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

Introduction The 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 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) had been originally defined to detect failure of point-to-point paths: single-hop , multihop . 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. extends 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.

Conventions used in this document

Terminology BFD: Bidirectional Forwarding Detection p2mp: Point-to-Multipoint VRRP: Virtual Router Redundancy Protocol

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 when, and only when, they appear in all capitals, as shown here.

Applicability of p2mp BFD 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 as a source IP address and the value of the My Discriminator field () 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 .
For IPv6, the Active Router uses the 32 least-significant bits of IPv6 source address, as described in Section 5.1.2.1 of .

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 , 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 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.

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 ;
MUST set TTL or Hop Limit value to 255 (Section 5 ). 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 ;
Source UDP port value selection follows the rules defined in Section 4 of ;
MUST use the Active Router IP address as the source IP address.

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

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

Operational Considerations

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.

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.

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.