Summary

This chapter has covered the problems that can occur when using subnet 0, discontiguous networks, and summarized routes with RIP and IGRP. The use of discontiguous networks to cause summarization was discussed. Summarized routes were compared to subnet 0 in order to explain how they can be confused with one another. Recall that IGRP worked in some scenarios because of the anomalous behavior that allowed it to accept a major net advertisement even though it had a local connection to the same major net. In fact, RIP and IGRP should behave exactly the same when faced with major net advertisements for networks they have direct connections to—they should ignore the major net advertisement.

EIGRP, OSPF, and RIP V2 were not covered because they have built-in methods for coping with these issues. However, even EIGRP, OSPF, and RIP V2 can fail when using subnet 0 and summarization if configured incorrectly. The information provided in this chapter also explained why RIP and IGRP fail when using discontiguous networks.

Using aggregation as a tool in designing networks was also discussed. It can greatly simplify routing tables and the debugging of IP connectivity problems. Chapter 4 continues this discussion with explanations of IP unnumbered and VLSM.