Quick Reference Guide

Layer 3 Routing | 271
Virtual Router Redundancy Protocol
In a static default routed environment, all hosts are configured with a single default gateway. The router
that owns this gateway IP address takes care of forwarding traffic from the LAN to the other networks.
When an end station is statically configured with the address of the router that will handle its routed traffic,
a single point of failure is introduced into the network. If the router goes down, the end station is unable to
communicate. Static configuration is a convenient way to assign router addresses, so Virtual Router
Redundancy Protocol (VRRP) was developed (RFC 3768 and RFC 2338) to provide a backup mechanism.
VRRP eliminates the single point of failure associated with static default routes by enabling a backup
router to take over from a “master” router without affecting the end stations using the route. The end
stations will use a “virtual” IP address that will be recognized by the backup router if the master router
fails. Participating routers use an election protocol to determine which router is the master router at any
given time.
A given port may appear as more than one virtual router to the network. Also, more than one port on the
S-Series router may be configured as a virtual router. Either a physical port or a routed VLAN may
participate. You can configure up to 20 virtual routers (VRRP groups) on a switch. A group can be
assigned one primary IP address and up to 31 secondary IP addresses. You can use any VRID up to 255.
The following example shows how to configure S-Series routers to support VRRP. Router 1 will be the
default master router for the virtual route, and Router 2 will be the backup router. Testing indicates that
they must be on the same subnet.
Note: RFC 3768 defines the failover algorithm used to transfer control to the backup router. Briefly,
VRRP elects the Master router by choosing the router with the highest priority. You configure the
priority of the virtual router with the ip vrrp priority command, or you can leave it at the default value
of 100.
If two routers in a VRRP group come up at the same time and contain the same priority value, the
interfaces’ physical IP addresses are used as tie-breakers to decide which is Master. The router
with the higher IP address becomes the Master.
For details, see “Configuring VRRP on the S50” in the S-Series Tech Tips section of iSupport.
That tech tip also describes how VRRP can provide load balancing.