Multicast and Routing Guide K/KA/KB.15.15

12 Virtual Router Redundancy Protocol (VRRP)
VRRP overview
In many networks, edge devices are often configured to send packets to a statically configured
default router. If this router becomes unavailable, the devices that use it as their first-hop router
become isolated from the network. Virtual Router Redundancy Protocol (VRRP) uses dynamic failover
to ensure the availability of an end node's default router. This is done by assigning the IP address
used as the default route to a "virtual router" or VR. The VR includes:
• An owner router assigned to forward traffic designated for the virtual router (If the owner is
forwarding traffic for the VR, it is the master router for that VR.)
• One or more prioritized backup routers (If a backup is forwarding traffic for the VR, it has
replaced the owner as the master router for that VR.)
This redundancy provides a backup for gateway IP addresses (first-hop routers) so that if a VR's
master router becomes unavailable, the traffic it supports will be transferred to a backup router
without major delays or operator intervention. This operation can eliminate single-point-of-failure
problems and provide dynamic failover (and failback) support. As long as one physical router in
a VR configuration is available, the IP addresses assigned to the VR are always available, and the
edge devices can send packets to these IP addresses without interruption.
Advantages to using VRRP include:
• Minimizing failover time and bandwidth overhead if a primary router becomes unavailable.
• Minimizing service disruptions during a failover.
• Providing backup for a load-balanced routing solution.
• Addressing failover problems at the router level instead of on the network edge.
• Avoiding the need to make configuration changes in the end nodes if a gateway router fails.
• Eliminating the need for router discovery protocols to support failover operation.
Both VRRPv2 and VRRPv3 are supported. IPv4 VRs can be configured for both version 2 and version
3. IPv6 VRs can only be configured for version 3.
For more information, see “General operation” (page 274).
Configuring VRRP
Enabling VRRP in the global configuration context
VRRP can be configured regardless of the global VRRP configuration status. However, enabling a
VR and running VRRP requires enabling it in the global configuration context.
Syntax:
[no] router vrrp IPv4|IPv6 enable|disable
Enables or disables VRRP operation in the global configuration context. for IPv4,
IP routing must be enabled before enabling VRRP on the router. For IPv6, IPv6
unicast-routing must be enabled before enabling VRRP on the router. Disabling
global VRRP halts VRRP operation on the router, but does not affect the current VRRP
configuration. Enabling or disabling VRRP generates an Event Log message.
Note: This command has been revised from the prior router vrrp
enablecommand.
To display the current global VRRP configuration, use show vrrp config
global.
VRRP overview 251