BGP-LS Extension for Inter-AS Topology Retrieval
draft-ietf-idr-bgpls-inter-as-topology-ext-18

IDR Working Group                                                A. Wang
Internet-Draft                                             China Telecom
Intended status: Standards Track                                 H. Chen
Expires: 14 May 2026                                          Individual
                                                           K. Talaulikar
                                                           Cisco Systems
                                                               S. Zhuang
                                                     Huawei Technologies
                                                                  C. Lin
                                                    New H3C Technologies
                                                        10 November 2025

            BGP-LS Extension for Inter-AS Topology Retrieval
             draft-ietf-idr-bgpls-inter-as-topology-ext-18

Abstract

   This document describes the process to distribute Border Gateway
   Protocol- Link State (BGP-LS) key parameters for inter-domain links
   between two Autonomous Systems.  This document defines a new type
   within the BGP-LS NLRI for a Stub Link and three new type-length-
   values (TLVs) for BGP-LS Link descriptor.  These additions to BGP-LS
   let Software Definition Network (SDN) controllers retrieve the
   network topology automatically under various inter-AS environments.

   Such extension and process can enable the network operator to collect
   the interconnect information between different domains and then
   calculate the overall network topology automatically based on the
   information provided by BGP-LS protocol.

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
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   This Internet-Draft will expire on 14 May 2026.

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Copyright Notice

   Copyright (c) 2025 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
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions used in this document . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Inter-AS Domain Scenarios . . . . . . . . . . . . . . . . . .   3
   5.  Stub Link NLRI  . . . . . . . . . . . . . . . . . . . . . . .   4
   6.  IGP Information for Inter-AS Link . . . . . . . . . . . . . .   6
   7.  BGP-LS Extensions for Inter-AS Link . . . . . . . . . . . . .   6
     7.1.  Remote AS Number TLV  . . . . . . . . . . . . . . . . . .   7
     7.2.  IPv4 Remote ASBR ID . . . . . . . . . . . . . . . . . . .   8
     7.3.  IPv6 Remote ASBR ID . . . . . . . . . . . . . . . . . . .   8
   8.  Topology Reconstruction.  . . . . . . . . . . . . . . . . . .   9
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
     10.1.  New BGP-LS NLRI type . . . . . . . . . . . . . . . . . .  10
     10.2.  New Stub Link Descriptors  . . . . . . . . . . . . . . .  10
   11. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  10
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     12.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   BGP-LS [RFC9552] describes the methodology that using BGP protocol to
   transfer the Link-State information.  Such method can enable SDN
   controller to collect the underlay network topology automatically,
   but normally it can only get the information within one Interior
   Gateway Protocol (IGP) domain.  If the operator has more than one IGP
   domain, and these domains interconnect with each other, there is no
   mechanic within current BGP- LS to transfer the interconnect topology
   information.

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   [RFC9086] defines some extensions for exporting BGP peering node
   topology information (including its peers, interfaces and peering
   ASs) in a way that is exploitable in order to compute efficient BGP
   Peering Engineering policies and strategies.  Such information can
   also be used to calculate the interconnection topology among
   different IGP domains, but it requires every border router to run
   BGP-LS protocol and report the information to SDN controller.
   Considering there will be plenty of border routers on the network
   boundary, such solution restricts its deployment flexibility.

   This draft analyzes the situations during which the SDN controller
   needs to get the interconnected topology information between
   different AS domains.  After describing these situations, this draft
   defines a new Stub Link type within the BGP-LS NLRI[RFC9552] to
   describe the Iner-AS link and some new TLVs for that new BGP-LS type.
   After that, the SDN controller can then deduce the multi-domain
   topology automatically based on the information from BGP-LS protocol.

2.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119] .

3.  Terminology

   The following terms are defined in this document:

   *  IDCs: Internet Data Centers

   *  MAN: Metrio-Area-Network

   *  SDN: Software Definition Network

4.  Inter-AS Domain Scenarios

   Figure 1 illustrates the multi-domain scenarios that this draft
   discusses.  Normally, SDN Controller can get the topology of IGP A
   and IGP B individually via the BGP-LS protocol, but it can't get the
   topology connection information between these two IGP domains because
   there is generally no IGP protocol run on the connected links.

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                             +-----------------+
                        +----+IP SDN Controller+----+
                        |    +-----------------+    |
                        |                           |
                        |BGP-LS                     |BGP-LS
                        |                           |
        +---------------+-----+               +-----+--------------+
        | +--+        +-++   ++-+           +-++   +|-+        +--+|
        | |S1+--------+S2+---+B1+-----------+B2+---+T1+--------+T2||
        | +-++   N1   +-++   ++-+           +-++   ++++   N2   +-++|
        |   |           |     |               |     ||           | |
        |   |           |     |               |     ||           | |
        | +-++        +-++   ++-+           +-++   ++++        +-++|
        | |S4+--------+S3+---+B3+-----------+B4+---+T3+--------+T4||
        | +--+        +--+   ++-+           +-++   ++-+        +--+|
        |                     |               |                    |
        |                     |               |                    |
        |       IGP A         |               |      IGP B         |
        +---------------------+               +--------------------+

                    Figure 1: Inter-AS Domain Scenarios

5.  Stub Link NLRI

   [RFC9552] defines four types within the BGP Link-State NLRI (Node
   NLRI, Link NLRI, IPv4 Topology Prefix NLRI, and IPv6 Topology Prefix
   NLRI) to transfer the topology and prefix information.  For inter-as
   link, the two ends of the link exist in different IGP domains, so it
   is not appropriate to transfer their information within the current
   defined NLRI types.

   This draft defines one new NLRI type 7, see__Section 10) within the
   BGP Link-State (BGP-LS) NLRI, called the Stub link NLRI.  The Stub
   link NLRI is encoded in the format shown in Figure 2 and explained
   below:

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        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
       +-+-+-+-+-+-+-+-+
       |  Protocol-ID  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Identifier                          |
       |                            (64 bits)                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       //              Local Node Descriptors (variable)              //
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       //               Stub Link Descriptors (variable)              //
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 2: Stub Link NLRI Format

   The "Protocol-ID" should be set to the value that indicates the
   source protocol of the stub link information, as indicated in
   [RFC9552] in section 5.2.

   Local Node descriptors: define the ASBRs that are attached to the
   Inter-AS stub link, and use the [RFC9552] “Local Node Descriptor” in
   section 5.2.1.4.  The following Node Descriptor sub-TLVs from
   [RFC9552] are valid for inclusion in the local Node descriptor: AS
   system, OSPF Area-ID, IGP Router-ID.

   Stub Link Descriptors: define the Stub link which has only one end
   Located in the IGP domain using the [RFC9552] “Link Descriptor
   definition” in section 5.2.2 with the exceptions noted below.

   The Stub link Descriptor supports the inclusion of the following
   subTLVs:

   • Link/Local Identifier (TLV 258, [RFC9552]),

   • IPv4 Interface Address (TLV 259, [RFC9552]),

   • IPv4 Neighbor Address (TLV 260,[RFC9552]),

   • IPv6 Interface Address (TLV 261, [RFC9552]),

   • IPv6 Neighbor Address (TLV 262, [RFC9552]),

   • Multi-topology identifier (TLV 263, [RFC9552]),

   • Remote-AS Number (TLV 270, [This document], section Section 7.1),

   • IPv4 Remote ASBR ID [TLV 271, [This document], section
   Section 7.2), and

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   • IPv6 Remote ASBR ID [TLV 272, [This document], section
   Section 7.3).

   This newly defined NLRI can be used to describe the link that has
   only one end located within the IGP domain, as described in the
   following sections.  The Node and Link Descriptor sub-TLVs and Node
   and Link attributes that are defined in [RFC9552] can be included in
   the NLRI if necessary.  The interface and neighbor address sub- TLVs
   SHOULD be included in the Local Node Descriptors to differentiate the
   parallel links between two ASBRs.

6.  IGP Information for Inter-AS Link

   [RFC9346] and [RFC5392] define IS-IS and OSPF extensions respectively
   to deal with the reasons for reporting inter-AS link.  Three sub-TLVs
   relating to Inter-Domain Links (Remote AS Number、IPv4 Remote ASBR ID、
   and IPv6 Remote ASBR ID) are defined in these documents. .

   These IGP TLVs are flooded within the IGP domain automatically.  This
   document specifies that these MAY also be carried within the newly
   defined Stub Link NLRI within the BGP-LS protocol, as the descriptors
   for the inter-AS stub link.  The "Local Node Descriptors" in the Stub
   Link NLRI within the BGP-LS NLRI should describe the characteristics
   of ASBRs that are connected across the inter-AS links.

   If the SDN controller knows these information via one of the interior
   router that runs BGP-LS protocol, the SDN controller can rebuild the
   inter-AS topology correctly according to the procedure described in
   Section 8

7.  BGP-LS Extensions for Inter-AS Link

   This section proposes to add three new TLVs to be supported in the
   Stub Link NLRI in the BGP-LS NLRI.  These new TLVs allow BGP-LS to
   transfer inter-AS information gathered by the SDN controller.

   The following Link Descriptor TLVs are added into the BGP-LS protocol
   :

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   +-----------+---------------------+--------------+----------------+
   |  TLV Code | Description         |IS-IS/OSPF TLV| Reference      |
   |   Point   |                     |   /Sub-TLV   | (RFC/Section)  |
   +-----------+---------------------+--------------+----------------+
   |    270    |Remote AS Number     |   24/21      | [RFC9346]/3.3.1|
   |           |                     |              | [RFC5392]/3.3.1|
   |    271    |IPv4 Remote ASBR ID  |   25/22      | [RFC9346]/3.3.2|
   |           |                     |              | [RFC5392]/3.3.2|
   |    272    |IPv6 Remote ASBR ID  |   26/24      | [RFC9346]/3.3.3|
   |           |                     |              | [RFC5392]/3.3.3|
   +-----------+---------------------+--------------+----------------+
                Figure 3: Stub Link Descriptor TLVs

   Detail encoding of these TLVs are synchronized with the corresponding
   parts in [RFC9346] and [RFC5392], which keeps the BGP-LS protocol
   agnostic to the underly protocol.

7.1.  Remote AS Number TLV

   A new TLV, the remote AS number TLV, is defined for inclusion in the
   link descriptor when advertising inter-AS links.  The remote AS
   number TLV specifies the AS number of the neighboring AS to which the
   advertised link connects.

   The remote AS number TLV is TLV type 270 (see Section 10 ) and is 4
   octets in length.  The format is as follows:

    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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Remote AS Number                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Figure 4: Remote AS Number TLV Format

   The Remote AS number field has 4 octets.  When only 2 octets are used
   for the AS number, as in current deployments, the left (high-order) 2
   octets MUST be set to 0.  The remote AS number TLV MUST be included
   when a router advertises an inter-AS link.

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7.2.  IPv4 Remote ASBR ID

   A new TLV, which is referred to as the IPv4 remote ASBR ID TLV, is
   defined for inclusion in the link descriptor when advertising inter-
   AS links.  The IPv4 remote ASBR ID TLV specifies the IPv4 identifier
   of the remote ASBR to which the advertised inter-AS link connects.
   This could be any stable and routable IPv4 address of the remote
   ASBR.  Use of the TE Router ID as specified in the Traffic
   Engineering router ID TLV [RFC9346] is RECOMMENDED.

   The IPv4 remote ASBR ID TLV is TLV type 271 (see Section 10) and is 4
   octets in length.  The format of the IPv4 remote ASBR ID TLV is as
   follows:

    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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Remote ASBR ID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               Figure 5:  IPv4 Remote ASBR ID TLV Format

   The IPv4 remote ASBR ID TLV MUST be included if the neighboring ASBR
   has an IPv4 address.  If the neighboring ASBR does not have an IPv4
   address (not even an IPv4 TE Router ID), the IPv6 remote ASBR ID TLV
   MUST be included instead.  An IPv4 remote ASBR ID TLV and IPv6 remote
   ASBR ID TLV MAY both be present in an inter-AS stub link NLRI.

7.3.  IPv6 Remote ASBR ID

   A new TLV, which is referred to as the IPv6 remote ASBR ID TLV, is
   defined for inclusion in the link descriptor when advertising inter-
   AS links.  The IPv6 remote ASBR ID TLV specifies the IPv6 identifier
   of the remote ASBR to which the advertised inter-AS link connects.
   This could be any stable and routable IPv6 address of the remote
   ASBR.  Use of the TE Router ID as specified in the IPv6 Traffic
   Engineering router ID TLV [RFC9346] is RECOMMENDED.

   The IPv6 remote ASBR ID TLV is TLV type 272 (see Section 10) and is
   16 octets in length.  The format of the IPv6 remote ASBR ID TLV is as
   follows:

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    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            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Remote ASBR ID                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Remote ASBR ID (continued)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Remote ASBR ID (continued)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Remote ASBR ID (continued)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Figure 6:  IPv6 Remote ASBR ID TLV Format

   The IPv6 remote ASBR ID TLV MUST be included if the neighboring ASBR
   has an IPv6 address.  If the neighboring ASBR does not have an IPv6
   address, the IPv4 remote ASBR ID TLV MUST be included instead.  An
   IPv4 remote ASBR ID TLV and IPv6 remote ASBR ID TLV MAY both be
   present in an inter-AS stub link NLRI.

8.  Topology Reconstruction.

   When SDN controller gets such information from BGP-LS protocol, it
   should find information from the associated router.  Based on this
   information it can create a logical topology that contains the link
   between these two border routers.  Iterating the above procedures for
   all of the stub links, the SDN controller can automatically retrieve
   the Inter-AS connection topology.

9.  Security Considerations

   BGP-LS security is described in [RFC9552].  This addition to BGP-LS
   focuses on the case when one network operated by a single entity has
   several IGP domains that are composited by its backbone network and
   several MANs (Metro-Area- Networks) and Internet Data Centers (IDCs).
   The configuration of these networks operated by the single
   administrative entity creates a “walled garden”. Within this single
   Administrative Domain, the network operator needs to monitor and
   engineer traffic flows that traverse such a network that spans
   multiple Autonomous Systems.  The network operators can obtain this
   information on inter-as topology via the process described in this
   draft.  Using the passive-interface features or configuring the
   Traffic Engineering (TE) parameters on the interconnect links will
   not provide the real-time Information for this single Administrative
   Domain.

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10.  IANA Considerations

   This document defines:

   *  A new BGP NLRI Type: Stub Link NLRI.  The codepoint is from the
      "BGP-LS NLRI Types"

   *  Three new Link Descriptors TLV: Remote AS Number TLV, IPv4 Remote
      ASBR ID, IPv6 Remote ASBR ID.  The codepoint are from "BGP-LS Node
      Descriptor, Link Descriptor, Prefix Descriptor, and Attribute
      TLVs" registry.

10.1.  New BGP-LS NLRI type

   This document defines a new value in the registry "BGP-LS NLRI
   Types":

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Code Point |   Description |           Status              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       7       | Stub Link NLRI|    Allocation from IANA       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Figure 7:  Stub Link NLRI Codepoint

10.2.  New Stub Link Descriptors

   This document defines three new values in the registry "BGP-LS NLRI
   and Attribute TLVs":

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Code Point |   Description         |             Status      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      270      | Remote AS Number      | Allocation from IANA    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      271      |  IPv4 Remote ASBR ID  | Allocation from IANA    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      272      |  IPv6 Remote ASBR ID  | Allocation from IANA    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Figure 8:  BGP-LS Link Descriptors TLV

11.  Acknowledgement

   The author would like to thank Susan Hares, Acee Lindem, Jie Dong,
   Shaowen Ma, Jeff Tantsura and Dhruv Dhody for their valuable comments
   and suggestions.

12.  References

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12.1.  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>.

   [RFC5392]  Chen, M., Zhang, R., and X. Duan, "OSPF Extensions in
              Support of Inter-Autonomous System (AS) MPLS and GMPLS
              Traffic Engineering", RFC 5392, DOI 10.17487/RFC5392,
              January 2009, <https://www.rfc-editor.org/info/rfc5392>.

   [RFC9346]  Chen, M., Ginsberg, L., Previdi, S., and D. Xiaodong, "IS-
              IS Extensions in Support of Inter-Autonomous System (AS)
              MPLS and GMPLS Traffic Engineering", RFC 9346,
              DOI 10.17487/RFC9346, February 2023,
              <https://www.rfc-editor.org/info/rfc9346>.

   [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>.

12.2.  Informative References

   [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>.

Authors' Addresses

   Aijun Wang
   China Telecom
   Beiqijia Town, Changping District
   Beijing
   Beijing, 102209
   China
   Email: wangaj3@chinatelecom.cn

   Huaimo Chen
   Individual
   Boston, MA
   United States of America
   Email: hchen.ietf@gmail.com

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   Ketan Talaulikar
   Cisco Systems
   India
   Email: ketant.ietf@gmail.com

   Shunwan Zhuang
   Huawei Technologies
   Huawei Building, No.156 Beiqing Rd.
   Beijing
   100095
   China
   Email: zhuangshunwan@huawei.com

   Changwang Lin
   New H3C Technologies
   Huawei Building, No.156 Beiqing Rd.
   Beijing
   China
   Email: linchangwang.04414@h3c.com

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