Cisco s proprietary Interior Gateway Routing Protocol (IGRP) represented a leap forward in routing technologies over its distance-vector kid-brother, the Routing Information Protocol (RIP). While IGRP maintained the underlying distance-vector route establishment process, it redefined exactly what distance meant. RIP simply established the distance metric as the number of hops to the destination without considering the nature of the hops. IGRP, however, established the nature of the path to the destination as the distance metric. This allowed the routing process to consider bandwidth and delay (and optionally load, reliability, and MTU) as factors in establishing an efficient routing architecture. However, what IGRP failed to improve significantly was the efficiency of distributing routing information and the required self-healing aspect of a well-designed network.

Recognizing the need in modern, complex internetworks for rapid healing and efficient use of expensive bandwidth, Cisco designed EIGRP. While the name enhanced may lead you to believe that EIGRP is IGRP with a few functional modifications, it is actually a complete overhaul. In fact, the only significant similarity between the two protocols is the metric used to establish path cost. The processes used to share routing information and to reconverge after a failure are vastly different.

IGRP uses a system of periodic updates to inform directly connected routers, referred to as neighbors, of the routes that it knows. When the update period expires (every 90 seconds by default), the router broadcasts the entire routing table to its neighbors. This is a very...