Applicability of Bidirectional Forwarding Detection (BFD) for Multi-point Networks in Virtual Router Redundancy Protocol (VRRP)

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Table of Contents

1. Introduction

The [RFC9568] is the current specification of the Virtual Router Redundancy Protocol (VRRP) for IPv4 and IPv6 networks. VRRPv3 allows for a faster switchover to a Backup Router. Using such capability with the software-based implementation of VRRP may prove challenging. But it still may 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 (p2p) paths: single-hop [RFC5881], multihop [RFC5883]. Single-hop BFD may be used to enable Backup Routers to detect a failure of the Active router within 100 msec or faster.¶

[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. The document also defines the extension to VRRP [RFC9568] to bootstrap a VRRP Backup Router to join in a p2mp BFD session.¶

2. Problem Statement

A router may be part of several Virtual Router Redundancy groups, as Active in some and as 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. BFD already has many implementations based on HW that are capable of supporting multiple sub-second sessions concurrently.¶

3. Applicability of p2mp BFD

[RFC8562] may provide an efficient and scalable solution for 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 root and Backup Routers being the tails of the p2mp BFD session. Figure 1 displays the extension of VRRP [RFC9568] to bootstrap a tail of the p2mp BFD session.¶

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Version| Type  | Virtual Rtr ID|   Priority    |Count IPvX Addr|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Rsvd |B|     Max Adver Int     |          Checksum             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    |                       IPvX Address(es)                        |
    +                                                               +
    +                                                               +
    +                                                               +
    +                                                               +
    |                                                               |
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                   Active Router Discriminator                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The new fields are interpreted as follows:¶

• B(FD) - a one-bit flag that indicates that the Active Router Discriminator field is appended to VRRP packet defined in [RFC9568];¶
• Active Router Discriminator - the four-octet field. The value MUST NOT be zero, and it equals the My Discriminator [RFC5880] value allocated by the root 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 locally allocated value as the value of the My Discriminator field ([RFC5880]). The same non-zero value of My Discriminator MUST be set as the value of the Active Router Discriminator field. The BFD flag MUST be set in the VRRP packet. 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 learns from the received VRRP packet. When a Backup router detects the failure of the Active router, according to the 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. If the former is the case, then the new Active router MUST select its new My Discriminator value, include that value in the VRRP packet to bootstrap a new p2mp BFD session, and start transmitting p2mp BFD control packets using the Active router IP address as the source IP address for p2mp BFD control packets and its new My Discriminator value. If the latter is the case, the Backup Router MUST wait for the VRRP packet from the new VRRP Active router that will bootstrap the new p2mp BFD session.¶

4. IANA Considerations

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

5. Security Considerations

This document defines an alternative way, to the one defined in [RFC9568], to accelerate detecting a failure that affects VRRP functionality using p2mp BFD. The operation of either protocol is not changed.¶

Security considerations discussed in [RFC9568], [RFC5880], [RFC5881], and [RFC8562], apply to this document.¶

Authors' Addresses