o far in this book, we have taken an in-depth look at the rout-
ing protocol OSPF and shown how a routing protocol is used to find routes through the network. We also learned how routing protocols are used to exchange IP address information between routers in an enterprise network. IP addressing schemes establish a hierarchy that makes path information both distinct and efficient. A router receives this routing information via a given interface. It then advertises the information it knows out the other physical interfaces. This routing process occurs at Layer 3 of the OSI model. In this chapter, in order to decide on the best routing protocol or protocols to use, we'll take a look at both the Interior Gateway Routing Protocol (IGRP) and its big brother, the Enhanced Interior Gateway Routing Pro- tocol (EIGRP).
Unlike OSPF, IGRP and EIGRP are proprietary Cisco protocols and run
on Cisco routers and internal route processors found in the Cisco Distribu- tion and Core layer switches. (I need to note here that Cisco has licensed IGRP to be used on other vendors' equipment, such as Compaq.) Each of these routing protocols also has its own identifiable functions, so we'll dis- cuss each routing protocol's features and differences. Once you understand how these protocols differ from OSPF and how they calculate routes, you will learn how to configure these protocols and fine-tune them with config- uration changes to make each perform at peak efficiency.