RFC 9183

Internet Engineering Task Force (IETF)                          M. Zhang
Request for Comments: 9183                                   Independent
Category: Standards Track                                D. Eastlake 3rd
ISSN: 2070-1721                                                Futurewei
                                                              R. Perlman
                                                               M. Cullen
                                                       Painless Security
                                                                 H. Zhai
                                                           February 2022

  Single Nickname for an Area Border RBridge in Multilevel Transparent
                Interconnection of Lots of Links (TRILL)


   A major issue in multilevel TRILL is how to manage RBridge nicknames.
   In this document, area border RBridges use a single nickname in both
   Level 1 and Level 2.  RBridges in Level 2 must obtain unique
   nicknames but RBridges in different Level 1 areas may have the same

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Acronyms and Terminology
   3.  Nickname Handling on Border RBridges
     3.1.  Actions on Unicast Packets
     3.2.  Actions on Multi-destination Packets
   4.  Per-Flow Load Balancing
     4.1.  L2-to-L1 Ingress Nickname Replacement
     4.2.  L1-to-L2 Egress Nickname Replacement
   5.  Protocol Extensions for Discovery
     5.1.  Discovery of Border RBridges in L1
     5.2.  Discovery of Border RBridge Sets in L2
   6.  One Border RBridge Connects Multiple Areas
   7.  E-L1FS/E-L2FS Backwards Compatibility
   8.  Manageability Considerations
   9.  Security Considerations
   10. IANA Considerations
   11. References
     11.1.  Normative References
     11.2.  Informative References
   Appendix A.  Level Transition Clarification

   Authors' Addresses

1.  Introduction

   TRILL (Transparent Interconnection of Lots of Links) [RFC6325]
   [RFC7780] multilevel techniques are designed to improve TRILL
   scalability issues.

   "Alternatives for Multilevel Transparent Interconnection of Lots of
   Links (TRILL)" [RFC8243] is an educational document to explain
   multilevel TRILL and list possible concerns.  It does not specify a
   protocol.  As described in [RFC8243], there have been two proposed
   approaches.  One approach, which is referred to as the "unique
   nickname" approach, gives unique nicknames to all the TRILL switches
   in the multilevel campus either by having the Level 1/Level 2 border
   TRILL switches 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.  [RFC8397] is
   the Standards Track document specifying a "unique nickname" flavor of
   TRILL multilevel.  The other approach, which is referred to in
   [RFC8243] as the "aggregated nickname" approach, involves assigning
   nicknames to the areas, and allowing nicknames to be reused inside
   different areas, by having the border TRILL switches rewrite the
   nickname fields when entering or leaving an area.  [RFC8243] makes
   the case that, while unique nickname multilevel solutions are
   simpler, aggregated nickname solutions scale better.

   The approach specified in this Standards Track document is somewhat
   similar to the "aggregated nickname" approach in [RFC8243] but with a
   very important difference.  In this document, the nickname of an area
   border RBridge is used in both Level 1 (L1) and Level 2 (L2).  No
   additional nicknames are assigned to represent L1 areas as such.
   Instead, multiple border RBridges are allowed and each L1 area is
   denoted by the set of all nicknames of those border RBridges of the
   area.  For this approach, nicknames in the L2 area MUST be unique but
   nicknames inside an L1 area can be reused in other L1 areas that also
   use this approach.  The use of the approach specified in this
   document in one L1 area does not prohibit the use of other approaches
   in other L1 areas in the same TRILL campus, for example the use of
   the unique nickname approach specified in [RFC8397].  The TRILL
   packet format is unchanged by this document, but data plane
   processing is changed at Border RBridges and efficient high volume
   data flow at Border RBridges might require forwarding hardware

2.  Acronyms and Terminology

   Area Border RBridge:  A border RBridge between a Level 1 area and
      Level 2.

   Data Label:  VLAN or Fine-Grained Label (FGL).

   DBRB:  Designated Border RBridge.

   IS-IS:  Intermediate System to Intermediate System [IS-IS].

   Level:  Similar to IS-IS, TRILL has Level 1 for intra-area and
      Level 2 for inter-area.  Routing information is exchanged between
      Level 1 RBridges within the same Level 1 area, and Level 2
      RBridges can only form relationships and exchange information with
      other Level 2 RBridges.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Familiarity with [RFC6325] is assumed in this document.

3.  Nickname Handling on Border RBridges

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

           Area {2,20}             Level 2             Area {3,30}
   +-------------------+     +-----------------+     +--------------+
   |                   |     |                 |     |              |
   | S--RB27---Rx--Rz----RB2---Rb---Rc--Rd---Re--RB3---Rk--RB44---D |
   |     27            |     |      39         |     |     44       |
   |                 ----RB20---             ----RB30---            |
   +-------------------+     +-----------------+     +--------------+

             Figure 1: An Example Topology for TRILL Multilevel

   In Figure 1, RB2, RB20, RB3, and RB30 are area border TRILL switches
   (RBridges).  Their nicknames are 2, 20, 3, and 30, respectively, and
   are used as TRILL switch identifiers in their areas [RFC6325].  Area
   border RBridges use the set of border nicknames to denote the L1 area
   that they are attached to.  For example, RB2 and RB20 use nicknames
   {2,20} to denote the L1 area on the left.

   A source S is attached to RB27 and a destination D is attached to
   RB44.  RB27 has a nickname (say, 27), and RB44 has a nickname (say,
   44).  (In fact, they could even have the same nickname, since the
   TRILL switch nickname will not be visible outside these Level 1

3.1.  Actions on Unicast Packets

   Let's say that S transmits a frame to destination D and let's say
   that D's location has been learned by the relevant TRILL switches
   already.  These relevant switches have learned the following:

   1)  RB27 has learned that D is connected to nickname 3.

   2)  RB3 has learned that D is attached to nickname 44.

   The following sequence of events will occur:

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

   2.  RB27 encapsulates with a TRILL header with ingress RBridge = 27
       and egress RBridge = 3 producing a TRILL Data packet.

   3.  RB2 and RB20 have announced in the Level 1 IS-IS area designated
       {2,20} that they are attached to the nicknames of all the border
       RBridges in the Level 2 area including RB3 and RB30.  Therefore,
       IS-IS routes the packet to RB2 (or RB20, if RB20 is on the least-
       cost route from RB27 to RB3).

   4.  RB2, when transitioning the packet from Level 1 to Level 2,
       replaces the ingress TRILL switch nickname with its own nickname,
       replacing 27 with 2.  Within Level 2, the ingress RBridge field
       in the TRILL header will therefore be 2, and the egress RBridge
       field will be 3.  (The egress nickname MAY be replaced with any
       area nickname selected from {3,30} such as 30.  See Section 4 for
       the detail of the selection method.  Here, suppose the egress
       nickname remains 3.)  Also, RB2 learns that S is attached to
       nickname 27 in area {2,20} to accommodate return traffic.  RB2
       SHOULD synchronize with RB20 using the End Station Address
       Distribution Information (ESADI) protocol [RFC7357] that MAC = S
       is attached to nickname 27.

   5.  The packet is forwarded through Level 2, to RB3, which has
       advertised, in Level 2, its L2 nickname as 3.

   6.  RB3, when forwarding into area {3,30}, replaces the egress
       nickname in the TRILL header with RB44's nickname (44) based on
       looking up D.  (The ingress nickname MAY be replaced with any
       area nickname selected from {2,20}. See Section 4 for the detail
       of the selection method.  Here, suppose the ingress nickname
       remains 2.)  So, within the destination area, the ingress
       nickname will be 2 and the egress nickname will be 44.

   7.  RB44, when decapsulating, learns that S is attached to nickname
       2, which is one of the area nicknames of the ingress.

3.2.  Actions on Multi-destination Packets

   Distribution trees for flooding of multi-destination packets are
   calculated separately within each L1 area and in L2.  When a multi-
   destination packet arrives at the border, it needs to be transitioned
   either from L1 to L2, or from L2 to L1.  All border RBridges are
   eligible for Level transition.  However, for each multi-destination
   packet, only one of them acts as the Designated Border RBridge (DBRB)
   to do the transition while other non-DBRBs MUST drop the received
   copies.  By default, the border RBridge with the smallest nickname,
   considered as an unsigned integer, is elected DBRB.  All border
   RBridges of an area MUST agree on the mechanism used to determine the
   DBRB locally.  The use of an alternative is possible, but out of the
   scope of this document; one such mechanism is used in Section 4 for
   load balancing.

   As per [RFC6325], multi-destination packets can be classified into
   three types: unicast packets with unknown destination MAC addresses
   (unknown-unicast packets), multicast packets, and broadcast packets.
   Now suppose that D's location has not been learned by RB27 or the
   frame received by RB27 is recognized as broadcast or multicast.  What
   will happen within a Level 1 area (as it would in TRILL today) is
   that RB27 will forward the packet as multi-destination, setting its M
   bit to 1 and choosing an L1 tree, which would flood the packet on
   that distribution tree (subject to potential pruning).

   When the copies of the multi-destination packet arrive at area border
   RBridges, non-DBRBs MUST drop the packet while the DBRB (say, RB2)
   needs to do the Level transition for the multi-destination packet.
   For an unknown-unicast packet, if the DBRB has learned the
   destination MAC address, it SHOULD convert the packet to unicast and
   set its M bit to 0.  Otherwise, the multi-destination packet will
   continue to be flooded as a multicast packet on the distribution
   tree.  The DBRB chooses the new distribution tree by replacing the
   egress nickname with the new tree root RBridge nickname from the area
   the packet is entering.  The following sequence of events will occur:

   1.  RB2, when transitioning the packet from Level 1 to Level 2,
       replaces the ingress TRILL switch nickname with its own nickname,
       replacing 27 with 2.  RB2 also MUST replace the egress RBridge
       nickname with an L2 tree root RBridge nickname (say, 39).  In
       order to accommodate return traffic, RB2 records that S is
       attached to nickname 27 and SHOULD use the ESADI protocol
       [RFC7357] to synchronize this attachment information with other
       border RBridges (say, RB20) in the area.

   2.  RB20 will receive the packet flooded on the L2 tree by RB2.  It
       is important that RB20 does not transition this packet back to L1
       as it does for a multicast packet normally received from another
       remote L1 area.  RB20 should examine the ingress nickname of this
       packet.  If this nickname is found to be a border RBridge
       nickname of the area {2,20}, RB2 must not forward the packet into
       this area.

   3.  The multi-destination packet is flooded on the Level 2 tree to
       reach all border routers for all L1 areas including both RB3 and
       RB30.  Suppose RB3 is the selected DBRB.  The non-DBRB RB30 will
       drop the packet.

   4.  RB3, when forwarding into area {3,30}, replaces the egress
       nickname in the TRILL header with the root RBridge nickname of a
       distribution tree of L1 area {3,30} -- say, 30.  (Here, the
       ingress nickname MAY be replaced with a different area nickname
       selected from {2,20}, the set of border RBridges to the ingress
       area, as specified in Section 4.)  Now suppose that RB27 has
       learned the location of D (attached to nickname 3), but RB3 does
       not know where D is because this information has fallen out of
       cache or RB3 has restarted or some other reason.  In that case,
       RB3 must turn the packet into a multi-destination packet and then
       floods it on a distribution tree in the L1 area {3,30}.

   5.  RB30 will receive the packet flooded on the L1 tree by RB3.  It
       is important that RB30 does not transition this packet back to
       L2.  RB30 should also examine the ingress nickname of this
       packet.  If this nickname is found to be an L2 Border RBridge
       Nickname, RB30 must not transition the packet back to L2.

   6.  The multicast listener RB44, when decapsulating the received
       packet, learns that S is attached to nickname 2, which is one of
       the area nicknames of the ingress.

   See also Appendix A.

4.  Per-Flow Load Balancing

   Area border RBridges perform ingress/egress nickname replacement when
   they transition TRILL Data packets between Level 1 and Level 2.  The
   egress nickname will again be replaced when the packet transitions
   from Level 2 to Level 1.  This nickname replacement enables the per-
   flow load balance, which is specified in the following subsections.
   The mechanism specified in Section 4.1 or that in Section 4.2 or both
   is necessary in general to load-balance traffic across L2 paths.

4.1.  L2-to-L1 Ingress Nickname Replacement

   When a TRILL Data packet from other L1 areas arrives at an area
   border RBridge, this RBridge MAY select one area nickname of the
   ingress area to replace the ingress nickname of the packet so that
   the returning TRILL Data packet can be forwarded to this selected
   nickname to help load-balance return unicast traffic over multiple
   paths.  The selection is simply based on a pseudorandom algorithm as
   discussed in Section 5.3 of [RFC7357].  With the random ingress
   nickname replacement, the border RBridge actually achieves a per-flow
   load balance for returning traffic.

   All area border RBridges for an L1 area MUST agree on the same
   pseudorandom algorithm.  The source MAC address, ingress area
   nicknames, egress area nicknames, and the Data Label of the received
   TRILL Data packet are candidate factors of the input of this
   pseudorandom algorithm.  Note that the value of the destination MAC
   address SHOULD be excluded from the input of this pseudorandom
   algorithm; otherwise, the egress RBridge could see one source MAC
   address flip-flopping among multiple ingress RBridges.

4.2.  L1-to-L2 Egress Nickname Replacement

   When a unicast TRILL Data packet originated from an L1 area arrives
   at an area border RBridge of that L1 area, that RBridge MAY select
   one area nickname of the egress area to replace the egress nickname
   of the packet.  By default, it SHOULD choose the egress area border
   RBridge with the least cost route to reach or, if there are multiple
   equal cost egress area border RBridges, use the pseudorandom
   algorithm as defined in Section 5.3 of [RFC7357] to select one.  The
   use of that algorithm MAY be extended to selection among some stable
   set of egress area border RBridges that include non-least-cost
   alternatives if it is desired to obtain more load spreading at the
   cost of sometimes using a non-least-cost Level 2 route to forward the
   TRILL Data packet to the egress area.

5.  Protocol Extensions for Discovery

   The following topology change scenarios will trigger the discovery
   processes as defined in Sections 5.1 and 5.2:

   *  A new node comes up or recovers from a previous failure.

   *  A node goes down.

   *  A link or node fails and causes partition of an L1/L2 area.

   *  A link or node whose failure has caused partitioning of an L1/L2
      area is repaired.

5.1.  Discovery of Border RBridges in L1

   The following Level 1 Border RBridge APPsub-TLV will be included in
   E-L1FS FS-LSP fragment zero [RFC7780] as an APPsub-TLV of the TRILL
   GENINFO-TLV.  Through listening for this APPsub-TLV, an area border
   RBridge discovers all other area border RBridges in this area.

   | Type = L1-BORDER-RBRIDGE      | (2 bytes)
   | Length                        | (2 bytes)
   | Sender Nickname               | (2 bytes)

   Type:  Level 1 Border RBridge (TRILL APPsub-TLV type 256)

   Length:  2

   Sender Nickname:  The nickname the originating IS will use as the L1
      Border RBridge Nickname.  This field is useful because the
      originating IS might own multiple nicknames.

5.2.  Discovery of Border RBridge Sets in L2

   The following APPsub-TLV will be included in an E-L2FS FS-LSP
   fragment zero [RFC7780] as an APPsub-TLV of the TRILL GENINFO-TLV.
   Through listening to this APPsub-TLV in L2, an area border RBridge
   discovers all groups of L1 border RBridges and each such group
   identifies an area.

   | Type = L1-BORDER-RB-GROUP     | (2 bytes)
   | Length                        | (2 bytes)
   | L1 Border RBridge Nickname 1  | (2 bytes)
   | ...                           |
   | L1 Border RBridge Nickname k  | (2 bytes)

   Type:  Level 1 Border RBridge Group (TRILL APPsub-TLV type 257)

   Length:  2 * k.  If length is not a multiple of 2, the APPsub-TLV is
      corrupt and MUST be ignored.

   L1 Border RBridge Nickname:  The nickname that an area border RBridge
      uses as the L1 Border RBridge Nickname.  The L1-BORDER-RB-GROUP
      TLV generated by an area border RBridge MUST include all L1 Border
      RBridge Nicknames of the area.  It's RECOMMENDED that these k
      nicknames are ordered in ascending order according to the 2-octet
      nickname considered as an unsigned integer.

   When an L1 area is partitioned [RFC8243], border RBridges will re-
   discover each other in both L1 and L2 through exchanging LSPs.  In
   L2, the set of border RBridge nicknames for this splitting area will
   change.  Border RBridges that detect such a change MUST flush the
   reachability information associated to any RBridge nickname from this
   changing set.

6.  One Border RBridge Connects Multiple Areas

   It's possible that one border RBridge (say, RB1) connects multiple L1
   areas.  RB1 SHOULD use a single area nickname for itself for all
   these areas to minimize nickname consumption and the number of
   nicknames being advertised in L2; however, such a border RBridge
   might have to hold multiple nicknames -- for example, it might be the
   root of multiple L1 or multiple L2 distribution trees.

   Nicknames used within one of these L1 areas can be reused within
   other areas.  It's important that packets destined to those
   duplicated nicknames are sent to the right area.  Since these areas
   are connected to form a layer 2 network, duplicated {MAC, Data Label}
   across these areas SHOULD NOT occur (see Section 4.2.6 of [RFC6325]
   for tie breaking rules).  Now suppose a TRILL Data packet arrives at
   the area border nickname of RB1.  For a unicast packet, RB1 can look
   up the {MAC, Data Label} entry in its MAC table to identify the right
   destination area (i.e., the outgoing interface) and the egress
   RBridge's nickname.  For a multicast packet for each attached L1
   area: either RB1 is not the DBRB and RB1 will not transition the
   packet, or RB1 is the DBRB.  If RB1 is the DBRB, RB1 follows the
   following rules:

   *  If this packet originated from an area out of the connected areas,
      RB1 replicates this packet and floods it on the proper Level 1
      trees of all the areas in which it acts as the DBRB.

   *  If the packet originated from one of the connected areas, RB1
      replicates the packet it receives from the Level 1 tree and floods
      it on other proper Level 1 trees of all the areas in which it acts
      as the DBRB except the originating area (i.e., the area connected
      to the incoming interface).  RB1 might also receive the
      replication of the packet from the Level 2 tree.  This replication
      MUST be dropped by RB1.  It recognizes such packets by their
      ingress nickname being the nickname of one of the border RBridges
      of an L1 area for which the receiving border RBridge is DBRB.

7.  E-L1FS/E-L2FS Backwards Compatibility

   All Level 2 RBridges MUST support E-L2FS [RFC7356] [RFC7780].  The
   Extended TLVs defined in Section 5 are to be used in Extended Level
   1/2 Flooding Scope (E-L1FS/E-L2FS) Protocol Data Units (PDUs).  Area
   border RBridges MUST support both E-L1FS and E-L2FS.  RBridges that
   do not support both E-L1FS or E-L2FS cannot serve as area border
   RBridges but they can appear in an L1 area acting as non-area-border

8.  Manageability Considerations

   If an L1 Border RBridge Nickname is configured at an RBridge and that
   RBridge has both L1 and L2 adjacencies, the multilevel feature as
   specified in this document is turned on for that RBridge and normally
   uses an L2 nickname in both L1 and L2 although, as provided below,
   such an RBridge may have to fall back to multilevel unique nickname
   behavior [RFC8397], in which case it uses this L1 nickname.  In
   contrast, unique nickname multilevel as specified in [RFC8397] is
   enabled by the presence of L1 and L2 adjacencies without an L1 Border
   RBridge Nickname being configured.  RBridges supporting only unique
   nickname multilevel do not support the configuration of an L2 Border
   RBridge Nickname.  RBridges supporting only the single-level TRILL
   base protocol specified in [RFC6325] do not support L2 adjacencies.

   RBridges that support and are configured to use single nickname
   multilevel as specified in this document MUST support unique nickname
   multilevel [RFC8397].  If there are multiple border RBridges between
   an L1 area and L2, and one or more of them only support or are only
   configured for unique nickname multilevel [RFC8397], any of these
   border RBridges that are configured to use single nickname multilevel
   MUST fall back to behaving as a unique nickname border RBridge for
   that L1 area.  Because overlapping sets of RBridges may be the border
   RBridges for different L1 areas, an RBridge supporting single
   nickname MUST be able to simultaneously support single nickname for
   some of its L1 areas and unique nickname for others.  For example,
   RB1 and RB2 might be border RBridges for L1 area A1 using single
   nickname while RB2 and RB3 are border RBridges for area A2.  If RB3
   only supports unique nicknames, then RB2 must fall back to unique
   nickname for area A2 but continue to support single nickname for area
   A1.  Operators SHOULD be notified when this fallback occurs.  The
   presence of border RBridges using unique nickname multilevel can be
   detected because they advertise in L1 the blocks of nicknames
   available within that L1 area.

   In both the unique nickname approach specified in [RFC8397] and the
   single nickname aggregated approach specified in this document, an
   RBridge that has L1 and L2 adjacencies uses the same nickname in L1
   and L2.  If an RBridge is configured with an L1 Border RBridge
   Nickname for any a Level 1 area, it uses this nickname across the
   Level 2 area.  This L1 Border RBridge Nickname cannot be used in any
   other Level 1 area except other Level 1 areas for which the same
   RBridge is a border RBridge with this L1 Border RBridge Nickname

   In addition to the manageability considerations specified above, the
   manageability specifications in [RFC6325] still apply.

   Border RBridges replace ingress and/or egress nickname when a TRILL
   Data packet traverses a TRILL L2 area.  A TRILL Operations,
   Administration, and Maintenance (OAM) message will be forwarded
   through the multilevel single nickname TRILL campus using a MAC
   address belonging to the destination RBridge [RFC7455].

9.  Security Considerations

   For general TRILL Security Considerations, see [RFC6325].

   The newly defined TRILL APPsub-TLVs in Section 5 are transported in
   IS-IS PDUs whose authenticity can be enforced using regular IS-IS
   security mechanism [IS-IS] [RFC5310].  Malicious devices may also
   fake the APPsub-TLVs to attract TRILL Data packets, interfere with
   multilevel TRILL operation, induce excessive state in TRILL switches
   (or in any bridges that may be part of the TRILL campus), etc.  For
   this reason, RBridges SHOULD be configured to use the IS-IS
   Authentication TLV (10) in their IS-IS PDUs so that IS-IS security
   [RFC5310] can be used to authenticate those PDUs and discard them if
   they are forged.

   Using a variation of aggregated nicknames, and the resulting possible
   duplication of nicknames between areas, increases the possibility of
   a TRILL Data packet being delivered to the wrong egress RBridge if
   areas are unexpectedly merged as compared with a scheme where all
   nicknames in the TRILL campus are, except as a transient condition,
   unique such as the scheme in [RFC8397].  However, in many cases, the
   data would be discarded at that egress RBridge because it would not
   match a known end station Data Label / MAC address.

10.  IANA Considerations

   IANA has allocated two new types under the TRILL GENINFO TLV
   [RFC7357] from the range allocated by Standards Action [RFC8126] for
   the TRILL APPsub-TLVs defined in Section 5.  The following entries
   have been added to the "TRILL APPsub-TLV Types under IS-IS TLV 251
   Application Identifier 1" registry on the TRILL Parameters IANA web

                 | Type | Name               | Reference |
                 | 256  | L1-BORDER-RBRIDGE  | RFC 9183  |
                 | 257  | L1-BORDER-RB-GROUP | RFC 9183  |

                                  Table 1

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC6325]  Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A.
              Ghanwani, "Routing Bridges (RBridges): Base Protocol
              Specification", RFC 6325, DOI 10.17487/RFC6325, July 2011,

   [RFC7356]  Ginsberg, L., Previdi, S., and Y. Yang, "IS-IS Flooding
              Scope Link State PDUs (LSPs)", RFC 7356,
              DOI 10.17487/RFC7356, September 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, DOI 10.17487/RFC7357,
              September 2014, <https://www.rfc-editor.org/info/rfc7357>.

   [RFC7455]  Senevirathne, T., Finn, N., Salam, S., Kumar, D., Eastlake
              3rd, D., Aldrin, S., and Y. Li, "Transparent
              Interconnection of Lots of Links (TRILL): Fault
              Management", RFC 7455, DOI 10.17487/RFC7455, March 2015,

   [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,

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8397]  Zhang, M., Eastlake 3rd, D., Perlman, R., Zhai, H., and D.
              Liu, "Transparent Interconnection of Lots of Links (TRILL)
              Multilevel Using Unique Nicknames", RFC 8397,
              DOI 10.17487/RFC8397, May 2018,

11.2.  Informative References

   [IS-IS]    International Organization for Standardization,
              "Information technology -- Telecommunications and
              information exchange between systems -- Intermediate
              System to Intermediate System intra-domain routeing
              information exchange protocol for use in conjunction with
              the protocol for providing the connectionless-mode network
              service (ISO 8473)", ISO 8473, ISO/IEC 10589:2002, Second
              Edition, November 2002.

   [RFC5310]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
              and M. Fanto, "IS-IS Generic Cryptographic
              Authentication", RFC 5310, DOI 10.17487/RFC5310, February
              2009, <https://www.rfc-editor.org/info/rfc5310>.

   [RFC8243]  Perlman, R., Eastlake 3rd, D., Zhang, M., Ghanwani, A.,
              and H. Zhai, "Alternatives for Multilevel Transparent
              Interconnection of Lots of Links (TRILL)", RFC 8243,
              DOI 10.17487/RFC8243, September 2017,

Appendix A.  Level Transition Clarification

   It's possible that an L1 RBridge is only reachable from a non-DBRB
   border RBridge.  If this non-DBRB RBridge refrains from Level
   transition, the question is, how can a multicast packet reach this L1
   RBridge?  The answer is, it will be reached after the DBRB performs
   the Level transition and floods the packet using an L1 distribution

   Take the following figure as an example.  RB77 is reachable from the
   border RBridge RB30 while RB3 is the DBRB.  RB3 transitions the
   multicast packet into L1 and floods the packet on the distribution
   tree rooted from RB3.  This packet is finally flooded to RB77 via

                 +--------------+          (root) RB3 o
                 |              |                      \
            -RB3 |              |                       o RB30
              |  |              |                      /
            -RB30-RB77          |                RB77 o

                 Example Topology               L1 Tree

   In the above example, the multicast packet is forwarded along a non-
   optimal path.  A possible improvement is to have RB3 configured not
   to belong to this area.  In this way, RB30 will surely act as the
   DBRB to do the Level transition.

Authors' Addresses

   Mingui Zhang

   Email: zhangmingui@qq.com

   Donald E. Eastlake, 3rd
   Futurewei Technologies
   2386 Panoramic Circle
   Apopka, FL 32703
   United States of America

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

   Radia Perlman
   2010 256th Avenue NE, #200
   Bellevue, WA 98007
   United States of America

   Email: radia@alum.mit.edu

   Margaret Cullen
   Painless Security
   356 Abbott Street
   North Andover, MA 01845
   United States of America

   Phone: +1-781-405-7464
   Email: margaret@painless-security.com
   URI:   https://www.painless-security.com

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

   Email: honjun.zhai@tom.com