method based on variations in the TRILL header but this
   would be even more complex.

   The requirement to change the TRILL header and fast path processing
   to support the Swap Nickname Field variant make it impractical for
   the foreseeable future.

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 TRILL switch, say RBx, in each area, attach to the
   Level 2 area.  RBx would forward all multi-destination packets
   between that 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 TRILL switch).

   This is the same issue as with multiple ingress TRILL switches
   injecting traffic from a pseudonode, and can be solved with the
   mechanism that was adopted for that purpose: the affinity TLV

R. Perlman, et al                                              [Page 17]

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   [RFC7783].  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 data originating locally in RBx's area,
   computation of the RPF check is done as today.  For multi-destination
   packets originating outside RBx's area, computation of the RPF check
   must be done based on which one of the border TRILL switches (say
   RB1, RB2, or RB3) injected the packet into the area.

   A TRILL switch, say RB4, located inside an area, must be able to know
   which of RB1, RB2, or RB3 transitioned the packet 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 TRILL switches in the area, or the Affinity
   TLV mechanism given in [RFC7783], or a 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 or can be identified by the set of border TRILL
   switches.  It is probably simpler to allocate a block of nicknames
   (say, the top 4000) to either (1) represent areas and not specific
   TRILL switches or (2) used by border TRILL switches if the set of
   such border TRILL switches represent the area.

   The nicknames used for area identification need to be advertised and
   acquired through Level 2.

   Within an area, all the border TRILL switches can discover each other
   through the Level 1 link state database, by using the IS-IS attach
   bit or by explicitly advertising in their LSP "I am a border

   Of the border TRILL switches, one will have highest priority (say
   RB7). RB7 can dynamically participate, in Level 2, to acquire a
   nickname for identifying the area.  Alternatively, RB7 could give the
   area a pseudonode IS-IS ID, such as RB7.5, within Level 2.  So an
   area would appear, in Level 2, as a pseudonode and the pseudonode
   could participate, in Level 2, to acquire a nickname for the area.

   Within Level 2, all the border TRILL switches for an area can
   advertise reachability to the area, which would mean connectivity to

R. Perlman, et al                                              [Page 18]

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   a nickname identifying the area.

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 TRILL switches announce themselves,
   together with their DBRB priority. (Note that the election of the
   DBRB cannot be done based on Hello messages, because the border TRILL
   switches 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 can acquire an area nickname (in the aggregated nickname
   approach) and may give the area a pseudonode IS-IS ID (just like the
   DRB would give a pseudonode IS-IS ID to a link) depending on how the
   area nickname is handled.  RB1 advertises, in area A1, an area
   nickname that RB1 has acquired (and what the pseudonode IS-IS ID for
   the area is if needed).

   Level 1 LSPs (possibly pseudonode) initiated by RB1 for the area
   include any information external to area A1 that should be input into
   area A1 (such as nicknames of external areas, or perhaps (in the
   unique nickname variant) all the nicknames of external TRILL switches
   in the TRILL campus and pruning information such as multicast
   listeners and labels).  All the other border TRILL switches for the
   area announce (in their LSP) attachment to that area.

   Within Level 2, RB1 generates a Level 2 LSP on behalf of the area.
   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 TRILL switches would
   announce, in their Level 2 LSPs, connection to the area.

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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.

   Area addresses might be configured to ensure two areas are not
   inadvertently connected.  Area addresses appear 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 data only reaching the portion of the TRILL campus
   reachable through the partition attached to the TRILL switch that
   transitions that packet.  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 packet with a scope that indicates the
   packet 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 packet 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 TRILL switch 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 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 packet is not transitioned between areas. Or
   one or more special trees could be built that were restricted to the
   local area.

4.1 Unicast to Multi-destination Conversions

   In the border learning variant of the aggregated nickname
   alternative, the following situation may occur:
   -  a unicast packet might be known at the Level 1 to Level 2
      transition and be forwarded as a unicast packet to the least cost
      border TRILL switch advertising connectivity to the destination
      area, but
   -  upon arriving at the border TRILL switch, it turns out to have an
      unknown destination { MAC, Data Label } pair.

   In this case, the packet must be converted into a multi-destination
   packet and flooded in the destination area.  However, if the border
   TRILL switch doing the conversion is not the border TRILL switch
   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 as described above.

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   Alternatively, a multi-destination packet intended only for the area
   could be tunneled (within the area) to the RBridge RBx, that is the
   appointed transitioner for that form of packet (say, based on VLAN or
   FGL), with instructions that RBx only transmit the packet within the
   area, and RBx could initiate the multi-destination packet within the
   area.  Since RBx 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 TRILL switch that receives the unicast packet and
   the appointed level transitioner for that packet, might not be an

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), leaving 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 TRILL
      switches injected this packet

   If TRILL were to adopt this new encoding, any of the TRILL switches
   in an edge group could inject a multi-destination packet. This would
   require all TRILL switches 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 TRILL switches in an edge group
   would be.

   While there are a number of advantages to this technique, it requires
   fast path logic changes and thus its deployment is not practical at
   this time. It is included here for completeness.

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, (Dynamic Host Configuration Protocol) the
   NetBIOS Service Location Protocol, and multicast DNS (Domain Name

R. Perlman, et al                                              [Page 22]

INTERNET-DRAFT                                          Multilevel TRILL

   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
   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,
   in a variety of ways including the following:

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

   -  Then place one, or a few for redundancy, protocol P proxies inside
      each area where protocol P may be in use. 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 TRILL
   switches 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

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5. Co-Existence with Old TRILL switches

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

   A possible solution, as long as any old TRILL switches exist within
   an area, is to have the border TRILL switches 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 TRILL
   switches on a link with one or more end stations, keeping in mind
   that end stations are TRILL ignorant. In particular, it is essential
   that only one TRILL switch ingress/egress any given data packet
   from/to an end station so that connectivity is provided to that end
   station without duplicating end station data and that loops are not
   formed due to one TRILL switch egressing data in native form (i.e.,
   with no TRILL header) and having that data re-ingressed by another
   TRILL switch on the link.

   With existing, single level TRILL, this is done by electing a single
   Designated RBridge per link, which appoints a single Appointed
   Forwarder per VLAN [RFC7177] [RFC8139]. This mechanism depends on the
   RBridges establishing adjacency.  But suppose there are two (or more)
   TRILL switches on a link in different areas, say RB1 in area A1 and
   RB2 in area A2, as shown below, and that the link also has one or
   more end stations attached.  If RB1 and RB2 ignore each other's
   Hellos because they are in different areas, as they are required to
   do under normal IS-IS PDU processing rules,  then they will not form
   an adjacency.  If they are not adjacent, they will ignore each other
   for the Appointed Forwarder mechanism and will both ingress/egress
   end station traffic on the link causing loops and duplication.

   The problem is not avoiding adjacency or avoiding TRILL Data packet
   transfer between RB1 and RB2. The area address mechanism of IS-IS or
   possibly the use of topology constraints or the like does that quite
   well. The problem stems from end stations being TRILL ignorant so
   care must be taken that multiple RBridges on a link do not ingress
   the same frame originated by an end station and so that an RBridge
   does not ingress a native frame egressed by a different RBridge
   because the RBridge mistakes the frame for a frame originated by an
   end station.

      |                   Level 2                  |
      |  Area A1 |                     |  Area A2  |
      |   +---+  |                     |   +---+   |
      |   |RB1|  |                     |   |RB2|   |
      |   +-+-+  |                     |   +-+-+   |
      |     |    |                     |     |     |
      +-----|----+                     +-----|-----+
            |                                |
          --+---------+-------------+--------+-- Link
                      |             |
               +------+------+   +--+----------+
               | End Station |   | End Station |
               +-------------+   +-------------+

R. Perlman, et al                                              [Page 25]

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   A simple rule, which is preferred, is to use the TRILL switch or
   switches having the lowest numbered area, comparing area numbers as
   unsigned integers, to handle all native traffic to/from end stations
   on the link. This would automatically give multilevel-ignorant legacy
   TRILL switches, that would be using area number zero, highest
   priority for handling end station traffic, which they would try to do

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

   These techniques require multilevel aware TRILL switches to take
   actions based on Hellos from RBridges in other areas even though they
   will not form an adjacency with such RBridges. However, the action is
   quite simple in the preferred case: if a TRILL switch sees Hellos
   from lower numbered areas, then they would not act as an Appointed
   Forwarder on the link until the Hello timer for such Hellos had

R. Perlman, et al                                              [Page 26]

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7. Summary

   This draft describes potential scaling issues in TRILL and discusses
   possible approaches to multilevel TRILL as a solution or element of a
   solution to most of them.

   The alternative using aggregated areas in multilevel TRILL has
   significant advantages in terms of scalability over using campus wide
   unique nicknames, not just in avoiding nickname exhaustion, but by
   allowing RPF Checks to be aggregated based on an entire area.
   However, the alternative of using unique nicknames is simpler and
   avoids the changes in border TRILL switches 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 multilevel TRILL issues are not difficult, such as dealing with
   partitioned areas.  Other issues are more difficult, especially
   dealing with old TRILL switches that are multilevel ignorant.

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8. Security Considerations

   This informational document explores alternatives for the design of
   multilevel IS-IS in TRILL and generally does not consider security

   If aggregated nicknames are used in two areas that have the same area
   address and those areas merge, there is a possibility of a transient
   nickname collision that would not occur with unique nicknames. Such a
   collision could cause a data packet to be delivered to the wrong
   egress TRILL switch but it would still not be delivered to any end
   station in the wrong Data Label; thus such delivery would still
   conform to security policies.

   For general TRILL Security Considerations, see [RFC6325].

9. IANA Considerations

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

R. Perlman, et al                                              [Page 28]

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Normative 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.

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

   [RFC7177] - Eastlake 3rd, D., Perlman, R., Ghanwani, A., Yang, H.,
         and V. Manral, "Transparent Interconnection of Lots of Links
         (TRILL): Adjacency", RFC 7177, May 2014, <http://www.rfc->.

   [RFC7780] - Eastlake 3rd, D., Zhang, M., Perlman, R., Banerjee, A.,
         Ghanwani, A., and S. Gupta, "Transparent Interconnection of
         Lots of Links (TRILL): Clarifications, Corrections, and
         Updates", RFC 7780, DOI 10.17487/RFC7780, February 2016,

   [RFC8139] - Eastlake, D., Li, Y., Umair, M., Banerjee, A., and F. Hu,
         "Transparent Interconnection of Lots of Links (TRILL):
         Appointed Forwarders", RFC 8139, DOI 10.17487/RFC8139, June
         2017, <>.

Informative References

   [InterCon] - Perlman, R., "Interconnections, Second Edition; Bridges,
         Routers, Switches, and Internetworking Protocols", Addison
         Wesley, ISBN 0-201-63448-1, September 1999.

   [RFC3194] - Durand, A. and C. Huitema, "The H-Density Ratio for
         Address Assignment Efficiency An Update on the H ratio", RFC
         3194, DOI 10.17487/RFC3194, November 2001, <http://www.rfc->.

   [RFC6361] - Carlson, J. and D. Eastlake 3rd, "PPP Transparent
         Interconnection of Lots of Links (TRILL) Protocol Control
         Protocol", RFC 6361, August 2011.

   [RFC7172] - Eastlake 3rd, D., Zhang, M., Agarwal, P., Perlman, R.,
         and D. Dutt, "Transparent Interconnection of Lots of Links
         (TRILL): Fine-Grained Labeling", RFC 7172, May 2014

   [RFC7176] - Eastlake 3rd, D., Senevirathne, T., Ghanwani, A., Dutt,

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         D., and A. Banerjee, "Transparent Interconnection of Lots of
         Links (TRILL) Use of IS-IS", RFC 7176, May 2014.

   [RFC7357] - Zhai, H., Hu, F., Perlman, R., Eastlake 3rd, D., and O.
         Stokes, "Transparent Interconnection of Lots of Links (TRILL):
         End Station Address Distribution Information (ESADI) Protocol",
         RFC 7357, September 2014, <http://www.rfc->.

   [RFC7781] - Zhai, H., Senevirathne, T., Perlman, R., Zhang, M., and
         Y. Li, "Transparent Interconnection of Lots of Links (TRILL):
         Pseudo-Nickname for Active-Active Access", RFC 7781, DOI
         10.17487/RFC7781, February 2016, <http://www.rfc->.

   [RFC7783] - Senevirathne, T., Pathangi, J., and J. Hudson,
         "Coordinated Multicast Trees (CMT) for Transparent
         Interconnection of Lots of Links (TRILL)", RFC 7783, DOI
         10.17487/RFC7783, February 2016, <http://www.rfc->.

   [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.

   [DraftUnique] - M. Zhang, D. Eastlake, R. Perlman, M. Cullen, H.
         Zhai, D. Liu, "TRILL Multilevel Using Unique Nicknames", draft-
         ietf-trill-multilevel-unique-nickname, Work In Progress.

   [SingleName] - Mingui Zhang, et. al, "Single Area Border RBridge
         Nickname for TRILL Multilevel", draft-ietf-trill-multilevel-
         single-nickname, Work in Progress.

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   The helpful comments and contributions of the following are hereby

      Alia Atlas, David Michael Bond, Dino Farinacci, Sue Hares, Gayle
      Noble, Alexander Vainshtein, and Stig Venaas.

   The document was prepared in raw nroff. All macros used were defined
   within the source file.

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Authors' Addresses

   Radia Perlman
   2010 256th Avenue NE, #200
   Bellevue, WA 98007 USA


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

   Phone: +1-508-333-2270

   Mingui Zhang
   Huawei Technologies
   No.156 Beiqing Rd. Haidian District,
   Beijing 100095 P.R. China


   Anoop Ghanwani
   5450 Great America Parkway
   Santa Clara, CA  95054 USA


   Hongjun Zhai
   Jinling Institute of Technology
   99 Hongjing Avenue, Jiangning District
   Nanjing, Jiangsu 211169  China


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R. Perlman, et al                                              [Page 33]