to avoid single point of failure of a gateway.

Virtual Router Redundancy Protocol (VRRP) is a non-
proprietary redundancy protocol described in RFC 3768 
designed to increase the availability of the default 
gateway servicing hosts on the same subnet. This increased 
reliability is achieved by advertising a "virtual router" 
(an abstract representation of master and backup routers 
acting as a group) as a default gateway to the host(s) 
instead of one physical router. Two or more physical 
routers are then configured to stand for the virtual 
router, with only one doing the actual routing at any given 
time. If the current physical router that is routing the 
data on behalf of the virtual router fails, an arrangement 
is made for another physical router to automatically 
replace it. The physical router that is currently 
forwarding data on behalf of the virtual router is called 
the master router. Physical routers standing by to take 
over from the master router in case something goes wrong 
are called backup routers