Applicability of Border Gateway Protocol - Link State (BGP-LS) with Multi-Topology (MT) for Segment Routing based Network Resource Partitions (NRPs)
draft-ietf-idr-bgpls-sr-vtn-mt-13

IDR Working Group                                                 C. Xie
Internet-Draft                                                     C. Li
Intended status: Informational                             China Telecom
Expires: 2 April 2026                                            J. Dong
                                                                   Z. Li
                                                     Huawei Technologies
                                                       29 September 2025

  Applicability of Border Gateway Protocol - Link State (BGP-LS) with
     Multi-Topology (MT) for Segment Routing based Network Resource
                           Partitions (NRPs)
                   draft-ietf-idr-bgpls-sr-vtn-mt-13

Abstract

   When Segment Routing (SR) is used for building Network Resource
   Partitions (NRPs), each NRP can be allocated with a group of Segment
   Identifiers (SIDs) to identify the topology and resource attributes
   of network segments in the NRP.  This document describes how BGP-Link
   State (BGP-LS) with Multi-Topology (MT) can be used to distribute the
   information of SR based NRPs to a network controller when each NRP is
   associated with a separate logical network topology identified by a
   Multi-Topology ID (MT-ID).  This document sets out the targeted
   scenarios for the approach suggested, and presents the scalability
   limitations that arise.

Status of This Memo

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Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Advertisement of Topology Information for SR-based NRP  . . .   3
     2.1.  Intra-domain Topology Advertisement . . . . . . . . . . .   3
     2.2.  Inter-Domain Topology Advertisement . . . . . . . . . . .   4
   3.  Advertisement of Resource related TE Attribute for SR-based
           NRP . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Scalability Considerations  . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   [RFC9543] discusses the general framework, components, and interfaces
   for requesting and operating network slices using IETF technologies.
   [RFC9543] also introduces the concept of the Network Resource
   Partition (NRP), which is defined as a subset of the buffer/queuing/
   scheduling resources and associated policies on each of a connected
   set of links in an underlay network.  An NRP can be associated with a
   logical network topology to select or specify the set of links and
   nodes involved.  [RFC9732] specifies the framework of NRP-based
   enhanced VPNs and describes the candidate component technologies in
   different network planes and network layers.  An NRP could be used as
   the underlay to meet the requirement of one or a group of network
   slice or enhanced VPN services.  The mechanism of enforcing NRP
   resource allocation and the mechanism of mapping one or group of
   enhanced VPN services to a specific NRP is outside the scope of this
   document.

   [I-D.ietf-spring-resource-aware-segments] introduces resource
   awareness to Segment Routing (SR) [RFC8402].  As described in
   [I-D.ietf-spring-sr-for-enhanced-vpn], a group of resource-aware SIDs
   can be used to build SR-based NRPs with the required network topology

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   and network resource attributes.  The group of resource-aware SR SIDs
   together with the associated topology and resource attributes of an
   NRP need to be distributed in the network using IGP, and BGP-Link
   State (BGP-LS) [RFC9552] can be used to advertise the SR SIDs and the
   resource related Traffic Engineering (TE) attributes (e.g., link
   bandwidth) of NRPs in each IGP area or AS to a network controller.

   In some network scenarios, the required number of NRPs could be
   small, each NRP can be associated with an separate logical topology,
   i.e., there is 1:1 mapping between an NRP and an Multi-Topology (MT)
   ID, and a set of dedicated or shared network resources is allocated
   to the NRP.  [I-D.ietf-lsr-isis-sr-vtn-mt] describes how IS-IS Multi-
   Topology (MT) [RFC5120] can be used to advertise an independent
   topology and the associated SR SIDs, together with the resource
   related TE attributes for each SR based NRP in the network.  This
   document describes the how BGP-LS with MT can be used distribute the
   information of SR based NRPs to a network controller.The limitation
   and the targeted scenario of this approach are described in section 4
   "scalability considerations".

2.  Advertisement of Topology Information for SR-based NRP

   [I-D.ietf-lsr-isis-sr-vtn-mt] describes the IS-IS Multi-Topology
   based mechanisms to distribute the topology and the SR SIDs
   associated with SR based NRPs.  This section describes the
   corresponding BGP-LS mechanism to distribute both the intra-domain
   and inter-domain topology information and the SR SIDs of SR based
   NRPs.  It is considered that in each domain, one data plane mechanism
   is used for one NRP, while for inter-domain SR based NRPs, different
   data plane mechanisms (either SR-MPLS or SRv6) may be used in
   different domains.  For the inter-domain SR based NRPs, the involved
   network domains should be under a common administration, or they
   belong to the same trusted domain as specified in section 8 of
   [RFC8402].

2.1.  Intra-domain Topology Advertisement

   Section 5.2.2.1 of [RFC9552] defines the Multi-Topology Identifier
   (MT-ID) TLV (Type 263), which can contain one or more Multi-Topology
   Identifiers for a link, node, or prefix.  The MT-ID TLV may be
   included as a Link Descriptor, as a Prefix Descriptor, or in the BGP-
   LS Attribute of a Node Network Layer Reachability Information (NLRI),
   the detailed rules of the usage of MT-ID TLV in BGP-LS is described
   in section 5.2.2.1 of [RFC9552] .

   [RFC9085] defines the BGP-LS extensions to carry the SR-MPLS
   information using TLVs of BGP-LS Attribute.  When Multi-Topology is
   used with the SR-MPLS data plane, topology-specific Prefix-SIDs and

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   topology-specific Adjacency Segment Identifiers (Adj-SIDs) can be
   carried in the BGP-LS Attribute associated with the Prefix NLRI and
   Link NLRI respectively, the MT-ID TLV carried in the prefix
   descriptor or link descriptor [RFC9552] can be used to identify the
   corresponding topology of the SIDs.

   [RFC9514] defines the BGP-LS extensions to advertise Segment Routing
   over IPv6 (SRv6) information along with their functions and
   attributes.  When Multi-Topology is used with the SRv6 data plane,
   the SRv6 Locator TLV is carried in the BGP-LS Attribute associated
   with the Prefix NLRI, the MT-ID TLV can be carried as a Prefix
   Descriptor to identify the corresponding topology of the SRv6
   Locator.  The SRv6 End.X SIDs are carried in the BGP-LS Attribute
   associated with the Link NLRI, the MT-ID TLV can be carried in the
   link descriptor to identify the corresponding topology of the End.X
   SIDs.  The SRv6 SID NLRI is defined to advertise other types of SRv6
   SIDs, in which the SRv6 SID descriptors can include the MT-ID TLV so
   as to advertise topology-specific SRv6 SIDs.

2.2.  Inter-Domain Topology Advertisement

   [RFC9086] defines the BGP-LS extensions for BGP Egress Peer
   Engineering (EPE) with SR-MPLS.  The BGP-LS extensions for Egress
   Peer Engineering with SRv6 are specified in [RFC9514].  Such
   information could be used by a network controller for the collection
   of inter-domain topology and SR SID information, which can be used
   for the computation and instantiation of inter-AS SR-TE paths.

   In some network scenarios, for instance, an operator's network
   consists of multiple network parts, such as metro area networks,
   backbone networks, or data center networks, each part being a
   different AS.  Thus there may be a need to create NRPs which span
   multiple ASes.  The inter-domain NRPs may have different inter-domain
   logical topology, and may be associated with different subsets of
   network resources in each domain and also on the inter-domain links.
   To build multi-domain SR based NRPs, the inter-domain connectivity
   and the BGP peering SIDs associated with each logical topology on the
   inter-domain links need to be advertised.  This section describes the
   applicability of multi-topology for the advertisement of inter-domain
   topology and the associated SR SIDs using BGP-LS.  It does not
   introduce multi-topology into the operation of BGP sessions on the
   inter-domain links.

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   When an MT-ID is configured consistently in multiple domains covered
   by an NRP, the MT-ID may also be carried in the link NLRI of the
   inter-domain links for the advertisement of inter-domain logical
   topology and the topology-specific BGP peering SIDs.  This can be
   achieved with the combination of existing mechanisms as defined in
   [RFC9552][RFC9086] and [RFC9514].

   Depending on the different scenarios of inter-domain SR based NRPs,
   the approach for the inter-domain topology advertisement can be one
   of the following:

   *  One External BGP (EBGP) session between two ASes can be
      established over multiple underlying links.  In this case,
      different underlying links may be used for different inter-domain
      NRPs.  In another similar case, the EBGP session is established
      over a single physical link, while the network resource (e.g.,
      bandwidth) on this link is partitioned into multiple pieces, each
      of which is instantiated as a logical sub-interface.  Each
      underlying physical or logical link is associated with the MT-ID
      of the NRP, and different BGP Peer-Adj-SIDs or SRv6 End.X SIDs
      need to be allocated to each underlying physical or logical link.
      The association between the underlying physical of logical link
      and the corresponding MT-ID, together with the BGP Peer-Adj-SIDs
      or SRv6 End.X SID need to be advertised by the ASBR to a network
      controller.

   *  For inter-domain connection between two ASes, multiple EBGP
      sessions can be established between different sets of peering
      ASBRs.  It is possible that some of these BGP peers are only used
      for one inter-domain NRP, while some other BGP peers are used for
      another inter-domain NRP.  In this case, different BGP Peer Node
      SIDs can be allocated to steer traffic to a specific peer within
      an inter-domain NRP.  The association between the link of the BGP
      peering session and the corresponding MT-ID, together with the BGP
      Peer Node SIDs need to be advertised by the ASBR to a network
      controller.

   *  At the level inter-AS topology, different inter-domain NRPs may
      have different inter-AS connectivity.  In this case, different BGP
      Peer Set SIDs may be allocated to represent a groups of BGP peers
      which can be used for load-balancing within each inter-domain NRP.
      The BGP Peer Set SIDs may be advertised in the BGP-LS attributes
      of the link NLRI which carries the MT-ID of the corresponding NRP.

   In network scenarios where consistent allocation of MT-ID among
   multiple domains can not be achieved, the MT-ID advertised by the two
   peering ASBRs to a network controller for the same inter-domain link
   could be different.  Some mapping mechanism may be needed by the

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   controller to match the MT-IDs of an inter-domain link in two
   directions (e.g., for one inter-domain link, MT-ID A in domain X will
   be matched with MT-ID B in domain Y), and concatenate the inter-
   domain topology of the NRP.  The detailed mechanism is out of the
   scope of this document.

3.  Advertisement of Resource related TE Attribute for SR-based NRP

   [I-D.ietf-lsr-isis-sr-vtn-mt] describes the applicability of IS-IS
   multi-topology for the advertisement of resource related TE
   attributes associated with each SR based NRP.  This section describes
   the applicability of BGP-LS with multi-topology for reporting
   resource related TE attributes of each SR based NRP to network
   controllers.

   The information of the network resources attributes associated with a
   link of an NRP can be specified by carrying the corresponding TE Link
   attribute TLVs in BGP-LS Attribute [RFC9552], with the associated MT-
   ID carried in the corresponding Link NLRI.

   For example, the amount of bandwidth resource allocated to an NRP on
   a link can be advertised by carrying the Maximum Link Bandwidth sub-
   TLV in the BGP-LS Attribute associated with the Link NLRI which
   carries the MT-ID of the NRP.  The bandwidth allocated to an NRP can
   be exclusive for traffic carried by the corresponding NRP.  The
   advertisement of other topology-specific TE attributes in BGP-LS for
   NRP is for further study.  The receiving BGP-LS speaker should be
   prepared to receive any TE attributes in BGP-LS Attribute with the
   associated MT-ID carried in the corresponding Link NLRI.

4.  Scalability Considerations

   The mechanism described in this document assumes that each NRP is
   associated with an independent topology, and for the inter-domain
   NRPs, the MT-IDs used in the involved domains are consistent, so that
   the associated MT-ID can be used to identify the NRP in the control
   plane.  Reusing MT-ID can avoid introducing new mechanisms with
   similar functionality in the control plane, while it also has some
   limitations.  For example, even if multiple NRPs share the same
   topology, each NRP still need to be identified using a unique MT-ID
   in the control plane.  Thus independent path computation needs be
   executed for each NRP.  The number of NRPs supported in a network may
   be dependent on the number of topologies supported, which is related
   to both the number of topologies supported in the protocol and the
   control plane overhead which the network could afford.  Since no new
   control protocol extension is required, the mechanism described in
   this document is considered useful for network scenarios in which the
   required number of NRPs is small (e.g., less than 10).  For network

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   scenarios where the number of required NRPs is large, more scalable
   solutions would be needed which may require further protocol
   extensions and enhancements.  A detailed analysis about the NRP
   scalability and the possible optimizations for supporting a large
   number of NRPs are described in [I-D.ietf-teas-nrp-scalability].

5.  Security Considerations

   The security considerations in [RFC9552] [RFC9085] and [RFC9514]
   apply to this document.

   This document introduces no additional security vulnerabilities to
   BGP-LS.  The mechanism proposed in this document is subject to the
   same vulnerabilities as any other protocol that relies on BGP-LS.

6.  IANA Considerations

   This document does not request any IANA actions.

7.  Acknowledgments

   The authors would like to thank Shunwan Zhuang, Adrian Farrel, Susan
   Hares, Jeffrey Haas and Ketan Talaulikar for the review and
   discussion of this document.

8.  References

8.1.  Normative References

   [I-D.ietf-lsr-isis-sr-vtn-mt]
              Xie, C., Ma, C., Dong, J., and Z. Li, "Applicability of
              IS-IS Multi-Topology (MT) for Segment Routing based
              Network Resource Partition (NRP)", Work in Progress,
              Internet-Draft, draft-ietf-lsr-isis-sr-vtn-mt-10, 13 April
              2025, <https://datatracker.ietf.org/doc/html/draft-ietf-
              lsr-isis-sr-vtn-mt-10>.

   [I-D.ietf-spring-resource-aware-segments]
              Dong, J., Miyasaka, T., Zhu, Y., Qin, F., and Z. Li,
              "Introducing Resource Awareness to SR Segments", Work in
              Progress, Internet-Draft, draft-ietf-spring-resource-
              aware-segments-15, 3 September 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-spring-
              resource-aware-segments-15>.

   [I-D.ietf-spring-sr-for-enhanced-vpn]
              Dong, J., Miyasaka, T., Zhu, Y., Qin, F., and Z. Li,
              "Segment Routing based Network Resource Partition (NRP)

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              for Enhanced VPN", Work in Progress, Internet-Draft,
              draft-ietf-spring-sr-for-enhanced-vpn-09, 10 June 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-spring-
              sr-for-enhanced-vpn-09>.

   [RFC5120]  Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
              Topology (MT) Routing in Intermediate System to
              Intermediate Systems (IS-ISs)", RFC 5120,
              DOI 10.17487/RFC5120, February 2008,
              <https://www.rfc-editor.org/info/rfc5120>.

   [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>.

   [RFC9085]  Previdi, S., Talaulikar, K., Ed., Filsfils, C., Gredler,
              H., and M. Chen, "Border Gateway Protocol - Link State
              (BGP-LS) Extensions for Segment Routing", RFC 9085,
              DOI 10.17487/RFC9085, August 2021,
              <https://www.rfc-editor.org/info/rfc9085>.

   [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", RFC 9086, DOI 10.17487/RFC9086, August
              2021, <https://www.rfc-editor.org/info/rfc9086>.

   [RFC9514]  Dawra, G., Filsfils, C., Talaulikar, K., Ed., Chen, M.,
              Bernier, D., and B. Decraene, "Border Gateway Protocol -
              Link State (BGP-LS) Extensions for Segment Routing over
              IPv6 (SRv6)", RFC 9514, DOI 10.17487/RFC9514, December
              2023, <https://www.rfc-editor.org/info/rfc9514>.

   [RFC9543]  Farrel, A., Ed., Drake, J., Ed., Rokui, R., Homma, S.,
              Makhijani, K., Contreras, L., and J. Tantsura, "A
              Framework for Network Slices in Networks Built from IETF
              Technologies", RFC 9543, DOI 10.17487/RFC9543, March 2024,
              <https://www.rfc-editor.org/info/rfc9543>.

   [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>.

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   [RFC9732]  Dong, J., Bryant, S., Li, Z., Miyasaka, T., and Y. Lee, "A
              Framework for NRP-Based Enhanced Virtual Private
              Networks", RFC 9732, DOI 10.17487/RFC9732, March 2025,
              <https://www.rfc-editor.org/info/rfc9732>.

8.2.  Informative References

   [I-D.ietf-teas-nrp-scalability]
              Dong, J., Li, Z., Gong, L., Yang, G., and G. S. Mishra,
              "Scalability Considerations for Network Resource
              Partition", Work in Progress, Internet-Draft, draft-ietf-
              teas-nrp-scalability-07, 2 March 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-teas-
              nrp-scalability-07>.

Authors' Addresses

   Chongfeng Xie
   China Telecom
   China Telecom Beijing Information Science & Technology, Beiqijia
   Beijing
   102209
   China
   Email: xiechf@chinatelecom.cn

   Cong Li
   China Telecom
   China Telecom Beijing Information Science & Technology, Beiqijia
   Beijing
   102209
   China
   Email: licong@chinatelecom.cn

   Jie Dong
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Road
   Beijing
   100095
   China
   Email: jie.dong@huawei.com

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   Zhenbin Li
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Road
   Beijing
   100095
   China
   Email: lizhenbin@huawei.com

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