While there are methods like proxy ARP and IRDP that can help PC clients find their default router, most of us just configure a static route to a single router on each PC because it is easiest. However, by doing that, if that single router goes down, the PC client will be unable to reach other networks (like the Internet).
Virtual Router Redundancy Protocol (VRRP) enables you to set up a group of routers as a default gateway router (VRRP Group) for backup or redundancy purposes. This way, the PC clients can actually point to the IP address of the VRRP virtual router as their default gateway. If one of the master routers in the group goes down, one of the other routers can take over.
Routers can function as master or backup routers, and you can actually configure up to 255 virtual routers on a router interface. Of course, there are platform constraints like router memory, for instance. Additionally, VRRP is intended for use with IPv4 routers only.
Is there a version of the IOS that works on your router? Refer to my article "Get to Know the Cisco IOS Feature Navigator" to see if your current router IOS offers VRRP or if you need to upgrade your IOS.
Virtual Router Redundancy Protocol in greater detailLet's take a close look at Figure A to understand a little better how VRRP works.
Graphic courtesy of Cisco Systems
In this case, because VRRP uses the IP address of the Ethernet interface, Router A has assumed the position of Master or IP address owner (this is configurable). Clients 1-3 are configured with the default gateway IP address of 10.0.0.1 and Routers B and C have become backup virtual routers. If Router A fails, then based on priority (this is also configurable), Router B automatically assumes the Master until Router A becomes available again. One of the main characteristics about VRRP is the router priority. You can set up different priorities for each of your routers, which would enable them to become masters or backup routers to keep your network up and running in case of a hardware or natural failure. This would also help you balance out the work load on your routers.
To configure the priority, use the following command in global configuration mode. In this example, I've set Router A with a priority of 300.
RouterA(conf-if)# vrrp 1 priority 300
What's the difference between VRRP and HSRP?
Does VRRP sound a little like Hot Standby Router Protocol (HSRP)? There are many similarities, such as load balancing and redundancy, but the greatest difference is that HSRP is a Cisco proprietary method whereas VRRP is an industry standard (based on RFC 2338). Both of these have some minor technical differences, but the resulting functionality is the same.
For more information on HSRP, please see the Cisco's "Configuring IP Services" documentation.
How do you configure VRRP?
Configuring VRRP can happen in a few commands. Here is the basic command syntax:
vrrp [group] [timers advertise msec] [timers learn] [preempt delay] [priority] [description]
To enable VRRP on an interface, follow these steps in global configuration mode:
Router(config)# interface Fa0/0
Router(config-if)# vrrp (group) ip (ip address of the virtual router)
You would, of course, do this on a minimum of two routers (the smallest amount of routers you would want in your VRRP group).
To view the status of your VRRP group, use show vrrp, like this:
Router# show vrrp
Ethernet1/0 - Group 1
Using VRRP with your routers can keep your network up and running by configuring the Virtual Router Redundancy Protocol. VRRP can very quickly move all traffic over to a backup router in the event that the master fails. With mission-critical applications like VoIP, video, and e-mail, you need to do whatever you can to ensure that your network never goes down.
For more information on VRRP, please see "Cisco Virtual Router Redundancy Protocol."