IDR Working Group C. Lin, Ed. Internet-Draft New H3C Technologies Intended status: Standards Track Z. Ali, Ed. Expires: 22 April 2027 Cisco Systems, Inc. Y. Liu China Mobile R. Chen ZTE A. Karboubi Ciena 19 October 2026 Advertisement of SR Policy Operational States using BGP Link-State draft-ietf-idr-bgp-ls-sr-policy-admin-flags-01 Abstract This document defines the extension of BGP Link-State to advertise the operational state of the candidate path or segment list, facilitating the operation and maintenance of the SR Policy. 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 22 April 2027. Copyright Notice Copyright (c) 2026 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 (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 Lin, et al. Expires 22 April 2027 [Page 1] Internet-Draft BGP-LS for SR Policy Admin Flags October 2026 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. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Extension of SR Candidate Path State . . . . . . . . . . . . 3 4. Extension of SR Segment List TLV . . . . . . . . . . . . . . 3 5. Extension of SR Candidate Path Constraints TLV . . . . . . . 4 5.1. SR Maximum Segment Lists Constraint Sub-TLV . . . . . . . 4 6. SR Policy Pre-emption Priority TLV . . . . . . . . . . . . . 5 7. Operations . . . . . . . . . . . . . . . . . . . . . . . . . 5 7.1. Maximum Segment Lists . . . . . . . . . . . . . . . . . . 6 7.2. Policy Pre-emption Priority . . . . . . . . . . . . . . . 6 8. Security Considerations . . . . . . . . . . . . . . . . . . . 7 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 9.1. Additional TLV Type Allocations . . . . . . . . . . . . . 8 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8 10.1. Normative References . . . . . . . . . . . . . . . . . . 8 10.2. Informative References . . . . . . . . . . . . . . . . . 10 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10 Appendix A. Cross WG Information . . . . . . . . . . . . . . . . 10 A.1. Link to Spring WG . . . . . . . . . . . . . . . . . . . . 10 A.2. Interaction with PCE WG . . . . . . . . . . . . . . . . . 10 A.3. Interaction with SRV6ops Recommendations . . . . . . . . 10 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 1. Introduction Segment routing (SR) [RFC8402] is a source routing paradigm that explicitly indicates the forwarding path for packets at the ingress node. The ingress node steers packets into a specific path according to the Segment Routing Policy (SR Policy) as defined in [RFC9256]. An SR Policy may have multiple candidate paths that are provisioned or signaled [RFC9830] [RFC8664] from one of more sources. [I-D.lin-idr-sr-policy-admin-flags] proposes an extension to the BGP SR Policy that sets the operational state of the candidate path or the segment list, facilitating the operation and maintenance of the SR Policy. This document defines extensions to BGP-LS to advertise defines the extension of BGP Link-State to advertise the operational state of the candidate path or segment list. Lin, et al. Expires 22 April 2027 [Page 2] Internet-Draft BGP-LS for SR Policy Admin Flags October 2026 This document also defines advertisements for the maximum number of Segment Lists in a candidate path and the SR Policy pre-emption priority. 2. Terminology The definitions of the basic terms are identical to those found in Segment Routing Policy Architecture [RFC9256]. 3. Extension of SR Candidate Path State As defined in [RFC9857], the SR Candidate Path State TLV provides the operational status and attributes of the SR Policy at the candidate path level. New bits in Flags field of the SR Candidate Path State TLV are defined: - BI-Flag (Bit TBD): Indicates the CP is configured as "backup ineligible". - IS-Flag(Bit TBD): Indicates the CP is configured as "Ignore service route's Prefix SID". It allows traffic to a BGP service route to be steered over an SR policy without imposing the service route's prefix label or SRv6 Service SID. - PE-Flag (Bit TBD): Indicates that the CP is set to "candidate path eligible" as defined in [I-D. ietf-spring-sr-policy- eligibility]. - IFN-flag (Bit TBD) (Install Final Node-sid flag) as defined in [I-D. ietf-spring-srv6-policy-sid-list-opt]: indicate the endpoint node SID is included in installing SID list(s) of the Candidate Path (CP) when set. If set to 1, the endpoint node SID MUST be included when installing the SR Policy SID list(s) used to carry the data traffic. If set to 0, the endpoint node SID MUST NOT be included when installing the SR Policy SID list(s) used to carry the data traffic. 4. Extension of SR Segment List TLV As defined in [RFC9857], The SR Segment List TLV is used to report a single SID-List of a candidate path. Multiple instances of this TLV may be used to report multiple SID-Lists of a candidate path. Two New bits in Flags field of the SR Segment List TLV is defined: Lin, et al. Expires 22 April 2027 [Page 3] Internet-Draft BGP-LS for SR Policy Admin Flags October 2026 - S-Flag(Bit TBD): Indicates the SR Segment is in an administrative shut state when set and not in administrative shut state when clear. - PE-Flag(Bit TBD): When set, indicates the Segment List is in an eligible state (capable of forwarding traffic). When cleared, indicates the Segment List is ineligible (cannot forward traffic). 5. Extension of SR Candidate Path Constraints TLV The following constraint sub-TLV extends the SR Candidate Path Constraints TLV (Type 1204) defined in Section 5.6 of [RFC9857]. The enclosing TLV is carried in the BGP-LS Attribute associated with the SR Policy Candidate Path NLRI (Type 5). 5.1. SR Maximum Segment Lists Constraint Sub-TLV The SR Maximum Segment Lists Constraint sub-TLV reports the maximum number of Segment Lists that may be computed for a candidate path. It is an optional sub-TLV of the SR Candidate Path Constraints TLV defined in Section 5.6 of [RFC9857]. It counts Segment Lists, not the SIDs within a Segment List. The sub-TLV has the following format: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Maximum Segment Lists | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: SR Maximum Segment Lists Constraint Sub-TLV Format Where: Type: TBD1. Length: 2 octets. Maximum Segment Lists: A 2-octet unsigned integer specifying the maximum number of Segment Lists. Valid values are 1 through 65535. The operational procedures for this sub-TLV are specified in Section 7.1. Lin, et al. Expires 22 April 2027 [Page 4] Internet-Draft BGP-LS for SR Policy Admin Flags October 2026 6. SR Policy Pre-emption Priority TLV The SR Policy Pre-emption Priority TLV reports the priority used to determine whether an SR Policy can pre-empt a lower-priority policy. It is an optional top-level TLV in the BGP-LS Attribute associated with the SR Policy Candidate Path NLRI (Type 5) defined in [RFC9857]. It is distinct from the recomputation Priority field of the SR Candidate Path State TLV (Type 1202) and from candidate path Preference. The TLV has the following format: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Priority | +-+-+-+-+-+-+-+-+ Figure 2: SR Policy Pre-emption Priority TLV Format Where: Type: TBD2. Length: 1 octet. Priority: A 1-octet unsigned integer with a valid range of 0 through 7. Value 0 is the highest priority and value 7 is the lowest. The configuration or request-input default is 0. The operational procedures for this TLV are specified in Section 7.2. 7. Operations The operations defined in Section 6 of [RFC9857] apply to this document. The following additional procedures apply to the constraint sub-TLV and the policy pre-emption priority TLV defined in this document. Lin, et al. Expires 22 April 2027 [Page 5] Internet-Draft BGP-LS for SR Policy Admin Flags October 2026 7.1. Maximum Segment Lists A dynamic candidate path computation request MUST specify a maximum of at least 1. If the parameter is absent or invalid, the computation server MUST reject the request and MUST NOT compute a path for that candidate path. The server MUST NOT compute more Segment Lists than the specified maximum. When advertised, this sub- TLV MUST carry the effective maximum for the candidate path. Omission from a BGP-LS advertisement does not remove the requirement for a valid maximum in a computation request. 7.2. Policy Pre-emption Priority Because the NLRI identifies a candidate path rather than a standalone policy, this policy attribute is advertised with each candidate path of the policy. This TLV MUST be advertised at most once per candidate path. For duplicates, the receiver uses the first syntactically valid instance and ignores later instances. All candidate paths of an SR Policy MUST advertise the same policy pre-emption priority. If candidate paths of the same policy advertise different valid values, the Consumer MUST use the minimum numerical value as the policy priority, irrespective of which candidate path is active. Candidate paths are grouped by the policy headend, color, and endpoint within the same administrative domain and authoritative producer view. An absent or invalid priority is unknown; it MUST NOT be replaced with the input default for a pre- emption decision. The server MUST validate the original input before encoding it. A value outside 0 through 7 is invalid input and MUST cause rejection. A value within 0 through 7 but outside the server's configured supported range is an unsupported constraint and MUST also cause rejection. These checks apply to the default value. In either case, the server MUST NOT perform the requested computation or policy pre- emption operation. A supporting producer MUST advertise the effective value, including a defaulted value of 0. A policy with a smaller numerical value may pre-empt a policy with a larger numerical value when authorized by the applicable resource- management policy; equal priority alone does not permit pre-emption. Advertisement of this attribute does not itself authorize pre- emption. Lin, et al. Expires 22 April 2027 [Page 6] Internet-Draft BGP-LS for SR Policy Admin Flags October 2026 8. Security Considerations Procedures and protocol extensions defined in this document do not affect the security considerations discussed in [RFC9857]. This document defines new bits in the Flags field of the SR Candidate Path State TLV to indicate the operational state of the candidate path. Similarly, new bits in the Flags field of the SR Segment List TLV are defined to report the operational state of the segment list. The values of these bits determine the state of the candidate path or segment list within the SR policy and may consequently affect packet forwarding in the network. Therefore, when configuring, querying, or reporting these administrative flags in the BGP control plane, appropriate measures MUST be taken to protect such mission-critical or commercially sensitive information. 9. IANA Considerations This document requests four bits in the flag field of "SR Candidate Path State TLV" [RFC9857] under the "BGP-LS Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute TLVs" registry. Bit Description Reference ----------------------------------------------------------------- TBA S-Flag: The CP is configured This document as "backup ineligible" ----------------------------------------------------------------- TBA IS-Flag: The CP is configured This document as "Ignore service route's Prefix SID" ----------------------------------------------------------------- TBA PE-Flag: The CP is configured This document as "candidate path eligible" ----------------------------------------------------------------- TBA IFN-flag: The CP is configured This document as "Install Final Node-sid" ----------------------------------------------------------------- This document requests two bits in the flag field of "SR Segment List TLV" [RFC9857] under the "BGP-LS Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute TLVs" registry. Lin, et al. Expires 22 April 2027 [Page 7] Internet-Draft BGP-LS for SR Policy Admin Flags October 2026 Bit Description Reference ------------------------------------------------------------------ TBA S-Flag: Indicates that the CP is in an This document administrative shut state when set and not in an administrative shut state when clear. ------------------------------------------------------------------ TBA PE-Flag: Indicates the Segment List is in This document an eligible state (capable of forwarding traffic) when set and not in an eligible state when clear. ------------------------------------------------------------------ 9.1. Additional TLV Type Allocations IANA is requested to allocate two distinct code points from the "BGP- LS NLRI and Attribute TLVs" registry in the "Border Gateway Protocol - Link State (BGP-LS) Parameters" registry group: +=======+=====================================+=============+ | Value | Description | Reference | +=======+=====================================+=============+ | TBD1 | SR Maximum Segment Lists Constraint | Section 5.1 | +-------+-------------------------------------+-------------+ | TBD2 | SR Policy Pre-emption Priority | Section 6 | +-------+-------------------------------------+-------------+ Table 1: Requested TLV Types 10. References 10.1. Normative References [RFC9857] Previdi, S., Talaulikar, K., Dong, J., Gredler, H., and Tantsura, J., "Advertisement of Segment Routing Policies using BGP Link-State", RFC 9857, DOI 10.17487/RFC9857, October 2025, . [RFC9830] Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P., and D. Jain, "Advertising Segment Routing Policies in BGP", RFC 9830, DOI 10.17487/RFC9830, September 2025, . [I-D. lin-idr-sr-policy-admin-flags] Lin, C., Li, J., and Chen, R., "BGP SR Policy Extensions for Administrative Flags", draft-lin-idr-sr-policy-admin-flags (work in progress), February 2025. Lin, et al. Expires 22 April 2027 [Page 8] Internet-Draft BGP-LS for SR Policy Admin Flags October 2026 [I-D. ietf-spring-sr-policy-eligibility] Karboubi, A., Shah, H., Sivalaban, S., Stone, A. and Schmutz, C., "Eligibility Concept in Segment Routing Policies", draft-ietf-spring- sr-policy-eligibility-00 (work in progress), January 2026. [I-D. draft-ietf-spring-srv6-policy-sid-list-opt] Z. Ali, C. Lin, Y. Liu, R. Chen and C. Li, "SRv6 Policy SID List Optimization", Work in Progress, Internet-Draft, draft- ietf-spring-srv6-policy-sid-list-opt, 25 November 2025,. [I-D. draft-all-pce-srv6-policy-sid-list-optimization] Z. Ali, C. Lin, Y. Liu, R. Chen and C. Li, "Path Computation Element Communication Protocol (PCEP) extensions for SRv6 Policy SID List Optimization", Work in Progress, Internet-Draft, draft-all-pce-srv6-policy-sid-list-optimization, 02 March 2026, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L., Decraene, B., Litkowski, S., and R. Shakir, "Segment Routing Architecture", RFC 8402, DOI 10.17487/RFC8402, July 2018, . [RFC9256] Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and P. Mattes, "Segment Routing Policy Architecture", RFC 9256, DOI 10.17487/RFC9256, July 2022, . [RFC8664] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W., and J. Hardwick, "Path Computation Element Communication Protocol (PCEP) Extensions for Segment Routing", RFC 8664, DOI 10.17487/RFC8664, December 2019, . Lin, et al. Expires 22 April 2027 [Page 9] Internet-Draft BGP-LS for SR Policy Admin Flags October 2026 10.2. Informative References TBD 11. Acknowledgments The authors would like to thank Aravind Babu MahendraBabu for his valuable contributions of this document. Appendix A. Cross WG Information This section describes cross-working group information for use during the IETF review process. This section will be removed by the RFC editor prior to publication. A.1. Link to Spring WG This document implements the features defined in Section 4 of [I- D.ietf-spring-sr-policy-eligibility], Section 8.2 of [RFC9256], and Section 3.2 of [I-D.draft-ietf-spring-srv6-policy-sid-list-opt] via BGP-LS. These SPRING features require the implementation of this document. A.2. Interaction with PCE WG The BGP extension defined by this document also may be implemented by a new TLV of PCEP according to Section 4.3 of [I-D. draft-karboubi- spring-sidlist-optimized-cs-sr]. The optimization feature implemented in BGP by this document also could be implemented in PCEP according to Section 3.1 of [I-D. draft- ietf-spring-srv6-policy-sid-list-opt]. [I-D. draft-all-pce-srv6- policy-sid-list-optimization] specifies a PCEP extension to indicate whether the endpoint's node SID needs to be included or excluded when installing the SRv6 Policy. A.3. Interaction with SRV6ops Recommendations This optimization feature could be distributed to external components using BGP-LS extension defined in this document. Authors' Addresses Changwang Lin (editor) New H3C Technologies China Lin, et al. Expires 22 April 2027 [Page 10] Internet-Draft BGP-LS for SR Policy Admin Flags October 2026 Email: linchangwang.04414@h3c.com Zafar Ali (editor) Cisco Systems, Inc. Email: zali@cisco.com Yisong Liu China Mobile 32 Xuanwumen West Street Beijing Xicheng District, 100053 China Email: liuyisong@chinamobile.com Ran Chen ZTE Corporation Email: chen.ran@zte.com.cn Amal Karboubi Ciena Email: akarboub@ciena.com Lin, et al. Expires 22 April 2027 [Page 11]