Dell PowerEdge M420, Reference Manual

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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
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