EIGRP uses a neighbor table, topology table, and IP routing table. The neighbor table maintains a state of neighbors. The topology table is used to store information about all known routes received from all neighbors. EIGRP Update messages send the routers’ EIGRP topology tables. The EIGRP topology table is a database of possible routes. Each router chooses its best routes and installs these routes in its respective IP routing table uses the information in the topology table.
Each EIGRP router maintains an EIGRP topology table for each IPv4 and IPv6. It also includes route entries for every destination that the router learns from its directly connected EIGRP neighbors. The figure below illustrates the continuance of the earlier route discovery process from the previous article “EIGRP Neighbor Adjacency”. When a router R1 receives an update from neighbor router R2, it adds the routing information into its EIGRP topology table and replies with an EIGRP acknowledgement. The figure now illustrates the update of the topology table.
- Router R1 replies with an EIGRP acknowledgement packet informing Router R2 about the confirmation of receiving the update of the routing information.
- Router R1 sends an EIGRP update to Router R2 advertising the routes in its topology table, except route learned from Router R2.
- Router R2 receives the EIGRP update from Router R1 and adds this information to its topology table.
- Router R2 replies to Router R1’s EIGRP update packet with an EIGRP acknowledgement packet.
When Router R1 receives the EIGRP update packets from Router R2, R1 updates its IP routing table using the information in the topology table with the best path to each destination, including the metric and the next-hop router. Similarly to Router R1, Router R2 updates its IP routing table with the best path routes to each network. This is the EIGRP converged state for both routers.