IDR Working Group C. Lin Internet-Draft New H3C Technologies Intended status: Standards Track K. Wang Expires: 3 May 2027 HPE Networking 30 October 2026 Interface Index Capability for BGP draft-lin-idr-interface-index-capability-01 Abstract In some scenarios using unnumbered links, it is necessary for the BGP protocol to advertise the link's Interface Index and obtain the peer's Remote Interface Index. This document defines a new BGP capability, as specified in RFC 5492, for advertising local interface indexes and obtaining neighbor interface indexes. 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/. 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." This Internet-Draft will expire on 3 May 2027. Copyright Notice Copyright (c) 2026 IETF Trust and the persons identified as the document authors. All rights reserved. Lin & Wang Expires 3 May 2027 [Page 1] Internet-Draft Interface Index Capability for BGP October 2026 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://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 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 2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Interface Index Capability . . . . . . . . . . . . . . . . . 4 4.1. Error Handling . . . . . . . . . . . . . . . . . . . . . 4 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 6. Security Considerations . . . . . . . . . . . . . . . . . . . 5 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 7.1. Normative References . . . . . . . . . . . . . . . . . . 5 7.2. Informative References . . . . . . . . . . . . . . . . . 6 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6 1. Introduction Here are several issues that arise when using unnumbered links due to BGP's lack of support for interface index. Scenario 1: BGP-LS-SPF BGP-LS-SPF [RFC9815] is a solution that leverages both the BGP protocol [RFC4271] and the BGP-LS extension [RFC9552]. In the BGP SPF routing domain, BGP routers collect network-wide topology information through the BGP-LS-SPF protocol and perform cross-domain path calculation to compute routes using the SPF algorithm. This approach aims to achieve intelligent traffic engineering and automated routing optimization in large-scale data center or carrier networks. When an IGP protocol advertises an unnumbered link, it uses the local interface index and the remote interface index to distinguish between different unnumbered links. BGP-LS-SPF reuses the BGP-LS information from the IGP. However, during the deployment of BGP-LS-SPF, if the remote interface index is unknown, it may lead to path calculation errors. Scenario 2: BGP Egress Peer Engineering (EPE) Lin & Wang Expires 3 May 2027 [Page 2] Internet-Draft Interface Index Capability for BGP October 2026 When deploying EPE, the BGP-LS peer link information in EPE is defined as follows (see Section 5.2 of [RFC9086]): Link Descriptors MUST include the following TLV, as defined in [RFC9552]: * Link Local/Remote Identifiers (TLV 258) contains the 4-octet Link Local Identifier followed by the 4-octet Link Remote Identifier. The value 0 is used by default when the link remote identifier is unknown. Currently, BGP has no mechanism to obtain the peer's Link Remote Identifier. Scenario 3: BGP neighbor monitoring During neighbor monitoring, if displaying bidirectional neighbor connectivity, without the Link Remote Identifier, the corresponding neighbor relationships on both sides cannot be matched. Therefore, fundamentally, in unnumbered BGP scenarios, BGP peer information should include: local router-id, peer address, peer AS number, local interface index, and remote interface index. This document defines a new BGP capability, as specified in [RFC5492], for advertising support of the interface index capability, thereby advertising local interface indexes and obtaining neighbor interface indexes. 2. 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. 3. Terminology The following terminology is used in this document. * BGP: Border Gateway Protocol * EPE: Egress Peer Engineering * BGP-LS-SPF: BGP Link State Shortest Path First Routing Lin & Wang Expires 3 May 2027 [Page 3] Internet-Draft Interface Index Capability for BGP October 2026 4. Interface Index Capability In the BGP protocol, the OPEN message carries the newly added Interface Index Capability TLV to exchange interface index information with neighbors. When sending an OPEN message, the Interface Index Capability is included to advertise the Interface Index of the interface used to establish the neighbor relationship. When receiving an OPEN message, if it contains the Interface Index Capability, the peer's Remote Interface Index is saved. If the Interface Index Capability is not carried, save the Remote Interface Index as 0. The format of the Interface Index Capability is shown in Figure 1. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Cap Code = TBD |Cap Length = 4 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Interface Index Value | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: Interface Index Capability 4.1. Error Handling A BGP speaker that receives an OPEN message containing the Interface Index Capability with a Capability Length (as defined in [RFC5492]) other than 4 MUST treat the capability as malformed and ignore it. In this case, the Remote Interface Index of the peer MUST be set to 0, and the BGP session SHOULD still be established. An implementation MAY log this condition for further analysis. If the Interface Index Value is 0, it indicates that the local interface index of the sender is unknown. The receiver MUST treat the Remote Interface Index as 0, and the BGP session SHOULD still be established. Per [RFC5492], a BGP speaker that does not recognize or does not support the Interface Index Capability MUST silently ignore it, and the procedures described in this document do not apply to that BGP session. Lin & Wang Expires 3 May 2027 [Page 4] Internet-Draft Interface Index Capability for BGP October 2026 5. IANA Considerations A new BGP Capability named "Interface Index Capability" will be requested from the "Capability Codes" registry within the "IETF Review" range [RFC5492]. +============+============================+===============+ | Code Point | Description | Reference | +============+============================+===============+ | TBD | Interface Index Capability | This document | +------------+----------------------------+---------------+ Table 1: Capability Codes 6. Security Considerations This document defines a new BGP capability that is carried in the OPEN message as specified in [RFC5492]; it does not introduce any new BGP messages or modify the BGP finite state machine. The Interface Index carried in this capability is not considered sensitive information, as it is already advertised by the routing protocols of the local node (e.g., via the Link Local/Remote Identifiers TLV in BGP-LS [RFC9552]). However, a malicious BGP peer could advertise an incorrect Interface Index, which could mislead the receiver in scenarios such as those described in Section 1 (e.g., causing mismatches when correlating bidirectional neighbor relationships or computing paths over unnumbered links). This document does not introduce any new mechanisms to protect the BGP session beyond those that already exist. Existing mechanisms to protect BGP sessions, such as the Generalized TTL Security Mechanism (GTSM) [RFC5082] and the TCP Authentication Option (TCP-AO) [RFC5925], SHOULD be used to protect the integrity of the BGP session. 7. References 7.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", March 1997, . [RFC4271] Rekhter, Y., Li, T., Ed., and S. Hares, Ed., "A Border Gateway Protocol 4 (BGP-4)", January 2006, . Lin & Wang Expires 3 May 2027 [Page 5] Internet-Draft Interface Index Capability for BGP October 2026 [RFC5492] Scudder, J. and R. Chandra, "Capabilities Advertisement with BGP-4", February 2009, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", May 2017, . 7.2. Informative References [RFC5082] Gill, V., Heasley, J., Meyer, D., and P. Savola, Ed., "The Generalized TTL Security Mechanism (GTSM)", October 2007, . [RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP Authentication Option", June 2010, . [RFC9086] Previdi, S., Talaulikar, K., Ed., Filsfils, C., Patel, K., Ray, S., and J. Dong, "Border Gateway Protocol - Link State (BGP-LS) Extensions for Segment Routing BGP Egress Peer Engineering", August 2021, . [RFC9552] Talaulikar, K., Ed., "Distribution of Link-State and Traffic Engineering Information Using BGP", December 2023, . [RFC9815] Patel, K., Lindem, A., Zandi, S., and W. Henderickx, "BGP Link State (BGP-LS) Shortest Path First (SPF) Routing", July 2025, . Authors' Addresses Changwang Lin New H3C Technologies Email: linchangwang.04414@h3c.com Kevin Wang HPE Networking Email: kfwang@juniper.net Lin & Wang Expires 3 May 2027 [Page 6]