7.4 The 0.0.0.0 Default Route
7.4.1 RIP and 0.0.0.0
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).

Lab Activity
  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.