Virtual Router Redundancy Protocol (VRRP) is a redundancy
protocol for establishing a fault-tolerant default gateway. The
protocol establishes a framework between network devices in order
to achieve default gateway failover if the primary gateway becomes
Protocol Independent Multicast (PIM) has no inherent redundancy capabilities and its operation is
completely independent of VRRP group states. As a result, IP
multicast traffic is forwarded not necessarily by the same device
as is elected by VRRP. The VRRP Aware PIM feature provides
consistent IP multicast forwarding in a redundant network with
virtual routing groups enabled.
In a multi-access segment (such as LAN), PIM designated router
(DR) election is unaware of the redundancy configuration, and the
elected DR and VRRP master router (MR) may not be the same router.
In order to ensure that the PIM DR is always able to forward PIM
Join/Prune message towards RP or FHR, the VRRP MR becomes the PIM
DR (if there is only one VRRP group). PIM is responsible for
adjusting DR priority based on the group state. When a fail over
occurs, multicast states are created on the new MR elected by the
VRRP group and the MR assumes responsibility for the routing and
forwarding of all the traffic addressed to the VRRP virtual IP
address. This ensures the PIM DR runs on the same gateway as the
VRRP MR and maintains mroute states. It enables multicast traffic
to be forwarded through the VRRP MR, allowing PIM to leverage VRRP
redundancy, avoid potential duplicate traffic, and enable fail over,
depending on the VRRP states in the device.
Virtual Router Redundancy Service (VRRS) provides public APIs
for a client to communicate with VRRP. VRRP Aware PIM is a feature
of VRRS that supports VRRPv3 (unified VRRP) in both IPv4 and
PIM, as a VRRS client, uses the VRRS client API to obtain
generic First Hop Redundancy Protocol (FHRP) state and
configuration information in order to provide multicast redundancy
PIM performs the following as a VRRS client:
- Listens to state change and update notification from VRRS server (i.e., VRRP).
- Automatically adjust PIM DR priority based on VRRP state.
- Upon VRRP fail over, PIM receives state change notification from VRRS for the tracked VRRP group and ensures traffic is forwarded through VRRP MR.