Skip to main content

Advertisement of Stub Link Attributes
draft-wang-lsr-stub-link-attributes-08

Document Type Active Internet-Draft (candidate for lsr WG)
Authors Aijun Wang , Zhibo Hu, Gyan Mishra , Jinsong Sun
Last updated 2024-01-05 (Latest revision 2023-11-24)
Replaces draft-wang-lsr-passive-interface-attribute
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Formats
Additional resources Mailing list discussion
Stream WG state Call For Adoption By WG Issued
Other - see Comment Log
Document shepherd Christian Hopps
IESG IESG state I-D Exists
Consensus boilerplate Yes
Telechat date (None)
Responsible AD (None)
Send notices to chopps@chopps.org
draft-wang-lsr-stub-link-attributes-08
LSR Working Group                                                A. Wang
Internet-Draft                                             China Telecom
Intended status: Standards Track                                   Z. Hu
Expires: 27 May 2024                                 Huawei Technologies
                                                               G. Mishra
                                                            Verizon Inc.
                                                                  J. Sun
                                                         ZTE Corporation
                                                        24 November 2023

                 Advertisement of Stub Link Attributes
                 draft-wang-lsr-stub-link-attributes-08

Abstract

   This document describes the mechanism that can be used to advertise
   the stub link attributes within the IS-IS or OSPF domain.

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 27 May 2024.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

Wang, et al.               Expires 27 May 2024                  [Page 1]
Internet-Draft    Advertisement of Stub Link Attributes    November 2023

   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
   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.  Conventions used in this document . . . . . . . . . . . . . .   3
   3.  Consideration for Identifying Stub Link . . . . . . . . . . .   3
   4.  OSPF Protocol Extension for Stub Link Attributes  . . . . . .   3
     4.1.  OSPF Stub-Link TLV  . . . . . . . . . . . . . . . . . . .   4
     4.2.  OSPF Stub Link IPv4 Prefix Sub-TLV  . . . . . . . . . . .   5
     4.3.  OSPF Stub Link IPv6 Prefix Sub-TLV  . . . . . . . . . . .   5
   5.  IS-IS Protocol Extension for Stub Link Attributes . . . . . .   6
     5.1.  IS-IS Stub-link TLV . . . . . . . . . . . . . . . . . . .   6
     5.2.  IS-IS Stub Link IPv4 Prefix Sub-TLV . . . . . . . . . . .   7
     5.3.  IS-IS Stub Link IPv6 Prefix Sub-TLV . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Applied Scenarios  . . . . . . . . . . . . . . . . .   9
     A.1.  Inter-AS topology recovery  . . . . . . . . . . . . . . .  10
     A.2.  Egress Engineering for Anycast Servers  . . . . . . . . .  11
     A.3.  Egress Engineering for the path to BGP Next-hop . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   Stub links are used commonly within enterprise or service provider
   networks.  One common use case is the inter-AS routing scenario where
   there are no IGP adjacencies between the adjacent BGP domains,
   another use case is at the network boundary that the interfaces are
   used to connect to the application servers.

Wang, et al.               Expires 27 May 2024                  [Page 2]
Internet-Draft    Advertisement of Stub Link Attributes    November 2023

   For operators that have multiple ASes interconnecting with each other
   via the stub links, there is a requirement to obtain the inter-AS
   topology information as described in
   [I-D.ietf-idr-bgpls-inter-as-topology-ext].  To achieve such goal, it
   is required that the BGP-LS to be enabled on every router that has
   the stub links, which is challenging for the network operation.  It
   is desirable to advertise the stub link info into the IGP to ease the
   deployment of BGP-LS on any router in the IGP domain.

   For stub links that are used to connect the servers, knowing the
   status of these stub links can facilitate the routers within the IGP
   to accomplish TE tasks in some scenarios.

   But OSPF and IS-IS have no capability to identify such stub links and
   their associated attributes now.

   This document defines the protocol extension for OSPFv2/v3 and IS-IS
   to indicate the stub links and their associated attributes.

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 [RFC2119] .

3.  Consideration for Identifying Stub Link

   OSPF[RFC5392] defines the Inter-AS-TE-v2 LSA and Inter-AS-TE-v3 LSA
   to carry the TE information about inter-AS links.  IS-IS[RFC9346]
   defines the Inter-AS Reachability TLV to carry the TE information
   about inter-AS links.  But they are normally being used under RSVP-
   TE, especially inter-domain RSVP-TE scenarios.  As illustrated in the
   potential scenarios that described in Appendix A, there is still the
   need for a generic solution which also covers non inter-AS stub
   links.

   Then, to solve the problems that described in the applied scenarios,
   this document defines the Stub-Link TLV and some additional sub-TLVs
   to identify the stub link and transmit the associated attributes for
   OSPF and IS-IS respectively.

4.  OSPF Protocol Extension for Stub Link Attributes

   The following sections define the protocol extension to indicate the
   stub link and its associated attributes in OSPFv2/v3.

Wang, et al.               Expires 27 May 2024                  [Page 3]
Internet-Draft    Advertisement of Stub Link Attributes    November 2023

4.1.  OSPF Stub-Link TLV

   This document defines the Stub-Link TLV to describe stub link of a
   single router.  This Stub-Link TLV is only applicable to the Inter-
   AS-TE-v2 LSA and Inter-AS-TE-v3 LSA [RFC5392].  Inclusion in other
   LSAs MUST be ignored.

   The OSPF Stub-Link TLV which is under the IANA codepoint "Top Level
   Types in TE LSAs" 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(Stub-Link)            |      Length                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |U|        Flags                |    Reserved                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   Stub Link Prefix Sub-TLVs                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                  Existing Sub-TLVs (variable)                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                 Figure 1: OSPF Stub-Link TLV

   Type: The TLV type.  The value is 7(TBD) for OSPF Stub-Link.

   Length: Variable, dependent on sub-TLVs

   Flags: Define the type of the stub-link:

   *  U bit(bit 0): Identify the unnumbered stub link if this bit is
      set.

   *  bit 1-15: Reserved

   Stub Link Prefix Sub-TLV: The prefix of the stub-link.  It's format
   is defined in Section 4.2 and Section 4.3.

   Existing Sub-TLVs: Sub-TLV that defined within "Open Shortest Path
   First (OSPF) Traffic Engineering TLVs" for TE Link TLV(Value 2) can
   be included if necessary.

   If the stub-link is identified as unnumbered stub link (U bit is
   set), then the "Remote IPv4 Address Sub-TLV" or "Remote Interface
   IPv6 Address Sub-TLV", which should be set to the identifier value of
   remote router, SHOULD be included to facilitate the pairing of inter-
   AS link.

Wang, et al.               Expires 27 May 2024                  [Page 4]
Internet-Draft    Advertisement of Stub Link Attributes    November 2023

   If this TLV is advertised multiple times in the same Inter-AS-TE-v2/
   v3 LSA, only the first instance of the TLV is used by receiving
   OSPFv2/v3 routers.  This situation SHOULD be logged as an error.

   If this TLV is advertised multiple times for the same link in
   different Inter-AS-TE-v2/v3 LSA originated by the same OSPFrouter,
   the OSPFStub-Link TLV in these LSAs with the smallest Opaque ID is
   used by receiving OSPFrouters.  This situation may be logged as a
   warning.

   It is RECOMMENDED that OSPF routers advertising OSPF Stub-Link TLVs
   in different OSPF Inter-AS-TE v2/v3 LSAs re-originate these LSAs in
   ascending order of Opaque ID to minimize the disruption.

   This document creates a registry for Stub-Link attributes in
   Section 7.

4.2.  OSPF Stub Link IPv4 Prefix Sub-TLV

   The OSPF Stub Link IPv4 Prefix 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              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Prefix Length |           IPv4 Prefix(variable)               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             Figure 2: OSPF Stub Link IPv4 Prefix Sub-TLV

   Type: IPv4 Prefix Sub-TLV codepoint.  Value is 37(TBD) for OSPF(under
   "Types for sub-TLVs of TE Link TLV (Value 2)")

   Length: The length of the value portion in octets.

   Prefix Length: the length of the IPv4 Prefix in bits.

   IPv4 Prefix: The IPv4 Prefix value of stub link.

4.3.  OSPF Stub Link IPv6 Prefix Sub-TLV

   The OSPF Stub Link IPv6 Prefix Sub-TLV has the following format:

Wang, et al.               Expires 27 May 2024                  [Page 5]
Internet-Draft    Advertisement of Stub Link Attributes    November 2023

      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              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Prefix Length |           IPv6 Prefix(variable)               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            Figure 3: OSPF Stub Link IPv6 Prefix Sub-TLV

   Type: IPv6 Prefix Sub-TLV codepoint.  Value is Value is 38(TBD) for
   OSPF(under "Types for sub-TLVs of TE Link TLV (Value 2)")

   Length: The length of the value portion in octets.

   IPv6 Prefix: The IPv6 Prefix value of stub link.

5.  IS-IS Protocol Extension for Stub Link Attributes

   The following sections define the protocol extension to indicate the
   stub link and its associated attributes in IS-IS.

5.1.  IS-IS Stub-link TLV

   This document defines the IS-IS Stub-Link TLV to describes stub link
   of a single router.

   The IS-IS Stub-Link 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(Stub-Link)|    Length     |U|       Flags                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |               Stub Link Prefix Sub-TLV                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |               Existing Sub-TLVs(Variable)                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                 Figure 4: IS-IS Stub-Link TLV

   Type: IS-IS TLV codepoint.  Value is 151 (TBD) for stub-link TLV.

   Length: Variable, dependent on sub-TLVs

   Flags: Define the type of the stub-link:

   *  0: U bit(bit 0): Identify the unnumbered stub link if this bit is
      set.

Wang, et al.               Expires 27 May 2024                  [Page 6]
Internet-Draft    Advertisement of Stub Link Attributes    November 2023

   *  bit 1-15: Reserved

   Stub Link Prefix Sub-TLV: The prefix of the stub-link.  It's format
   is defined in Section 5.2 and Section 5.3.

   Existing Sub-TLVs: Sub-TLVs that defined within "IS-IS Sub-TLVs for
   TLVs Advertising Neighbor Information" can be included if necessary.

   If the stub-link is identified as unnumbered stub link type (U bit is
   set), then the "IPv4 Remote ASBR ID" or "IPv6 Remote ASBR ID" Sub-TLV
   SHOULD be included to facilitate the pairing of inter-AS link.

5.2.  IS-IS Stub Link IPv4 Prefix Sub-TLV

   The IS-IS Stub Link IPv4 Prefix 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    |      Reserved   |Prefix Length|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    IPv4 Prefix(Variable)                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              Figure 5: IS-IS Stub Link IPv4 Prefix Sub-TLV

   Type: IPv4 Prefix Sub-TLV codepoint.  Value is 46(TBD) for IS-
   IS(under "IS-IS Sub-TLVs for TLVs Advertising Neighbor Information")

   Length: Length: The length of the value portion in octets.

   Prefix Length: the length of the IPv4 Prefix in bits.

   IPv4 Prefix: The IPv4 Prefix value of stub link.

5.3.  IS-IS Stub Link IPv6 Prefix Sub-TLV

   The IS-IS Stub Link IPv6 Prefix 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    |      Reserved   |Prefix Length|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |               IPv6 Prefix(Variable)                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             Figure 6: IS-IS Stub Link IPv6 Prefix Sub-TLV

Wang, et al.               Expires 27 May 2024                  [Page 7]
Internet-Draft    Advertisement of Stub Link Attributes    November 2023

   Type: IPv6 Prefix Sub-TLV codepoint.  Value is 47(TBD) for IS-
   IS(under "IS-IS Sub-TLVs for TLVs Advertising Neighbor Information")

   Length: Length: The length of the value portion in octets.

   Prefix Length: the length of the IPv6 Prefix in bits.

   IPv6 Prefix: The IPv6 Prefix value of stub link.

6.  Security Considerations

   Security concerns for IS-IS are addressed in [RFC5304] and[RFC5310]

   Security concern for OSPFv3 is addressed in [RFC4552]

   Advertisement of the additional information defined in this document
   introduces no new security concerns.

7.  IANA Considerations

   IANA is requested to the allocation in following registries:

+===========================================+============+===============================+
| Registry                                  | Type       |       Meaning                 |
|                                           |(suggested) |                               |
+===========================================+============+===============================+
|Top Level Types in TE LSAs                 |   7        |OSPF Stub-Link                 |
+-------------------------------------------+------------+-------------------------------+
|IS-IS Top-Level TLV                        |   151      |IS-IS Stub-Link                |
+-------------------------------------------+------------+-------------------------------+
|Types for sub-TLVs of TE Link TLV (Value 2)|   37       |OSPF Stub Link IPv4 Prefix     |
+-------------------------------------------+------------+-------------------------------+
|Types for sub-TLVs of TE Link TLV (Value 2)|   38       |OSPF Stub Link IPv6 Prefix     |
+-------------------------------------------+------------+-------------------------------+
|IS-IS Sub-TLVs for TLVs                    |            |                               |
|Advertising Neighbor Information           |   46       |IS-IS Stub Link IPv4 Prefix    |
+-------------------------------------------+------------+-------------------------------+
|IS-IS Sub-TLVs for TLVs                    |            |                               |
|Advertising Neighbor Information           |   47       |IS-IS Stub Link IPv6 Prefix    |
+-------------------------------------------+------------+-------------------------------+
   Figure 7: IANA Allocation for newly defined TLVs and Sub-TLVs

8.  Acknowledgement

   Thanks Ketan Talaulikar, Acee Lindem, Shunwan Zhang, Peter Psenak,
   Tony Li, Les Ginsberg, Dhruv Dhody, Jeff Tantsura and Robert Raszuk
   for their suggestions and comments on this idea.

Wang, et al.               Expires 27 May 2024                  [Page 8]
Internet-Draft    Advertisement of Stub Link Attributes    November 2023

9.  References

9.1.  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>.

   [RFC4552]  Gupta, M. and N. Melam, "Authentication/Confidentiality
              for OSPFv3", RFC 4552, DOI 10.17487/RFC4552, June 2006,
              <https://www.rfc-editor.org/info/rfc4552>.

   [RFC5304]  Li, T. and R. Atkinson, "IS-IS Cryptographic
              Authentication", RFC 5304, DOI 10.17487/RFC5304, October
              2008, <https://www.rfc-editor.org/info/rfc5304>.

   [RFC5310]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
              and M. Fanto, "IS-IS Generic Cryptographic
              Authentication", RFC 5310, DOI 10.17487/RFC5310, February
              2009, <https://www.rfc-editor.org/info/rfc5310>.

   [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>.

9.2.  Informative References

   [I-D.ietf-idr-bgpls-inter-as-topology-ext]
              Wang, A., Chen, H., Talaulikar, K., and S. Zhuang, "BGP-LS
              Extension for Inter-AS Topology Retrieval", Work in
              Progress, Internet-Draft, draft-ietf-idr-bgpls-inter-as-
              topology-ext-13, 3 April 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-idr-
              bgpls-inter-as-topology-ext-13>.

Appendix A.  Applied Scenarios

   The following sections describe the scenarios that knowing the stub
   link related attributes information can help solve the corresponding
   necessity in questions.

Wang, et al.               Expires 27 May 2024                  [Page 9]
Internet-Draft    Advertisement of Stub Link Attributes    November 2023

A.1.  Inter-AS topology recovery

   Figure 1 describes the scenario that the necessity of inter-AS
   topology recovery for Native IP point-to-point stub link scenario.

   R10, R11 and R12 are located in AS1.  R20, R21,R22 are located in
   AS2.  The controller runs BGP-LS with R10 in AS1 and R20 in AS2
   respectively.

   There is one BGP session among the border router R11 and R21, which
   are connected by several stub links(passive interfaces) between them.
   The situation within the R21 and R22 are the same.

   Since the links between the border routers are passive, there will be
   no IGP neighbors between them.  The BGP-LS information carried in
   each AS will not report these stub links,and the controller can't
   recovery the inter-AS topology automatically.

                               +----------+
                +--------------+Controller+--------------+
                ^              +----------+              ^
                |                                        |
          BGP-LS|                                        |BGP-LS
                |            +---+BGP+----+              |
           +-----------------+            +-------------------+
           |    |            |            |              |    |
           |    |        +---+-+        +-+---+          |    |
           |    |        |     +--------+     |          |    |
           |    +--------+R11  |--------|  R21+----------+    |
           |    |        |     +--------+     |          |    |
           |    |        +---+-+        +-+---+          |    |
           | +--+--+         |            |           +--+--+ |
           | |     |         |            |           |     | |
           | |R10  |         |            |           |R20  | |
           | |     |         |            |           |     | |
           | +--+--+     +---+-+        +-+---+       +--+--+ |
           |    |        |     +--------+     |          |    |
           |    +--------+R12  |--------|  R22+----------+    |
           |             |     +--------+     |               |
           |             +---+-+        +-+---+               |
           |                 |            |                   |
           +--------AS1------+            +----------AS2------+
                             +---+BGP+----+

              Figure 1: Inter-AS Topology Recovery(P2P Scenario)

Wang, et al.               Expires 27 May 2024                 [Page 10]
Internet-Draft    Advertisement of Stub Link Attributes    November 2023

   Figure 2 describes the similar situation but in LAN environment.  The
   border routers of AS1, AS2 and AS3 are connected via one LAN
   interfaces(that is to say, the corresponding interfaces on R1, R2 and
   R3 are on the same subnet).  There are three different BGP sessions
   from the loopback address of the border routers among them
   respectively.  It is necessary to recovery the underlying inter-AS
   topology automatically.

           +---------+                      +---------+
          |         |                      |         |
          |      +--+--+                +--+--+      |
          |      |     |                |     |      |
          |      |R1   +-------+ +------+R2   |      |
          |      |     |       | |      |     |      |
          |      +--+--+       | |      +--+--+      |
          |         |          | |         |         |
          +---AS1---+          | |         +----AS2--+
                             +-+-+-+
                             |     |
                             | SW  |
                             |     |
                             +--+--+
                                |
                             +--+--+
                          +--+     +--+
                          |  |  R3 |  |
                          |  +-----+  |
                          |           |
                          |           |
                          +-----AS3---+

              Figure 2: Inter-AS Topology Recovery(LAN Scenario)

A.2.  Egress Engineering for Anycast Servers

   Figure 3 describes the scenario that the stub link information can be
   used for egress engineering for Anycast servers that connected to the
   network.  In the example, the R1, R2 and R3 are border routers which
   are connected directly the server S1, S2 and S3 that have the same IP
   address IPa.  The characteristics of the stub links that connected to
   these Anycast servers are different.  It will be help for the router
   R0, to know the attributes of the stub links and select the optimal
   Anycast server to serve the customer's application.

Wang, et al.               Expires 27 May 2024                 [Page 11]
Internet-Draft    Advertisement of Stub Link Attributes    November 2023

            +----------------+
            |                |
            |            +---+-+      +-----+
            |            |     |      | S1  |
            |    +-------+R1   +------+     |
            |    |       |     +---+  |(IPa)|
            |    |       +---+-+   |  +-----+
            |    |           |     |
            | +--+--+    +---+-+   |  +-----+
            | |     |    |     |   +--| S2  |
            | |R0   +----+R2   +------+     |
            | |     |    |     |   +--|(IPa)|
            | +--+--+    +---+-+   |  +-----+
            |    |           |     |
            |    |       +---+-+   |  +-----+
            |    |       |     |---+  | S3  |
            |    +-------+R3   +------+     |
            |            |     |      |(IPa)|
            |            +---+-+      +-----+
            |                |
            +----------------+

                Figure 3: Egress Engineering for Anycast Server

A.3.  Egress Engineering for the path to BGP Next-hop

   Figure 4 describes the scenario that the stub link information can
   facilitate the selection of path to BGP next hop.  The router R10 and
   R20 which are located in different AS establish the BGP session
   directly.The attributes of the stub links among the border routers
   are vary.  It is certainly will be helpful for the router R10 and R20
   to select the optimized forwarding path to BGP next hop, that is via
   the stub links among them, to reach each other.

Wang, et al.               Expires 27 May 2024                 [Page 12]
Internet-Draft    Advertisement of Stub Link Attributes    November 2023

                |<---------------+BGP+------------------>|
                |                                        |
                |                                        |
           +-----------------+            +-------------------+
           |    |            |            |              |    |
           |    |        +---+-+        +-+---+          |    |
           |    |        |     +--------+     |          |    |
           |    +--------+R11  |--------|  R21+----------+    |
           |    |        |     +--------+     |          |    |
           |    |        +---+-+        +-+---+          |    |
           | +--+--+         |            |           +--+--+ |
           | |     |         |            |           |     | |
           | |R10  |         |            |           |R20  | |
           | |     |         |            |           |     | |
           | +--+--+     +---+-+        +-+---+       +--+--+ |
           |    |        |     +--------+     |          |    |
           |    +--------+R12  |--------|  R22+----------+    |
           |             |     +--------+     |               |
           |             +---+-+        +-+---+               |
           |                 |            |                   |
           +-------+AS1+-----+            +---------+AS2+-----+

               Figure 4: Optimized BGP next hop selection

Authors' Addresses

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

   Zhibo Hu
   Huawei Technologies
   Huawei Bld., No.156 Beiqing Rd.
   Beijing
   100095
   China
   Email: huzhibo@huawei.com

Wang, et al.               Expires 27 May 2024                 [Page 13]
Internet-Draft    Advertisement of Stub Link Attributes    November 2023

   Gyan S. Mishra
   Verizon Inc.
   13101 Columbia Pike
   Silver Spring,  MD 20904
   United States of America
   Email: gyan.s.mishra@verizon.com

   Jinsong Sun
   ZTE Corporation
   No. 68, Ziijnhua Road
   Nan Jing
   210012
   China
   Email: sun.jinsong@zte.com.cn

Wang, et al.               Expires 27 May 2024                 [Page 14]