Enhanced Interior Gateway Routing Protocol (EIGRP) discovers neighbors using hello messages. It then builds a topology table containing the networks learned from different neighbors along with the metric's advertised distance. This is the distance advertised by the neighbor for the particular network, and the locally calculated distance, which is the sum of the advertised distance reported by the neighbor and the link cost to reach the neighbor.
The route with the lowest locally calculated metric known as feasible distance is chosen and installed in the routing table. The neighbor advertising that route becomes the successor. All other routes for the same network with their advertised distances less than the feasible distance are considered backup routes. The neighbors advertising backup routes are considered feasible successors. The backup routes are immediately used by installing them into the routing table. If the primary route through the current successor goes down, it provides fast convergence.
If an EIGRP route is not installed in the routing table, it may be due to one of these reasons:
The neighbors are not discovered.
The neighbors are discovered, but the networks are not available in the topology table.
The neighbors are discovered and the networks are available in the topology table. However, the route is not installed in the routing table. This is due to feasible distance considered inaccessible, which could either be due to the metric of the route being infinity, or the network being learned through a routing protocol with a better administrative distance. Administrative distance is the parameter used to select routes installed in the routing table when the same network is learned through more than one routing protocol. The lesser the administrative distance, the more preferred the route.
To find out if the neighbors are being discovered by EIGRP, issue the show ip eigrp neighbors command in privileged EXEC mode. To determine if the prefix exists in the EIGRP topology table, issue the show ip eigrp topology [prefix mask] command in privileged EXEC mode. To look at the routes available in the IP routing table, issue the show ip route command in privileged EXEC mode.
If the feasible distance value of a route is shown as inaccessible, check the routing table to see if the same network is learned through a different routing protocol with a lower administrative distance. By default, EIGRP internal routes have an administrative distance value of 90, and external routes have a value of 170. To select the EIGRP route, issue the distance command in router configuration mode and lower the administrative distance value of EIGRP, or increase the value of the other protocol.