Applicability of Bidirectional Forwarding Detection (BFD) for Multi-point Networks in Virtual Router Redundancy Protocol (VRRP)
draft-ietf-rtgwg-vrrp-p2mp-bfd-15

RTGWG                                                          G. Mirsky
Internet-Draft                                                Individual
Intended status: Standards Track                             J. Tantsura
Expires: 3 February 2027                                          NVIDIA
                                                               G. Mishra
                                                              Individual
                                                           2 August 2026

  Applicability of Bidirectional Forwarding Detection (BFD) for Multi-
      point Networks in Virtual Router Redundancy Protocol (VRRP)
                   draft-ietf-rtgwg-vrrp-p2mp-bfd-15

Abstract

   This document specifies the applicability of Bidirectional Forwarding
   Detection in multipoint networks to support sub-second failure
   detection for Virtual Router Redundancy Protocol Router Role
   election.  The mechanism enables faster determination of the Active
   Router without requiring any modification to the protocol behavior or
   message formats defined in RFC 9568.

Status of This Memo

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   This Internet-Draft will expire on 3 February 2027.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions used in this document . . . . . . . . . . . .   3
       1.1.1.  Terminology . . . . . . . . . . . . . . . . . . . . .   3
       1.1.2.  Requirements Language . . . . . . . . . . . . . . . .   3
   2.  Applicability of p2mp BFD . . . . . . . . . . . . . . . . . .   3
     2.1.  Multipoint BFD Encapsulation  . . . . . . . . . . . . . .   4
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  Operational Considerations  . . . . . . . . . . . . . . . . .   5
     5.1.  Mixed-mode Operation  . . . . . . . . . . . . . . . . . .   5
     5.2.  Scaling Considerations in Multi-tenant Environment  . . .   6
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   6
   8.  Informative References  . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   The [RFC9568] is the current Virtual Router Redundancy Protocol
   (VRRP) specification for IPv4 and IPv6 networks.  VRRPv3 allows for a
   faster switchover to a Backup Router.  A router may be part of
   several Virtual Router Redundancy groups, such as Active in some and
   Backup in others.  Supporting sub-second mode for VRRPv3 [RFC9568]
   for all these roles without specialized support in the data plane may
   prove challenging because of the increased load on the control plane.
   However, it may still be possible to deploy VRRP and provide sub-
   second detection of Active Router failure by Backup Routers.

   Bidirectional Forwarding Detection (BFD) [RFC5880] had been
   originally defined to detect failure of point-to-point paths: single-
   hop [RFC5881], multihop [RFC5883].  Single-hop BFD may enable a
   Backup router to detect an Active router failure within sub-seconds.
   But this approach may present a scaling challenge in a multi-Backup
   router environment.

   [RFC8562] extends [RFC5880] for multipoint and multicast networks,
   which matches the deployment scenarios for VRRP over the LAN segment.
   This document demonstrates how point-to-multipoint (p2mp) BFD can
   enable faster detection of the Active Router failure and thus
   minimize service disruption in a VRRP domain.

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1.1.  Conventions used in this document

1.1.1.  Terminology

   BFD: Bidirectional Forwarding Detection

   p2mp: Point-to-Multipoint

   VRRP: Virtual Router Redundancy Protocol

1.1.2.  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.  Applicability of p2mp BFD

   [RFC8562] may provide an efficient and scalable solution for a fast-
   converging environment that uses the default route rather than
   dynamic routing.  Each redundancy group presents itself as a p2mp BFD
   session, with its Active Router being the head and Backup Routers
   being the tails of the p2mp BFD session.

   The Active Router, configured to use p2mp BFD to support faster
   convergence of VRRP, starts transmitting BFD control packets with
   IPvX address associated with the Virtual Router [RFC9568] as a source
   IP address and the value of the My Discriminator field ([RFC5880])
   locally selected according to the following rules:

      For IPv4, the Active Router uses 32 bits of the IPv4 source
      address as described in Section 5.1.1.1 of [RFC9568].

      For IPv6, the Active Router uses the 32 least-significant bits of
      IPv6 source address, as described in Section 5.1.2.1 of [RFC9568].

   A Backup Router demultiplexes p2mp BFD test sessions based on IPvX
   address associated with the Virtual Router that it has been
   configured with and the non-zero My Discriminator value, it deduces
   from the received VRRP Advertisement packet according to the rules
   listed above.  When a Backup router detects the failure of the Active
   Router, according to Section 5.11 [RFC8562], it re-evaluates its role
   in the Virtual Router.  As a result, the Backup Router may become the
   Active router of the given Virtual Router or continue as a Backup
   Router.

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      If the former is the case, then the new Active router will start
      transmitting p2mp BFD control packets using the Active Router IP
      address as the source IP address for p2mp BFD control packets and
      thus bootstraps a new p2mp BFD session on a Backup Router.

      If the latter is the case, the Backup Router MUST close and remove
      the p2mp BFD session associated with the failed Active Router.
      The VRRP Advertisement packet from the new VRRP Active Router will
      bootstrap the new p2mp BFD session.

2.1.  Multipoint BFD Encapsulation

   The MultipointHead of p2mp BFD session when transmitting BFD control
   packet:

      Set the source MAC address according to rules in Section 7.3 of
      [RFC9568];

      MUST set TTL or Hop Limit value to 255 (Section 5 [RFC5881]).
      Similarly, all received BFD Control packets that are demultiplexed
      to the session MUST be discarded if the received TTL or Hop Limit
      is not equal to 255;

      SHOULD use group address VRRP ('224.0.0.18' for IPv4 and
      'FF02:0:0:0:0:0:0:12' for IPv6) as destination IP address;

      MAY use network broadcast address for IPv4 or link-local all nodes
      multicast group for IPv6 as destination IP address;

      MUST set destination UDP port value to 3784 when transmitting BFD
      control packets, as defined in [RFC8562];

      Source UDP port value selection follows the rules defined in
      Section 4 of [RFC5881];

      MUST use the Active Router IP address as the source IP address.

3.  IANA Considerations

   This document makes no requests for IANA allocations.  This section
   may be deleted by RFC Editor.

4.  Security Considerations

   This document defines an alternative mechanism, complementary to the
   one specified in [RFC9568], for accelerating the detection of
   failures that affect VRRP functionality by using p2mp BFD.  The
   operation of either protocol is not modified.

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   Security considerations discussed in [RFC9568], [RFC5880], [RFC5881],
   and [RFC8562], apply to this document.  Using p2mp BFD in the VRRP
   context does not introduce additional security considerations beyond
   those already covered in the referenced RFCs.

5.  Operational Considerations

5.1.  Mixed-mode Operation

   In deployments where a VRRP group contains routers that support the
   use of p2mp BFD as described in this specification and others that do
   not, the mechanism continues to operate correctly, but convergence
   characteristics will differ among routers.  Routers that support p2mp
   BFD will detect failure of the Active Router based on BFD session
   state and may transition to Active more quickly than routers relying
   solely on VRRP Advertisement timers.  Conversely, if the Active
   Router does not support p2mp BFD, all routers, regardless of BFD
   capability, will converge based on VRRP Advertisement timeout.

   In a VRRP group where both VRRP-native and BFD-based mechanisms are
   used for detecting failure of the Active Router, the system can be
   viewed as a multi-layer OAM environment, with p2mp BFD operating as a
   lower-layer and faster failure-detection mechanism.  In such an
   environment, operators should ensure that VRRP Advertisement
   intervals and BFD timer values are coordinated so that the expected
   convergence behavior is well understood.

   Consistent with multi-layer OAM design principles, the failure-
   detection interval at the lower layer is typically configured to be
   at least three times shorter than the failure-detection interval at
   the layer above it.  This relationship ensures that the faster
   mechanism (p2mp BFD) reliably triggers failover before the VRRP-
   native mechanism, while still allowing VRRP Advertisements to provide
   a backup detection method.

   In mixed-mode deployments, routers within the VRRP group may detect
   failure of the Active Router at different times, depending on whether
   they rely on p2mp BFD or VRRP Advertisements.  From the host's
   perspective, however, failover is determined by the fastest detection
   mechanism that results in a new Active Router being elected and
   advertising the virtual router address.

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5.2.  Scaling Considerations in Multi-tenant Environment

   In multi-tenant deployments, multiple VRRP groups may exist on the
   same segment, each maintaining its own p2mp BFD session.  The scaling
   impact depends primarily on the operator's convergence objectives.
   If sub-second convergence is required, an operator may choose either
   sub-second VRRP Advertisement intervals or sub-second BFD
   transmission intervals.  Using p2mp BFD for failure detection allows
   VRRP Advertisement intervals to remain relatively large (e.g., one
   second), thereby reducing the overall volume of VRRP control traffic
   even when many VRRP groups are present.

   Because p2mp BFD uses a single multipoint session per VRRP group, the
   incremental overhead scales linearly with the number of groups.  In
   environments with a large number of VRRP groups, operators should
   ensure that BFD transmission intervals and VRRP Advertisement
   intervals are configured to balance convergence requirements with the
   control plane load.

6.  Acknowledgements

   The authors appreciate the comments and suggestions provided by Acee
   Lindem and Alexander "Sasha" Vainshtein, which helped simplify and
   improve the solution.  The authors also express their gratitude to
   Joe Clarke for his valuable insights into the application and
   deployment aspects of p2mp BFD in VRRP.

7.  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>.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
              <https://www.rfc-editor.org/info/rfc5880>.

   [RFC5881]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881,
              DOI 10.17487/RFC5881, June 2010,
              <https://www.rfc-editor.org/info/rfc5881>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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   [RFC8562]  Katz, D., Ward, D., Pallagatti, S., Ed., and G. Mirsky,
              Ed., "Bidirectional Forwarding Detection (BFD) for
              Multipoint Networks", RFC 8562, DOI 10.17487/RFC8562,
              April 2019, <https://www.rfc-editor.org/info/rfc8562>.

   [RFC9568]  Lindem, A. and A. Dogra, "Virtual Router Redundancy
              Protocol (VRRP) Version 3 for IPv4 and IPv6", RFC 9568,
              DOI 10.17487/RFC9568, April 2024,
              <https://www.rfc-editor.org/info/rfc9568>.

8.  Informative References

   [RFC5883]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for Multihop Paths", RFC 5883, DOI 10.17487/RFC5883,
              June 2010, <https://www.rfc-editor.org/info/rfc5883>.

Authors' Addresses

   Greg Mirsky
   Individual
   Email: gregimirsky@gmail.com

   Jeff Tantsura
   NVIDIA
   Email: jefftant.ietf@gmail.com

   Gyan Mishra
   Individual
   Email: hayabusagsm@gmail.com

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