The Enhanced Interior Routing Protocol (EIGRP) is a hybrid routing protocol (it has characteristics of both distance vector and link state routing protocols) and is based on the Interior Gateway Routing Protocol (IGRP) which was its predecessor. Any Router which runs EIGRP will voluntarily resend its routing information to its EIGRP neighbors and also to its Interior Gateway Routing Protocol (IGRP) neighbors by changing its 32 bit EIGRP metric to a 24 bit metric for IGRP.
EIGRP is based on the DUAL (Diffusing Update Algorithm) routing algorithm which is the algorithm that works to guarantee that there are loop-free routing updates in the network and a fast convergence time is achieved. EIGRP collects routing data and stores it in three tables. Unlike many other distance vector routing protocols, EIGRP will not need any periodic route dumps in order to maintain the network topology table. The routing information is only exchanged when there is an establishment of new neighbor adjacencies and after this, routing updates will be sent only when there are routing changes in the network.
This routing protocol is Cisco proprietary. Distance vector routing protocols find the shortest path to a destination. What they do is exchange information about all the known destinations to their neighbors on the network. Information about the topology is never exchanged. Each node would then know about all the destinations present in the network and would be able to tell the distance to any destination through any of the neighbors. It does not need to know about the physical network topology though.
The Enhanced Interior Gateway Routing Protocol (EIGRP) is designed with certain enhancements over the classical distance vector protocols (like RIP and IGRP) so as to minimize the routing instability which usually occurs after changes in the topology and also to more efficiently utilize the processing power and bandwidth of the router.
As we said before, EIGRP uses three tables to store network information (neighbor table, topology table and routing table). The first is the neighbor table. This table has information about the neighbor routers which are directly connected.
Next it stores data in the topology table. The topology table will not, as the name suggests, store data about the entire network topology but rather it has the aggregated information about routing tables which are taken from all the directly linked neighbors.
The topology table will contain all the destination networks included in the network along with their metrics respectively. All destinations in the topology table will have a successor along with its feasible successor if they are available. The successor and feasible successor will serve as the next hop router for destination networks. Moreover, each destination can be shown as either Passive or Active. A destination is in the Passive state when the other routers know about a way to reach this destination. When a destination route is in the Active state it means that a topology change has just occurred for that particular destination network. The router now actively updates the routes for that particular destination.
There is also the routing table. In the routing table all the real routes to destinations are stored. This table is created by data coming out of the topology table and includes all the destinations and also the successor and feasible successor to those destinations.
To configure EIGRP on a router to include the 10.10.60.0 and 10.10.77.0 networks you would perform the follow commands (for example purposes only):
Router #config terminal
Router (config)# router eigrp 1
Router (config-router)#network 10.10.60.0
Router (config-router)#network 10.10.77.0
Router (config-router)#no auto-summary