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The 0.0.0.0 default
route has special meaning to RIP and IGRP. This section explains
what this meaning is and how RIP and IGRP treat it slightly
differently.
The special address 0.0.0.0 is used
to describe a default route. A default route is used when it is not
convenient to list every possible network in the RIP updates, and
when one or more closely-
connected routers in the system
are prepared to handle traffic destined for networks that are not
listed explicitly in the routing table. These routers should create
RIP entries for the address 0.0.0.0, just as if it were a network to
which they are connected. The decision as to how routers create
entries for 0.0.0.0 is left to the network administrator. The method
used to have a RIP router advertise a default route is to use the
command default-information originate. (see
show running-config
from router A in main figure).
The routing
table from router C in the main figure shows that router A is
advertising a route to 0.0.0.0. It appears that router A is the
origin of this route, as well as the next router C hop, because it
has a metric of 1. (see RouterC#show
ip route command output).
The partial
configuration from router A shows how the route to 0.0.0.0 was
created. In this case, null0 was used as the ultimate destination.
Router B and router C used 0.0.0.0 as their gateway of last resort
and sent all packets for unknown nonlocal domain networks to router
A. (see RouterA#show
running-config command
output).
Any packets for
nonlocal domain networks arriving at router A that router A doesn't
have a route for are sent to null0. Notice that it was not necessary
to configure redistribute static or a default metric. The metric is
assumed to be that of a connected route. In the output of the debug
ip rip command from router A, you
can see router A advertising 0.0.0.0 with a metric of 1. (see RouterA#debug
ip rip command output).
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Lab
Activity |
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In this lab,
you will learn how to use a default route with
RIP to setup a gateway of last resort on all RIP routers. |
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