7.2 Default Routing
7.2.4 The solution

The solution to this problem is to enter a static route for the summarized major net that points to the gateway of last resort. In this case, the fact that 10.0.0.0 is tagged as a gateway of last resort is not relevant. This scenario would still work if router A advertised 10.0.0.0 without also flagging it as a candidate default route (capable of being used as a gateway of last resort). The configuration from router C shows how to add the static route that will point to the gateway of last resort. (see RouterC# show running-config command output).

The routing table from router C shows that router C has installed the static route for 168.71.0.0 via 10.0.0.0. (see RouterC#show ip route command output).

The output from the show ip route command on router C shows that router C now has a route it can use to the subnet 168.71.6.0. When queried about its knowledge of a route, it does a longest-match lookup in its routing table. In this case, the longest match is to the major net itself. (see RouterC#show ip route 168.71.6.1 command output). 

Note: To ensure that you receive the output for the full destination subnet, use the full IP address of a host on the destination subnet when using the show ip route command. You might find that a subnet exists in the routing table that you did not know existed. It is not uncommon to have a routing table with thousands of entries in it. It can be easy to miss a subnet when doing a visual scan. Using the show ip route command to scan the table is much easier and less prone to failure.

In the output from router C, you can see that router C can now ping 168.71.6.1. (see RouterC#ping 168.71.6.1 command output).