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HSRP

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ICMP Router Discovery Protocol (IRDP)

A third solution to the IP default gateway problem is ICMP Router Discov-
ery Protocol (IRDP). This will allow hosts to use the Internet Control Mes-
sage Protocol (ICMP) to find a new path when the primary router becomes
unavailable.

IRDP is an extension of the ICMP protocol and not a dynamic routing

protocol. This ICMP extension allows routers to advertise default routes to
end stations. This saves processing on the end stations because they don't
need to run routing protocols and no configuration of hosts by an adminis-
trator is required. However, they need to run the IRDP daemon. You can
find an IRDP daemon on the Cisco FTP site (ftp.cisco.com).

Hosts running the IRDP daemon listen for the IRDP hello messages,

which are sent out from a router as multicast packets. The hello messages are
sent out every 7 to 10 minutes, with a holdtime of 30 minutes.

The problem with this topology is that it is very broadcast intensive. IRDP

is not as broadcast intensive as routing protocols running on the worksta-
tions, but it still uses broadcasts to communicate to the router.

HSRP

ot Standby Routing Protocol (HSRP) can solve the default gateway

problems we listed earlier. HSRP will enable end stations to continue to com-
municate through the network if a router fails without placing excessive
broadcasts on a network.

HSRP works by creating a single virtual standby router that really is a set

of at least two routers or more. To the workstations, it appears that the vir-
tual router is just one router interface. If a router fails in the group, another
router will automatically assume the function of the default router. There is
a drawback to all this, and maybe you have already figured it out. You need
more than one router to run HSRP. So, although it doesn't have the broad-
cast problems the solutions we mentioned earlier in this chapter have, it does
cost money.

What HSRP does is create a Hot Standby router group with a lead router

that services all packets that are sent to the Hot Standby address. All of the
other routers in the group monitor the lead router, and if the lead router fails,

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