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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 master (active) and the others are backups. If the master fails, one of the backup routers becomes the new master, 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 master router within a few seconds and without any interaction with the hosts.

Routing devices on which VRRP is enabled dynamically elect the master and backup devices. You can also configure the assignment of the master and the backup routers by specifying the priorities from 1 through 255 for master election, with 255 being the highest priority. VRRP functions by the default master 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 master and begins forwarding packets. To minimize network traffic, VRRP is designed in such a way that only the device that is acting as the master 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 master.

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 master 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 master router, router A forwards packets sent to its IP address. If the master virtual router fails, the backup router configured with the higher priority becomes the master virtual router and provides uninterrupted service for the LAN hosts. When router A recovers, it becomes the master virtual router again.

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