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Virtual Router Redundancy Protocol Commands
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Virtual Router Redundancy Protocol 
Commands
An end station running IP needs to know the address of its first hop router. 
While some network administrators choose to install dynamic router 
discovery protocols such as DHCP, others prefer to statically allocate router 
addresses. If the router identified by such a statically allocated address goes 
down, the end station loses connectivity. The Virtual Router Redundancy 
Protocol (VRRP) is designed to provide backup for the failing router without 
requiring any action on the part of the end station. It is based on the concept 
of having more than one router recognize the same IP address. One of the 
routers is elected the master router and handles all traffic sent to the specified 
virtual router IP address. If the master router fails, one of the backup routers 
is elected in its place and starts handling traffic sent to the address. This 
change is transparent to end stations.
VRRP increases the availability of the default path without requiring 
configuration of dynamic routing or router discovery protocols on every end 
station.
Multiple virtual routers can be defined on a single router interface.
Pingable VRRP Interface
RFC 3768 specifies that a router may only accept IP packets sent to the 
virtual router’s IP address if the router is the address owner. In practice, this 
restriction makes it more difficult to troubleshoot network connectivity 
problems. When a host cannot communicate, it is common to ping (send an 
ICMP Echo Request) the host’s default gateway to determine whether the 
problem is in the first hop of the path to the destination. When the default 
gateway is a virtual router that does not respond to pings, the operator cannot 
use this troubleshooting technique. Because of this, it has been common for 
VRRP implementations to respond to pings, in spite of the prohibition in the 
RFC. The IETF has recognized the issue, and a draft revision of the VRRP 
2CSPC4.X8100-SWUM102.book  Page 1379  Friday, March 15, 2013  8:56 AM