INTERNET-DRAFT                                                  M. Zhang
Intended Status: Proposed Standard                                Huawei
                                                             D. Eastlake
                                                              R. Perlman
                                                               M. Cullen
                                                       Painless Security
                                                                 H. Zhai
Expires: January 6, 2021                                    July 7, 2020

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

   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 nicknames.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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M. Zhang, et al                                                 [Page 1]

INTERNET-DRAFT                                Multilevel Single Nickname

Table of Contents

      1. Introduction............................................3
      2. Acronyms and Terminology................................4

      3. Nickname Handling on Border RBridges....................5
      3.1. Actions on Unicast Packets............................5
      3.2. Actions on Multi-Destination Packets..................6

      4. Per-flow Load Balancing.................................9
      4.1. Ingress Nickname Replacement..........................9
      4.2. Egress Nickname Replacement...........................9

      5. Protocol Extensions for Discovery......................10
      5.1. Discovery of Border RBridges in L1...................10
      5.2. Discovery of Border RBridge Sets in L2...............10

      6. One Border RBridge Connects Multiple Areas.............12
      7. E-L1FS/E-L2FS Backwards Compatibility..................13

      8. Manageability Considerations...........................13
      9. Security Considerations................................15
      10. IANA Considerations...................................15

      11. References............................................16
      11.1. Normative References................................16
      11.2. Informative References..............................17

      Appendix A. Level Transition Clarification................18

      Authors' Addresses........................................19

M. Zhang, et al                                                 [Page 2]

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1. Introduction

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

   Informational [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 change.

M. Zhang, et al                                                 [Page 3]

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2. Acronyms and Terminology

   Data Label: VLAN or FGL 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

   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.

M. Zhang, et al                                                 [Page 4]

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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            |     |                 |     |     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
   (and in fact, they could even have the same nickname, since the TRILL
   switch nickname will not be visible outside these Level 1 areas).

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:

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

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

   -  RB2 and RB20 have announced in the Level 1 IS-IS instance in area
      {2,20}, that they are attached to all those area nicknames,
      including {3,30}. Therefore, IS-IS routes the packet to RB2 (or
      RB20, if RB20 on the least-cost route from RB27 to RB3).

M. Zhang, et al                                                 [Page 5]

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   -  RB2, when transitioning the packet from Level 1 to Level 2,
      replaces the ingress TRILL switch nickname with its own nickname,
      so replaces 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 an area
      nickname selected from {3,30}. See Section 4 for the detail of the
      selection method. Here, suppose nickname 3 is used.) Also RB2
      learns that S is attached to nickname 27 in area {2,20} to
      accommodate return traffic. RB2 SHOULD synchronize with RB20 using
      ESADI protocol [RFC7357] that MAC = S is attached to nickname 27.

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

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

   -  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. All border RBridges of an area MUST agree on a pseudorandom
   algorithm as the tie-breaker to locally determine the DBRB. The same
   pseudorandom algorithm will be reused in Section 4 for the purpose of
   load balancing. It's also possible to implement a certain election
   protocol to elect the DBRB. However, such kind of implementations are
   out the scope of this document. By default, the border RBridge with
   the smallest nickname, considered as an unsigned integer, is elected

   As per [RFC6325], multi-destination packets can be classified into
   three types: unicast packet with unknown destination MAC address
   (unknown-unicast packet), multicast packet and broadcast packet. 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

M. Zhang, et al                                                 [Page 6]

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   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, flooding the packet on the distribution
   tree, subject to possible 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 a unknown-unicast packet, if the DBRB has learnt 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 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. The following sequence of
   events will occur:

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

   -  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 forwarded the packet into this

   -  The packet is flooded on the Level 2 tree to reach both RB3 and
      RB30. Suppose RB3 is the selected DBRB. The non-DBRB RB30 will
      drop the packet.

   -  RB3, when forwarding into area {3,30}, replaces the egress
      nickname in the TRILL header with a root RBridge nickname, say 3,
      of the distribution tree of L1 area {3,30}. (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. In that case, RB3 must turn the packet into a multi-
      destination packet and floods it on the distribution tree of L1
      area {3,30}.

   -  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.

M. Zhang, et al                                                 [Page 7]

INTERNET-DRAFT                                Multilevel Single Nickname

      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.

   -  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.

M. Zhang, et al                                                 [Page 8]

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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 as follows.

4.1. Ingress Nickname Replacement

   When a TRILL data packet from other 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. The selection is simply based on a pseudorandom algorithm
   as defined 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 in 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 will see one source MAC
   address flip flopping among multiple ingress RBridges.

4.2. Egress Nickname Replacement

   When a TRILL data packet originated from an L1 area arrives at an
   area border RBridge of that 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.

M. Zhang, et al                                                 [Page 9]

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5. Protocol Extensions for Discovery

   The following topology change scenarios will trigger the discover
   processes as defined in Sections 5.1 and Section 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 have 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
   an 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)

   o  Type: Level 1 Border RBridge (TRILL APPsub-TLV type tbd1)

   o  Length: 2

   o  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.

M. Zhang, et al                                                [Page 10]

INTERNET-DRAFT                                Multilevel Single Nickname

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

   o  Type: Level 1 Border RBridge Group (TRILL APPsub-TLV type tbd2)

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

   o  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.

M. Zhang, et al                                                [Page 11]

INTERNET-DRAFT                                Multilevel Single Nickname

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 all these areas.

   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: suppose RB1 is not the
   DBRB, RB1 will not transition the packet; otherwise, RB1 is the DBRB,

   -  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 to which the receiving border RBridge is attached.

M. Zhang, et al                                                [Page 12]

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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) 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 RBridges.

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. 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.

   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 used single nickname multilevel as specified in
   this document MUST support [RFC8397] and 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.

   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 configured.

   Other than 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 TRILL L2 area. A TRILL OAM message will be
   forwarded through the multilevel single nickname TRILL campus using a

M. Zhang, et al                                                [Page 13]

INTERNET-DRAFT                                Multilevel Single Nickname

   MAC address belonging to the destination RBridge [RFC7455].

M. Zhang, et al                                                [Page 14]

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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]. This document raises no new
   security issues for IS-IS.

   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. 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 is requested to allocate two new types under the TRILL GENINFO
   TLV [RFC7357] from the range allocated by standards action for the
   TRILL APPsub-TLVs defined in Section 5. The following entries are
   added to the "TRILL APPsub-TLV Types under IS-IS TLV 251 Application
   Identifier 1" Registry on the TRILL Parameters IANA web page.

      Type       Name                     Reference
      ---------  ----                     ---------
      tbd1[256]  L1-BORDER-RBRIDGE        [This document]
      tbd2[257]  L1-BORDER-RB-GROUP       [This document]

M. Zhang, et al                                                [Page 15]

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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, <http://www.rfc-

   [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, <http://www.rfc-

   [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, <>.

   [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,

   [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119
             Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May
             2017, <>.

   [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, <https://www.rfc-

M. Zhang, et al                                                [Page 16]

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11.2. Informative References

   [IS-IS]   International Organization for Standardization, ISO/IEC
             10589:2002, "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, 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,

   [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, <https://www.rfc-

M. Zhang, et al                                                [Page 17]

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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.

M. Zhang, et al                                                [Page 18]

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Authors' Addresses

      Mingui Zhang
      Huawei Technologies
      No. 156 Beiqing Rd. Haidian District
      Beijing  100095


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

      Phone: +1-508-333-2270

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


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

      Phone: +1-781-405-7464

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


M. Zhang, et al                                                [Page 19]

INTERNET-DRAFT                                Multilevel Single Nickname

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   described in the Simplified BSD License.

M. Zhang, et al                                                [Page 20]