Advertisement of SR Policy Operational States using BGP Link-State
draft-ietf-idr-bgp-ls-sr-policy-admin-flags-01
| Document | Type | Active Internet-Draft (idr WG) | |
|---|---|---|---|
| Authors | Changwang Lin , Zafar Ali , Yisong Liu , Ran Chen , Amal Karboubi | ||
| Last updated | 2026-10-20 | ||
| Replaces | draft-ali-idr-bgp-ls-sr-policy-sl-opt-distribution, draft-lin-idr-bgp-ls-sr-policy-admin-flag | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| On agenda | idr at IETF-127 | ||
| Document shepherd | (None) | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-idr-bgp-ls-sr-policy-admin-flags-01
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
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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.
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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:
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- 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.
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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.
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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.
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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.
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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, <https://www.rfc-editor.org/info/rfc9857>.
[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,
<https://www.rfc-editor.org/info/rfc9830>.
[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.
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[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,<https://datatracker.ietf.org/doc/html/draft-ietf-
spring-srv6-policy-sid-list-opt-00>.
[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, <https://datatracker.ietf.org/doc/html/draft-all-
pce-srv6-policy-sid-list-optimization-04>.
[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>.
[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, <https://www.rfc-editor.org/info/rfc8402>.
[RFC9256] Filsfils, C., Talaulikar, K., 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>.
[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, <https://www.rfc-editor.org/info/rfc8664>.
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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
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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
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