Understanding the network.pdf - Back to Home

ehave this way by default. With IP, it is a little more difficult, because some
end-node configuration is needed. However, most major operating systems provide
support for RIP to operate in a "snooping" mode to build a local dynamic routing
table (some also support OSPF in this mode). Unfortunately, the feasibility of having
every IP host run a routing protocol is quite limited, because of the technical support
required, additional processing overhead, and peripheral support availability
needed to implement and manage such processes.
There is really no efficient alternative for providing IP gateway assignments, so
statically assigned gateways continue to be the most common method of providing
a default gateway for external network access. They are often preferred over the
possibility of running a dynamic protocol on an end-station. The static default is
universally supported, easy to maintain, and has no possibility of failing—unless, of
course, the router goes down. Regardless of how much redundancy you build into
your network infrastructure, when the router fails the end-stations are stranded and
cut off from the rest of the network until the router is recovered.
Gateway failures are perhaps the largest single point of failure for any enterprise
network. Cisco and other vendors have developed router redundancy protocols to
address this problem. The idea behind router redundancy protocols is simple. Two
or more gateway routers collectively share a virtual IP and MAC address, which is
set as the default gateway for the network segment. The routers sharing the IP
address use an election protocol to determine who is the gateway and who, in effect,
stands by to take over when a failure occurs. There are three such protocols used to
perform this service:
• Cisco Systems' Hot Standby Router Protocol (HSRP)
• Digital Equipment Corporation's IP Standby Protocol (IPSTB)
• A new open standard called Virtual Router Redundancy Protocol (VIRP)
IPSTB and VIRP are not supported currently in the Cisco IOS, so we will only focus
on HSRP. IPSTB and VIRP are supported by other router vendors, so you should at
least be aware of their existence. IPSTB and VIRP are not compatible with Cisco's
HSRP, and Cisco has no plans to support VIRP (the newer and open standard) in the
future. More information on IPSTB and VIRP is available in RFC 2338.
HSRP
HSRP works by sharing a virtual IP address between two routers, where one is the
primary and the other the standby. When HSRP is active, all traffic and operational
activity (ping responses, VTY sessions, and so on) is handled by the primary router.
HSRP is activated using the
interface configuration subcommand. In situations where only one HSRP instance is
in use on a router, the group identifier is unnecessary. If more than one HSRP group