INTERNET-DRAFT M. Zhang Intended Status: Proposed Standard Huawei D. Eastlake Futurewei R. Perlman EMC M. Cullen Painless Security H. Zhai JIT Expires: December 25, 2020 June 26, 2020 Transparent Interconnection of Lots of Links (TRILL) Single Area Border RBridge Nickname for Multilevel draft-ietf-trill-multilevel-single-nickname-11.txt Abstract 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. Distribution of this document is unlimited. Comments should be sent to the authors or the IDR 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- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. 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................................14 10. IANA Considerations...................................14 11. References............................................15 11.1. Normative References................................15 11.2. Informative References..............................15 Appendix A. Level Transition Clarification................17 Authors' Addresses........................................18 M. Zhang, et al [Page 2] INTERNET-DRAFT Multilevel Single Nickname 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] INTERNET-DRAFT Multilevel Single Nickname 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 RBridges. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "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] INTERNET-DRAFT Multilevel Single Nickname 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] INTERNET-DRAFT Multilevel Single Nickname - 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 DBRB. 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] INTERNET-DRAFT Multilevel Single Nickname 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 area. - 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] INTERNET-DRAFT Multilevel Single Nickname 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] INTERNET-DRAFT Multilevel Single Nickname 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 ought not occur. 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] INTERNET-DRAFT Multilevel Single Nickname 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 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. M. Zhang, et al [Page 13] INTERNET-DRAFT Multilevel Single Nickname 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 14] INTERNET-DRAFT Multilevel Single Nickname 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, . [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, . 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., M. Zhang, et al [Page 15] INTERNET-DRAFT Multilevel Single Nickname 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, . [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, . M. Zhang, et al [Page 16] INTERNET-DRAFT Multilevel Single Nickname 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 tree. 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 RB30. Area{3,30} +--------------+ (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 17] INTERNET-DRAFT Multilevel Single Nickname Authors' Addresses Mingui Zhang Huawei Technologies No. 156 Beiqing Rd. Haidian District Beijing 100095 China Email: zhangmingui@huawei.com Donald E. Eastlake, 3rd Futurewei Technologies 2386 Panoramic Circle Apopka, FL 32703 United States Phone: +1-508-333-2270 Email: d3e3e3@gmail.com Radia Perlman EMC 2010 256th Avenue NE, #200 Bellevue, WA 98007 United States Email: radia@alum.mit.edu Margaret Cullen Painless Security 356 Abbott Street North Andover, MA 01845 United States Phone: +1-781-405-7464 Email: margaret@painless-security.com URI: http://www.painless-security.com Hongjun Zhai Jinling Institute of Technology 99 Hongjing Avenue, Jiangning District Nanjing, Jiangsu 211169 China Email: honjun.zhai@tom.com M. Zhang, et al [Page 18] INTERNET-DRAFT Multilevel Single Nickname Copyright, Disclaimer, and Additional IPR Provisions Copyright (c) 2020 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 (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. M. Zhang, et al [Page 19]