Supplement of BGP-LS Distribution for SR Policies and State

Internet-Draft	BGP-LS SR Policy	February 2026
Liu, et al.	Expires 30 August 2026	[Page]

Abstract

This document supplements additional information of the segment list in the BGP-LS advertisement for SR Policy state information. A new flag and a new sub-TLV are introduced in the SR Segment List TLV of BGP-LS SR Policy Candidate Path NLRI.¶

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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This Internet-Draft will expire on 30 August 2026.¶

Copyright Notice

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1. Introduction

SR Policy architecture details are specified in [RFC9256]. An SR Policy comprises one or more candidate paths (CP) of which at a given time one and only one may be active. Each CP in turn may have one or more SID-List of which one or more may be active; when multiple are active then traffic is load balanced over them.¶

[RFC9857] describes a mechanism to collect the SR policy information that is locally available in a node and advertise it into BGP Link State (BGP-LS) updates. Various TLVs are defined to enable the headend to report the state at the candidate path level and the segment list level.¶

Currently, the following segment-list-related information is not yet included in [RFC9857]:¶

Whether the segment list is in administrative shut state. For the candidate path, there's already an S-Flag in the SR Candidate Path State TLV in [RFC9857] indicating the CP is in an administrative shut state. In some usecases, the segment list may also be shut by an administrator for traffic engineering or power saving purpose, e.g, the network administrator may shut certain segment list when the load on the SR Policy is light. This information may also be needed and reported via BGP-LS.¶
The 32-bit MPLS LSE information, especially, the MPLS Network Actions (MNA) sub-stack. Traditional MPLS LSE consists of 20-bit MPLS label, 3-bit TC, 1-bit S(bottom of stack indication) and 8-bit TTL. Accordingly, SR Segment List TLV [RFC9857] only supports carrying MPLS labels with the TC, S and TTL fields set to 0 in SR Segment Sub-TLV. However, [I-D.ietf-mpls-mna-hdr] defines the MPLS Network Actions sub-stack (NAS) solution for carrying Network Actions and Ancillary Data in the MPLS label stack, unlike traditional MPLS LSE, some LSEs defined for MNA repurposed the TC and TTL field to carry additional information. MNA such as Network Resource Partition (NRP) [I-D.ietf-mpls-mna-nrp-selector], IOAM [I-D.ietf-mpls-mna-ioam] may be inserted in the SID list in the format of LSEs. The contents of the LSEs inserted in the SID-lists may be required by the controller when the headend reports the state of SR Policies via BGP-LS.¶

This document supplements some additional information of the segment list state as mentioned above in the BGP-LS advertisement for SR Policy state information.¶

3. BGP-LS Extensions for Distributing Segment List States

3.1. New Flag in SR Segment List TLV

SR Segment List TLV is defined in [RFC9857] to report the SID-List(s) of a candidate path. As show in Figure 1,this document introduces a new flag in the flag field of SR Segment List TLV, where,¶

       0                   1
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | | | | | | | | | |S|           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: New Flags in the Flag Field of SR Segment List TLV

S-Flag: Indicates the segment list is in administrative shut state when set. The segment list may be shut by the administrator via CLI or other methods, and it is out of the scope of this document.¶

3.2. MNA Sub-Stack Sub-TLV

The MNA Sub-Stack Sub-TLV is defined in this section to carry the MNA Sub-Stack information.¶

 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            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~                       MNA Sub-Stack                           ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2: MNA Sub-Stack Sub-TLV

Type: TBA¶
Length: Variable, the total length (in octets) of MNA Sub-Stack portion in octets, MUST be the multiple of 4. The value indicates the number of the LSEs in this sub-TLV.¶
MNA Sub-Stack: one or more 4-octet-field carrying the MNA Sub-Stack defined in [I-D.ietf-mpls-mna-hdr]¶

The MNA Sub-Stack is an optional sub-TLV of SR Segment List TLV, and can appear more than once in the SR Segment List TLV. It may be used as the sub-TLV of other TLVs, for the latter case, the detailed usage is out of the scope of this document.¶

4. IANA Considerations

This document requests bit 9 in the flag field of "SR Segment List TLV" [RFC9857] under the "BGP-LS Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute TLVs" registry.¶

       Bit     Description                                Reference
      ------------------------------------------------------------------
        9     Administrative Shut State Flag(S-Flag)      This document

This document requests a new type sub-TLV of "SR Segment List TLV" [RFC9857] under the "BGP-LS Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute TLVs" registry.¶

       Type     Description                                Reference
      ------------------------------------------------------------------
       TBA     MPLS LSE Sub-TLV                         This document

5. Manageability Considerations

The considerations as specified in [RFC9552] apply to this document. In general, unknown and unsupported types MUST be preserved and propagated within both the NLRI and the BGP-LS Attribute. The presence of unknown or unexpected TLVs MUST NOT result in the NLRI or the BGP-LS Attribute being considered malformed.¶

If the receiver doesn't recognize the new sub-TLV type defined in this document, it SHOULD skip it and process the remaining part in the SR Segment List TLV normally.¶

7. References

7.1. 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, <https://www.rfc-editor.org/info/rfc2119>.
[RFC8174]: Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC9552]: Talaulikar, K., Ed., "Distribution of Link-State and Traffic Engineering Information Using BGP", RFC 9552, DOI 10.17487/RFC9552, December 2023, <https://www.rfc-editor.org/info/rfc9552>.
[RFC9857]: Previdi, S., Talaulikar, K., Ed., Dong, J., Gredler, H., and J. Tantsura, "Advertisement of Segment Routing Policies Using BGP - Link State", RFC 9857, DOI 10.17487/RFC9857, October 2025, <https://www.rfc-editor.org/info/rfc9857>.

7.2. Informative References

[I-D.ietf-mpls-mna-hdr]: Rajamanickam, J., Gandhi, R., Zigler, R., Song, H., and K. Kompella, "MPLS Network Action (MNA) Sub-Stack Specification including In-Stack Network Actions and Data", Work in Progress, Internet-Draft, draft-ietf-mpls-mna-hdr-21, 24 February 2026, <https://datatracker.ietf.org/doc/html/draft-ietf-mpls-mna-hdr-21>.
[I-D.ietf-mpls-mna-ioam]: Gandhi, R., Mirsky, G., Li, T., Song, H., and B. Wen, "Supporting In Situ Operations, Administration and Maintenance Using MPLS Network Actions", Work in Progress, Internet-Draft, draft-ietf-mpls-mna-ioam-04, 20 November 2025, <https://datatracker.ietf.org/doc/html/draft-ietf-mpls-mna-ioam-04>.
[I-D.ietf-mpls-mna-nrp-selector]: Li, T., Beeram, V. P., Drake, J., Saad, T., and I. Meilik, "MPLS Network Actions for Network Resource Partition Selector", Work in Progress, Internet-Draft, draft-ietf-mpls-mna-nrp-selector-03, 23 December 2025, <https://datatracker.ietf.org/doc/html/draft-ietf-mpls-mna-nrp-selector-03>.
[RFC9256]: Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov, A., and P. Mattes, "Segment Routing Policy Architecture", RFC 9256, DOI 10.17487/RFC9256, July 2022, <https://www.rfc-editor.org/info/rfc9256>.