BESS Yisong Liu Internet Draft China Mobile Intended status: Standards Track C. Lin Expires: April 28, 2025 M. Chen New H3C Technologies Y. Liu ZTE October 21, 2024 No Further Fast Reroute for SRv6 Service SID draft-liu-bess-srv6-service-sid-nffrr-flag-02 Abstract In some multihoming SRv6 L3VPN and EVPN scenarios, once fast reroute has taken place, a second fast reroute is undesirable and may cause looping. This document proposes a mechanism to prevent further fast reroutes by advertising No-Further-FRR flags for SRv6 Service SIDs in BGP messages. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. 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Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction...................................................2 1.1. Requirements Language.....................................2 2. Use Case.......................................................3 2.1. SRv6 L3VPN Multihoming....................................3 2.2. SRv6 EVPN Multihoming.....................................5 3. Solution.......................................................5 3.1. Consideration for EVPN Single-Active Mode.................7 4. Extensions for BGP.............................................7 5. Backward Compatibility.........................................8 6. Security Considerations........................................8 7. IANA Considerations............................................9 8. References.....................................................9 8.1. Normative References......................................9 Authors' Addresses................................................9 1. Introduction [RFC9252] defines procedures and messages for SRv6-based BGP services, including Layer 3 Virtual Private Network (L3VPN), Ethernet VPN (EVPN), and Internet services. In some multihoming scenarios, two egress PEs may establish a backup path between them and use it as the protection of PE-CE link failure. Once fast reroute (FRR) has taken place, a second fast reroute is undesirable and may cause looping. This document defines the No-Further-FRR flag for SRv6 Service SIDs carried in BGP messages and proposes a mechanism using the No- Further-FRR flag to prevent further fast reroutes. 1.1. Requirements Language 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. liu, et al. Expires April 28, 2025 [Page 2] Internet-Draft SRv6 Service SID No-Further-FRR Flag October 2024 2. Use Case 2.1. SRv6 L3VPN Multihoming In the multihoming SRv6 L3VPN scenarios, two egress PEs may establish a backup path between them and use it as the protection of PE-CE link failure. Take the network in Figure 1 as an example. When traffic goes from CE1 to CE2, it may be load-balanced between PE2 and PE3 or only forwarded to the main egress PE. If the link PE2-CE2 fails, PE2 can still forward the traffic for CE2 by sending it over the backup path to PE3 (and similarly for PE3 if link2 fails). +-----+ | CE1 | +-----+ | | +-----+ ------------------- | PE1 |*************** ^ +-----+ * | / \ * | / \ * | P1 P2 * | . . +------+ SRv6 VPN . *************.*******|BGP-RR| | . * . +------+ | P3 * P4 * | | * | * | | * | * v +-----+ Backup +-----+ * --------- | PE2 |#############| PE3 |***** +-----+ Path +-----+ \ / \ / +-----+ | CE2 | +-----+ Figure 1 Examples of BGP routes advertised by PE2 and PE3 are as following: liu, et al. Expires April 28, 2025 [Page 3] Internet-Draft SRv6 Service SID No-Further-FRR Flag October 2024 BGP Route by PE2: VPN Prefix of CE2: BGP Prefix SID Attr: SRv6 L3 Service TLV: SRv6 SID Information sub-TLV: SID: SID-2 Behavior: End.DT46 BGP Route by PE3: VPN Prefix of CE2: BGP Prefix SID Attr: SRv6 L3 Service TLV: SRv6 SID Information sub-TLV: SID: SID-3 Behavior: End.DT46 Examples of FIB entries for L3VPN service SID on PE2 and PE3 are as following: FIB on PE2: SID-2: Primary Next-hop: CE2 Backup Next-hop: Service SRv6 SID-3 FIB on PE3: SID-3: Primary Next-hop: CE2 Backup Next-hop: Service SRv6 SID-2 However, suppose CE2 is down. PE2 will think PE2-CE2 link is down and send traffic to PE3 over the backup path. PE3 will also think PE3-CE3 link is down and send the traffic back to PE2 over the backup path. So, traffic will loop between PE2 and PE3 until BGP convergence. The traffic forwarding when CE2 fails is as following: liu, et al. Expires April 28, 2025 [Page 4] Internet-Draft SRv6 Service SID No-Further-FRR Flag October 2024 +======+=============+=======+==============+ | Node | Packet | Next | Comment | +======+=============+=======+==============+ | PE1 | pkt | PE2 | | +------+-------------+-------+--------------+ | PE2 | pkt | CE2 | PE2-CE2 down | +------+-------------+-------+--------------+ | PE2 | pkt | PE3 | FRR | +------+-------------+-------+--------------+ | PE3 | pkt | CE2 | PE3-CE2 down | +------+-------------+-------+--------------+ | PE3 | pkt | PE2 | FRR | +------+-------------+-------+--------------+ | PE2 | -- | CE2 | PE2-CE2 down | +------+-------------+-------+--------------+ | PE2 | pkt | PE3 | FRR | +------+-------------+-------+--------------+ | ... | | | Loop! | +------+-------------+-------+--------------+ 2.2. SRv6 EVPN Multihoming The EVPN services include Designated Forwarder (DF) election procedure. In All-Active mode, all PEs are allowed to forward unicast traffic, which is similar with the L3VPN case in Section 2.1. In Single-Active mode, only DF is allowed to forward unicast traffic, and it requires additional considerations in FRR. 3. Solution Each egress PE advertises an additional SRv6 Service SID in BGP routes which is called No-Further-FRR SID. The owner of No-Further-FRR SID will not provide local FRR for it. When the next-hop of No-Further-FRR SID is down, like PE-CE link failure or CE node failure, the PE will drop packets rather than apply FRR. The No-Further-FRR SID can used by other PE as the protection of local PE-CE link failure, without worrying about the looping problem. To support backwards compatibility and BGP RR deployment, both the normal SRv6 Service SID and the No-Further-FRR SID MAY be advertised liu, et al. Expires April 28, 2025 [Page 5] Internet-Draft SRv6 Service SID No-Further-FRR Flag October 2024 together. A No-Further-FRR flag is used to indicate the No-Further- FRR SID. Detailed BGP extensions will be described in Section 4. Still taking the network in Figure 1 as an example, the BGP routes advertised by PE2 and PE3 are as following: BGP Route by PE2: VPN Prefix of CE2: BGP Prefix SID Attr: SRv6 L3 Service TLV: SRv6 SID Information sub-TLV: SID: SID-21 Behavior: End.DT46(L3VPN) or End.DX2/End.DT2U(EVPN) SRv6 SID Information sub-TLV: SID: SID-22 Behavior: End.DT46(L3VPN) or End.DX2/End.DT2U(EVPN) Flag: No-Further-FRR BGP Route by PE3: VPN Prefix of CE2: BGP Prefix SID Attr: SRv6 L3 Service TLV: SRv6 SID Information sub-TLV: SID: SID-31 Behavior: End.DT46(L3VPN) or End.DX2/End.DT2U(EVPN) SRv6 SID Information sub-TLV: SID: SID-32 Behavior: End.DT46(L3VPN) or End.DX2/End.DT2U(EVPN) Flag: No-Further-FRR The FIB entries for L3VPN service SID on PE2 and PE3 are as following: liu, et al. Expires April 28, 2025 [Page 6] Internet-Draft SRv6 Service SID No-Further-FRR Flag October 2024 FIB on PE2: SID-21: Primary Next-hop: CE2 Backup Next-hop: Service SRv6 SID-32 SID-22 (No-Further-FRR): Primary Next-hop: CE2 FIB on PE3: SID-31: Primary Next-hop: CE2 Backup Next-hop: Service SRv6 SID-22 SID-32 (No-Further-FRR): Primary Next-hop: CE2 After adopting the proposed solution, if CE fails, PE2 will think PE2-CE2 link is down and send traffic to PE3 by using the No- Further-FRR SID-32. PE3 will also think PE3-CE3 link is down, but PE3 will drop the packets rather than apply FRR. The traffic forwarding when CE2 fails is as following: +======+==============+=======+==============+ | Node | Packet | Next | Comment | +======+==============+=======+==============+ | PE1 | pkt | PE2 | | +------+--------------+-------+--------------+ | PE2 | pkt | CE2 | PE2-CE2 down | +------+--------------+-------+--------------+ | PE2 | pkt | PE3 | FRR | +------+--------------+-------+--------------+ | PE3 | pkt | CE2 | PE3-CE2 down | +------+--------------+-------+--------------+ | PE3 | - | - | Drop | +------+--------------+-------+--------------+ 3.1. Consideration for EVPN Single-Active Mode The processing of the No-Further-FRR SID should apply an override to EVPN DF-Election and bypass the local blocking state on the AC, until EVPN control plane reconverges. 4. Extensions for BGP This document defines a new flag in the SRv6 Service SID Flags field of the SRv6 SID Information Sub-TLV [RFC9252]: liu, et al. Expires April 28, 2025 [Page 7] Internet-Draft SRv6 Service SID No-Further-FRR Flag October 2024 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SRv6 Service | SRv6 Service | | | Sub-TLV | Sub-TLV | | | Type=1 | Length | RESERVED1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SRv6 SID Value (16 octets) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Svc SID Flags | SRv6 Endpoint Behavior | RESERVED2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SRv6 Service Data Sub-Sub-TLVs // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Svc SID Flags: 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |N| | +-+-+-+-+-+-+-+-+ o N-flag: No-Further-FRR flag. When set, the associated SID has no fast reroute protection. The new-defined flag can be used for the SRv6 Service SIDs of L3 and L2 services, such as End.DX4, End.DT4, End.DX6, End.DT6, End.DT46. End.DX2, End.DX2V, End.DT2U, etc. 5. Backward Compatibility According to [RFC9252], o Any unknown flags in the SRv6 Service SID Flags field MUST be ignored by the receiver. o When multiple SRv6 SID Information Sub-TLVs are present, the ingress PE SHOULD use the SRv6 SID from the first instance of the Sub-TLV. When the egress PE advertises multiple service SIDs, the normal service SID SHOULD be carried in the first instance of Sub-TLV. If there are some other PE routers not supporting the flag defined in this document, the egress PE MAY expect those routers to use the first SID and ignore the new-defined flag. 6. Security Considerations TBD. liu, et al. Expires April 28, 2025 [Page 8] Internet-Draft SRv6 Service SID No-Further-FRR Flag October 2024 7. IANA Considerations This document defines the following bit in the SRv6 Service SID Flags field of SRv6 SID Information Sub-TLV: TLV Code Point Value -------------------------------------------------------- TBD N-flag 8. References 8.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, May 2017 [RFC9252] Dawra, G., Ed., Talaulikar, K., Ed., Raszuk, R., Decraene, B., Zhuang, S., and J. Rabadan, "BGP Overlay Services Based on Segment Routing over IPv6 (SRv6)", RFC 9252, DOI 10.17487/RFC9252, July 2022, . Authors' Addresses Yisong Liu China Mobile China Email: liuyisong@chinamobile.com Changwang Lin New H3C Technologies China Email: linchangwang.04414@h3c.com Mengxiao Chen New H3C Technologies China Email: chen.mengxiao@h3c.com liu, et al. Expires April 28, 2025 [Page 9] Internet-Draft SRv6 Service SID No-Further-FRR Flag October 2024 Yao Liu ZTE China Email: liu.yao71@zte.com.cn liu, et al. Expires April 28, 2025 [Page 10]