IP Routing

Route Summarization

IGRP summarizes network numbers when crossing a major network-number boundary, just like RIP does. Route summarization reduces the number of routes that need to be exchanged, processed, and stored.

However, route summarization does not work well in discontiguous networks. Consider the discontiguous network in Figure 3-5. Router X will receive advertisements for 10.0.0.0 from both routers A and B. If X sent packets with the destination 10.1.1.1 to B, the packet would be lost -- B would have to drop the packet because it would not have a route for 10.1.1.1 in its table. Likewise, if X sent packets with the destination 10.2.1.1 to A, the packet would be lost -- A would have to drop the packet because it would not have a route for 10.2.1.1.

Figure 3-5. Contiguous and discontiguous networks

Both IGRP and RIP networks must be designed in contiguous blocks of major network numbers.