3.2 Default Routing
3.2.4 Default route caveats
A router does not use a gateway of last resort for addresses that are part of its local domain. A local domain is a major network to which the router is connected. In the figure, RTX has two interfaces configured with IP addresses that belong to the major network, 172.16.0.0.

If all three routers are running IGRP, RTX will not learn about the subnet 172.16.1.1/30 because a variable-length subnet mask is used. (IGRP does not support VLSM.) So, after statically configuring a 0.0.0.0/0 route, RTX's routing table is shown in the figure. (Click on the topology to view the table).

What happens when you issue a ping to 172.16.1.1 from RTX? Because the network 172.16.1.0/30 is not in RTX's routing table, you may expect RTX to use its default route to send the ping to RTZ. However, because RTX has interfaces connected to the major net 172.16.0.0, RTX considers 172.16.0.0 a local domain and will not use a default route to reach 172.16.1.0 or any other local domain address. Without additional configuration, RTX's ping to 172.16.1.1 will fail.

You can solve this problem in several ways. The first and best approach is to configure the router with the ip classless global configuration command. With ip classless enabled (which is the default in Cisco IOS versions 11.3 and greater), the router uses the best prefix match available, including a supernet route, such as 172.0.0.0/8 or, ultimately, 0.0.0.0/0. By enabling ip classless you can get RTX to use the 0.0.0.0/0 route to reach unknown subnets within its local domain, 172.16.0.0.

A second approach is to give RTX an explicit route for the major network 172.16.0.0, as shown:

RTX(config)#ip route 172.16.0.0 255.255.0.0 172.16.3.1

Without a more specific route available for 172.16.1.1, RTX uses this static route to the major network number 172.16.0.0/16 and will successfully route packets destined for 172.16.1.1.