EIGRP follows three general steps to be able to add routes to the IP routing table:

1. Neighbor discovery: EIGRP routers send Hello messages to discover potential neighboring EIGRP routers and perform basic parameter checks to determine which routers should become neighbors.

2. Topology exchange: Neighbors exchange full topology updates when the neighbor relationship comes up, and then only partial updates as needed based on changes to the network topology.

3. Choosing routes: Each router analyzes its respective EIGRP topology tables, choosing the lowest-metric route to reach each subnet.

IOS maintains three important EIGRP tables. The EIGRP neighbor table lists the neighboring routers and is viewed with the show ip eigrp neighbor command. The EIGRP topology table holds all the topology information learned from EIGRP neighbors and is displayed with the show ip eigrp topology command. Finally, the IP routing table holds all the best routes and is displayed with the show ip route command.

EIGRP Neighbors

An EIGRP neighbor is another EIGRP-speaking router, connected to a common subnet, with which the router is willing to exchange EIGRP topology information.  EIGRP uses EIGRP Hello messages, sent to multicast IP 224.0.0.10, to dynamically discover potential neighbors.  A router learns of potential neighbors by receiving a Hello.

A potential neighbor is a router from which an EIGRP Hello has been received.  Then the router checks the following settings to determine if the router should be allowed to be a neighbor:

* It must pass the authentication process.

* It must use the same configured AS number.

* The source IP address used by the neighbor’s Hello must be in the same subnet.

If authentication is configured, the two routers must use hte same type and the same key.  EIGRP configuration includes a parameter called an autonomous system number (ASN), which must be the same on two neighboring routers.  The IP addresses used to send the EIGRP Hello messages – the routers’ respective interface IP address – must be in the range of addresses on the other routers’ respective connected subnet.

EIGRP does not have a concept of  being fully adjacent like OSPF, and are no neighbor states like OSPF.  As soon as an EIGRP neighbor is discovered and passes the basic verification checks, the router becomes a neighbor.  At that point, the two routers can begin exchanging topology information.  The neighbors send Hellos every EIGRP Hello interval.  A router considers its EIGRP neighbor to no longer be reachable after the neighbor’s Hellos cease to occur for the number of seconds defined by the EIGRP Hold Timer.