Understanding VRRP Profiles
Virtual Router Redundancy Protocol (VRRP) enables hosts on a LAN to make use of redundant routing devices on that LAN without requiring more than the static configuration of a single default route on the hosts. The routing device on which VRRP is enabled share the IP address corresponding to the default route configured on the hosts. At any time, one of the routing devices is the primary (active) and the others are backups. If the primary fails, one of the backup routers becomes the new primary, providing a virtual default routing platform and enabling traffic on the LAN to be routed without relying on a single routing device. Using VRRP, a backup routing device can take over a failed primary router within a few seconds and without any interaction with the hosts.
Routing devices on which VRRP is enabled dynamically elect the primary and backup devices. You can also configure the assignment of the primary and the backup routers by specifying the priorities from 1 through 255 for primary-role election, with 255 being the highest priority. VRRP functions by the default primary sending advertisements to the backup devices at regular intervals. The default interval is 1 second, but you can set this interval. If a backup device does not receive an advertisement for the set period, the backup device with the next highest priority takes over as primary and begins forwarding packets. To minimize network traffic, VRRP is designed in such a way that only the device that is acting as the primary sends out VRRP advertisements at any given point in time. The backup devices do not send any advertisement until and unless they take over as the primary.
The following figure illustrates a basic VRRP topology. In this example, routers A, B, and C are running VRRP and together they function as a virtual router. The IP address of this virtual router is 10.10.0.1 (the same address as the physical interface of Router A).
Because the virtual router uses the IP address of the physical interface of router A, router A is the primary router, while routers B and C function as backup VRRP routers. Clients 1 through 3 are configured with the default gateway IP address of 10.10.0.1. As the primary router, router A forwards packets sent to its IP address. If the primary virtual router fails, the backup router configured with the higher priority becomes the primary virtual router and provides uninterrupted service for the LAN hosts. When router A recovers, it becomes the primary virtual router again.