Cisco IOS Cookbook, 2nd Edition

Introduction

Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco proprietary routing protocol. Because it is proprietary, you can use it only in an all-Cisco network. But EIGRP more than makes up for this deficiency by being easy to configure, fast, and reliable. A detailed discussion of the protocol’s theory and operation is out of the scope of this book. So if you are unfamiliar with EIGRP in general, or if you need more detail on how the protocol works, we recommend reading the relevant sections from IP Routing by Ravi Malhotra (O’Reilly).

Like RIP, EIGRP is based on a distance vector algorithm to determine the best path to a destination. But EIGRP uses a more complex metric than RIP’s simple hop count. The EIGRP metric, which is based on the minimum bandwidth and net delay along each possible path, means that EIGRP can accommodate larger networks than RIP. But it also means that EIGRP needs a different algorithm for loop removal. This is because in EIGRP it isn’t possible to simply increment the hop count to infinity to eliminate a loop, as RIP does. EIGRP uses a more sophisticated algorithm is called Diffusing Update Algorithm (DUAL).

The DUAL algorithm ensures that every router can individually make sure that its routing table is always free from loops. It allows the router to take advantage of several different possible paths, if they all have the same metric. This facilitates load sharing among equal cost links. Further, the EIGRP topology database on each router ...