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| 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. The IGRP does not support VLSM. After statically configuring a 0.0.0.0/0 route, RTX routing table is shown in the example.

A

ping issued to 172.16.1.1 from RTX would be expected to use its default route to send the ping to RTZ. RTX has interfaces connected to the major net 172.16.0.0, and so 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, the ping will fail.

This problem may be solved in a number of ways. One approach is to configure the router with the

ip classless global configuration command. With ip classless enabled, 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 0.0.0.0/0. By enabling ip classless RTX will 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 configure RTX with an explicit route for the major network 172.16.0.0.

> 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 the static route to the major network number 172.16.0.0/16 and successfully routes packets destined for 172.16.1.1.

| Lab Activity e-Lab Activity: Default Route Caveats This lab is to create an explicit static default route in an environment where VLSM is used and IGRP cannot learn routes due to it.