Extensions to RT-Constrain in Hierarchical Route Reflection Scenarios
draft-ietf-idr-rtc-hierarchical-rr-05

Network Working Group                                            J. Dong
Internet-Draft                                                   M. Chen
Intended status: Standards Track                     Huawei Technologies
Expires: 20 January 2027                                       R. Raszuk
                                                                  Arrcus
                                                            19 July 2026

 Extensions to RT-Constrain in Hierarchical Route Reflection Scenarios
                 draft-ietf-idr-rtc-hierarchical-rr-05

Abstract

   The Route Target (RT) Constrain mechanism specified in RFC 4684 is
   used to build a route distribution graph in order to restrict the
   propagation of Virtual Private Network (VPN) routes.  In network
   scenarios where hierarchical route reflection (RR) is used, the
   existing RT-Constrain mechanism cannot guarantee a correct route
   distribution graph.  This document describes the problem scenario and
   proposes a solution to address the RT-Constrain issue in hierarchical
   RR scenarios.

Requirements Language

   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].

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 20 January 2027.

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Copyright Notice

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Potential Solutions . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Add-path Based Solution . . . . . . . . . . . . . . . . .   4
     3.2.  Allowing Duplicated CLUSTER_ID  . . . . . . . . . . . . .   5
     3.3.  Disjoint Path Selection . . . . . . . . . . . . . . . . .   5
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   The Route Target (RT) Constrain mechanism specified in [RFC4684] is
   used to build a route distribution graph in order to restrict the
   propagation of Virtual Private Network (VPN) routes.  In network
   scenarios where hierarchical route reflection (RR) is used, the
   existing advertisment rules of RT membership information as defined
   in section 3.2 of [RFC4684] cannot guarantee a correct route
   distribution graph.

   This document describes the problem scenario and proposes a solution
   to address the RT-Constrain issue in hierarchical RR scenarios.

2.  Problem Statement

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                                +-----+
                        Clu-1   | RR-1|
                                +-----+
                                / / \ \
                        ------/  /   \  \--------
                       /        /     \          \
                     /  Clu-2  /       \   Clu-3  \
                 +-/---+  +---/-+      +\----+  +--\--+
                 |RR-21|  |RR-22|      |RR-31|  |RR-32|
                 +-----+  +-----+      +-----+  +-----+
                    | \    / |            | \    / |
                    |   \/   |            |   \/   |
                    |   /\   |            |   /\   |
                    | /    \ |            | /    \ |
                 +-----+  +-----+      +-----+  +-----+
                 | PE-1|  | PE-2|      | PE-3|  | PE-4|
                 +-----+  +-----+      +-----+  +-----+

                   RT-1     RT-1         RT-1     RT-1
                 +-----+  +-----+      +-----+  +-----+
                 |VPN-1|  |VPN-1|      |VPN-1|  |VPN-1|
                 +-----+  +-----+      +-----+  +-----+
                Figure 1. RT-Constrain with Hierarchical RRs

   As shown in Figure 1, hierarchical RRs are deployed in the network,
   RR-21, RR-22 and RR-31, RR-32 are level-1 route-reflectors which
   connect to the PEs, and are also the clients of the level-2 route-
   reflector RR-1.  RR-21 and RR22 are in RR cluster 1, RR-31 and RR-32
   are in RR cluster 2.  If each PE advertises RT membership information
   of RT-1 to the upstream RRs, after the best path selection, the
   level-1 RRs (RR-21, RR-22, RR-31 and RR-32 would create the
   CLUSTER_LIST attribute, prepend their local CLUSTER_ID and then
   advertise the best path to RR-1 and their clients respectively.

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   On receipt of the RT-Constrain routes from the level-1 RRs, RR-1
   selects one of the received routes as the best route, here assume the
   route received from RR-21 is selected by RR-1 as the best route.
   Then RR-1 needs to advertise the best RT-Constrain route to RR-21,
   RR-22, RR31 and RR-32 to create the route distribution graph of VPN-
   1.  RR-1 would prepend its CLUSTER_ID to the CLUSTER_LIST of the
   path.  According to the rules in Section 3.2 of [RFC4684], it sets
   the ORIGINATOR_ID to its own router-id, and sets the NEXT_HOP to the
   local address for the session.  Then RR-1 would advertise this route
   to RR-21, RR-22, RR-31 and RR-32 respectively.  On receipt of the RT-
   Constrain route from RR-1, RR-21 and RR-22 checks the CLUSTER_LIST
   and find its own CLUSTER_ID in the list, so this route will be
   ignored by RR-21 and RR-22.  As a result, RR-21 and RR-22 will not
   form the outbound filter of RT-1 towards RR-1, hence it will not
   advertise the VPN routes of VPN-1 to RR-1.

3.  Potential Solutions

   This document specifies 3 potential solutions for the RTC issue in
   hierarchical RR scenario.

3.1.  Add-path Based Solution

   This section provides one solution which is based on the add-path
   mechanism defined in [RFC7911].  It makes use of the add-path
   mechanism for RTC route advertisement between the hierarchical RRs.
   The solution is summerized as follows:

   *  The route-reflector clients which themselves are also route-
      reflectors SHOULD be identified, then BGP add-paths [RFC7911]
      SHOULD be enabled for RT membership NLRI on the BGP sessions
      between the higher layer RR and the lower layer RRs to ensure that
      sufficient RT-Constrain routes can be advertised by the higher
      layer RR to the lower layer RRs to pass BGP loop detection.  In
      this case normal BGP path advertisement rules as defined in
      [RFC4271] SHOULD be applied.  The number of RT-Constrain routes to
      be advertised with add-path mechanism is a local decision of
      operators.  To ensure that sufficient RT-Constrain routes are
      advertised to build the distribution graph, the RECOMMENDED add-
      path number is the maximum number of the BGP client sessions in
      the same cluster plus 1.

   *  When advertising an RT membership NLRI to a route-reflector client
      which is not a lower layer RR, the advertisement rule as defined
      in section 3.2 of [RFC4684] SHOULD be applied.

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   With the above advertisement rule, RR-1 in figure 1 SHOULD advertise
   to RR-21 the RT-Constrain routes received from RR-21, RR-22 and RR-
   31, then the RTC route from RR-31 will pass the BGP loop detection on
   RR-21 and RR-22, and the route from RR-21 will pass the BGP loop
   detection on RR-31 and RR-32, thus the route distribution graph can
   be set up correctly.

3.2.  Allowing Duplicated CLUSTER_ID

   This section provides another solution for the distribution of RT-
   Constrain routes in hierarchical RR scenarios.  As the RT-Constrain
   route may be ignored due to duplicated CLUSTER_ID in the CLUSTER_LIST
   of the received BGP Update, one knob can be introduced to control
   whether CLUSTER_ID duplication is allowed or not, and the times of
   duplication of any CLUSTER_ID allowed in the received CLUSTER_LIST
   can be configurable.

   In the scenario shown in Figure 1, this knob can be enabled on all
   the level-1 RRs (RR-21, RR-22, RR-31 and RR-32) respectively, so that
   the RT-Constrain route received from the RR-1 can pass the route
   selection, and the route distribution graph can be set up correctly.

3.3.  Disjoint Path Selection

   This section specifies one possible solution which proposes
   modifications to the intra-AS advertisement rule of RTC route.

   Since the advertisement of RT-Constrain route is to set up a route
   distribution graph and not to guide the data packet forwarding,
   actually all the available RT-Constrain routes should be considered
   in setting up the route distribution graph, not just the best one.
   Thus the following advertisment rule for RT membership information is
   proposed to replace the rule i and ii in section 3.2 of [RFC4684]:

   *  When advertising an RT membership NLRI to a route-reflector peer
      (either client or non-client), the route which has the most
      disjoint path comparing to the route received from this peer
      SHOULD be selected and advertised.  The most disjoint path is the
      path whose CLUSTER_LIST and ORIGINATOR_ID attributes are diverse
      from the attributes of the path received from this peer.  This is
      analagous to the mechanism specified in [RFC9107], where per-peer
      route selection is perfomed by the route reflector.

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   With the above advertisement rule, RR-1 in figure 1 would advertise
   to RR-21 the RT-Constrain route received from RR-31 or RR-32, which
   is the most disjoint alternative route compared with the route
   received from RR-21.  In this way, RR-21 will not discard the RT-
   constrain route received from RR-1, and the route distribution graph
   can be set up correctly.

4.  IANA Considerations

   This document makes no request of IANA.

5.  Security Considerations

   This document does not change the security properties of BGP based
   VPNs and [RFC4684].

6.  Acknowledgements

   The authors would like to thank Yaqun Xiao for the discussion of RT-
   Constrain issue in hierarchical RR scenario.  Many people have made
   valuable comments and suggestions, including Susan Hares, Jeffrey
   Haas, Stephane Litkowski, Vitkovský Adam, Xiaohu Xu, Uttaro James,
   Shyam Sethuram, Saikat Ray and Bruno Decraene.

7.  References

7.1.  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>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [RFC4684]  Marques, P., Bonica, R., Fang, L., Martini, L., Raszuk,
              R., Patel, K., and J. Guichard, "Constrained Route
              Distribution for Border Gateway Protocol/MultiProtocol
              Label Switching (BGP/MPLS) Internet Protocol (IP) Virtual
              Private Networks (VPNs)", RFC 4684, DOI 10.17487/RFC4684,
              November 2006, <https://www.rfc-editor.org/info/rfc4684>.

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   [RFC7911]  Walton, D., Retana, A., Chen, E., and J. Scudder,
              "Advertisement of Multiple Paths in BGP", RFC 7911,
              DOI 10.17487/RFC7911, July 2016,
              <https://www.rfc-editor.org/info/rfc7911>.

7.2.  Informative References

   [RFC9107]  Raszuk, R., Ed., Decraene, B., Ed., Cassar, C., Åman, E.,
              and K. Wang, "BGP Optimal Route Reflection (BGP ORR)",
              RFC 9107, DOI 10.17487/RFC9107, August 2021,
              <https://www.rfc-editor.org/info/rfc9107>.

Authors' Addresses

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

   Mach(Guoyi) Chen
   Huawei Technologies
   Huawei Campus, No. 156 Beiqing Rd.
   Beijing
   100095
   China
   Email: mach.chen@huawei.com

   Robert Raszuk
   Arrcus
   Email: robert@raszuk.net

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