IGP Color-Aware Shortcut
draft-cheng-lsr-igp-shortcut-enhancement-09

Network Working Group                                          W. Cheng
Internet Draft                                                  L. Gong
Intended status: Informational                             China Mobile
Expires: April 17, 2026                                          C. Lin
                                                   New H3C Technologies
                                                                R. Chen
                                                        ZTE Corporation
                                                       October 14, 2025

                         IGP Color-Aware Shortcut
                draft-cheng-lsr-igp-shortcut-enhancement-09

Abstract

   IGP shortcut mechanism allows calculating routes to forward traffic
   over Traffic Engineering tunnels. This document specifies the
   enhancement of IGP shortcut which can steer routes onto TE-tunnels
   based on colors.

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Table of Contents

   1. Introduction...................................................2
      1.1. Requirements Language.....................................3
   2. Colors of IGP Prefixes.........................................3
   3. Colors of TE-Tunnels...........................................4
   4. IGP Color-Aware Shortcut.......................................4
   5. Use Case.......................................................5
   6. Management Considerations......................................6
   7. Security Considerations........................................6
   8. IANA Considerations............................................6
   9. References.....................................................6
      9.1. Normative References......................................6
      9.2. Informational References..................................7
   Authors' Addresses................................................7

1. Introduction

   [RFC3906] describes how IGP calculate routes to forward traffic over
   Traffic Engineering tunnels. Such mechanism is also referred to as
   IGP shortcut.

   The granularity of IGP shortcut is based on nodes. If the first-hop
   of a node is determined to be a TE-tunnel during the SPF
   computation, all routes to IP prefixes advertised by that node will
   be over that TE-tunnel. For example, in the following topology, X1
   and X2 are IP prefixes advertised by rtrC, and Y1 and Y2 are IP
   prefixes advertised by rtrD. Using IGP shortcut, all routes to X1,
   X2, Y1 and Y2 will be steered onto T1 and T2.

           === T1(8) ===>
           === T2(8) ===>
         rtrA -- rtrB -- rtrC -- rtrD
              10      10  |   10  |
                         X1,X2   Y1,Y2

                      Figure 1:  IGP shortcut Topology

   However, in some scenarios, there may be requirements to steer the
   routes to different prefixes of the same node onto different TE-
   tunnels. For example, the traffic flows to X1 and Y1 need to be

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   forwarded over low delay tunnel T1, but the traffic flows to X2 and
   Y2 need to be forwarded over high bandwidth tunnel T2.

   In the BGP-based service, "color" is often used to indicate the
   intent of forwarding [RFC9012] [RFC9252]. The Color Extended
   Community can be attached to BGP routes, and the associated flows
   will be steered into tunnels with the same color.

   This document specifies the enhancement of IGP shortcut which can
   steer routes onto TE-tunnels based on colors.

   In [RFC3906], the term "TE-tunnel" mainly refers to Label Switched
   Path, such as MPLS RSVP-TE tunnel. With the development of Segment
   Routing (SR) technology, SR Policy [RFC9256] becomes a useful tool
   for Traffic Engineering. In the context of this document, SR
   Policies are also included as TE-tunnels.

            1.1. Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2. Colors of IGP Prefixes

   A prefix can be associated with one or more colors. Administrative
   tags are used to advertising the colors for IGP prefixes.

   For IS-IS, the 32-bit Administrative Tag Sub-TLV defined in
   [RFC5130] can be used to associate one or more 32-bit tags with a
   prefix. For OSPFv2 and OSPFv3, the 32-bit Administrative Tag Sub-TLV
   defined in [RFC9825] provides the similar function.

   Since multiple tags for different applications may be attached to
   one prefix, there has to be a mechanism for a receiver to know which
   tags are used as colors. For example, the tag space can be split and
   some of it can be used to signal colors.

   The value of a tag and the value of color indicated by that tag can
   simply be equal. Alternatively, there can be a mapping relationship
   between them. For example, the value of color can be calculated from
   the value of tag by applying a mask over it.

   How to obtain the color(s) of a prefix from the associated tags, is
   governed by local policy and uniform within the same IGP domain.

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3. Colors of TE-Tunnels

   The main idea of this document is to steer the flows to colored
   prefixes into tunnels with the same color.

   Some kinds of TE-tunnels, such as SR Policy [RFC9256], have inherent
   color values which can be directly used to match the colors of
   prefixes. For the TE-tunnels which have no inherent color, the color
   values may be determined by local configurations.

4. IGP Color-Aware Shortcut

   The SPF computation for IGP shortcuts is described in Section 2 of
   [RFC3906], and the metric adjustment for IGP shortcuts is described
   in Section 4 of [RFC3906].

   This document does not modify the SPF calculation for IGP shortcuts.
   Instead, it introduces the following additional steps when
   calculating next hops for prefixes advertised by a node:

   o If a prefix is colored, we look up the first-hop information of
      the advertiser node for TE-tunnels with the same color.

        - If there are eligible TE-tunnels, we compare the costs of
          paths over those TE-tunnels, and use the next-hop of the TE-
          tunnel with the lowest path cost.

        - If there is no eligible TE-tunnel, we use the native
          adjacency next-hop.

   o If a prefix has no color, we use the next-hop with the lowest
      path cost.

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5. Use Case

      TE-tunnel1(SR Policy1)       ===============================
      (color100, low delay)        |SLA requirements:            |
      +-------------------------+  |  Prefix-1,3: low delay      |
      |          8              |  |  Prefix-2,4: high bandwidth |
      |TE-tunnel2(SR Policy2)   |  ===============================
      |(Color200,high bandwidth)|
      |  +------------------+   |        Prefix-3
      |  |       8          |   |        Prefix-4
     ++--++                 ++--++        +----+        +----+
     | R1 +-----------------+ R2 +--------+ R3 +--------+ R4 |
     +----+      10         +----+   10   +----+   10   +----+
                            Prefix-1
                            Prefix-2

   Routing calculation on R1:
     Traditional Shortcut:
       Prefix-1,2,3,4
         Next-hop1: SR Policy 1
        Next-hop2: SR Policy 2

     Color Aware Shortcut:
       Prefix-1,3
         Next-hop: SR Policy 1
       Prefix-2,4
         Next-hop: SR Policy 2

                       Figure 2:  IGP Color Aware Shortcut

   Between R1 and R3, there are two SP policies: SR Policy 1, which has
   a color of 100 and corresponds to a low-latency SLA (Service-level
   Agreement) required path, and SR Policy 2, which has a color of 200
   and corresponds to a high-bandwidth SLA-required path.

   In the case of a traditional TE Shortcut, after R1's route
   calculation, the next hops for Prefix-1, Prefix-2, Prefix-3, and
   Prefix-4 all point to SR Policy 1 and SR Policy 2.

   In the case of a Color-aware TE Shortcut, the next hops for Prefix-1
   and Prefix-3 point to SR Policy 1, while the next hops for Prefix-2
   and Prefix-4 point to SR Policy 2.

   This approach ensures that traffic is routed according to the
   specific SLA requirements, improving network efficiency and
   performance.

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6. Management Considerations

   Implementations MAY allow configuration of policies that:

   o Assign color values to prefixes.

   o Map these colors to prefix tags for advertisement
      [RFC5130][RFC9825].

   During prefix calculation, implementations MAY support local
   conversion of prefix tags to colors through configuration.

   When a TE tunnel is not associated with a color, implementations MAY
   permit manual configuration of the color attribute.

   Furthermore, implementations MAY support:

   o Color-based TE tunnel matching during IGP shortcut computation.

   o Policy-based control over which prefixes are included in the
      computation.

7. Security Considerations

   This document does not change the security aspects of IS-IS or OSPF.
   Security considerations specific to each protocol still apply.

8. IANA Considerations

   This document has no IANA actions.

9. References

            9.1. Normative References

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3906] Shen, N. and H. Smit, "Calculating Interior Gateway
             Protocol (IGP) Routes Over Traffic Engineering Tunnels",
             RFC 3906, DOI 10.17487/RFC3906, October 2004,
             <https://www.rfc-editor.org/info/rfc3906>.

   [RFC5130] Previdi, S., Shand, M., Ed., and C. Martin, "A Policy
             Control Mechanism in IS-IS Using Administrative Tags", RFC
             5130, DOI 10.17487/RFC5130, February 2008,
             <https://www.rfc-editor.org/info/rfc5130>.

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   [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
             2119 Key Words", BCP 14, RFC 8174, May 2017

   [RFC9256] Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,
             A., and P. Mattes, "Segment Routing Policy Architecture",
             RFC 9256, DOI 10.17487/RFC9256, July 2022,
             <https://www.rfc-editor.org/info/rfc9256>

   [RFC9825] Lindem, A., Psenak, P., and Y. Qu, "Extensions to OSPF for
             Advertising Prefix Administrative Tags", RFC9825, DOI
             10.17487/RFC9825, July 2025, < https://www.rfc-
             editor.org/info/rfc9825>.

            9.2. Informational References

   [RFC9012] Patel, K., Van de Velde, G., Sangli, S., and J. Scudder,
             "The BGP Tunnel Encapsulation Attribute", RFC 9012, DOI
             10.17487/RFC9012, April 2021, <https://www.rfc-
             editor.org/info/rfc9012>.

   [RFC9252] Dawra, G., Ed., Talaulikar, K., Ed., Raszuk, R., Decraene,
             B., Zhuang, S., and J. Rabadan, "BGP Overlay Services
             Based on Segment Routing over IPv6 (SRv6)", RFC 9252, DOI
             10.17487/RFC9252, July 2022, <https://www.rfc-
             editor.org/info/rfc9252>.

Authors' Addresses

   Weiqiang Cheng
   China Mobile
   China
   Email: chengweiqiang@chinamobile.com

   Liyan Gong
   China Mobile
   China
   Email: gongliyan@chinamobile.com

   Changwang Lin
   New H3C Technologies
   China
   Email: linchangwang.04414@h3c.com

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   Ran Chen
   ZTE Corporation
   Email: chen.ran@zte.com.cn

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