3.11 Exchange of Routing Knowledge in EIGRP

A review of the different tables in EIGRP.

Neighbor Table – Maintains state information about adjacent neighbors. Tracks hold timers and reliable transport information (SRTT, Q Cnt, etc). When EIGRP is enabled on a router, it sends hello packets out its interface(s) and EIGRP-capable routers send hello packets back. The state information is built in this table and includes all the necessary information to track the relationship with those neighbors. Without this table, since EIGRP only sends updates when routes change, how would a router know if its neighbors either aren’t experiencing updates, or if they have simply just disappeared?

Topology Table – Contains the complete view of the routing table obtained either through exchange of information with neighbors, or redistribution of connected, static, or other routing protocols into the local EIGRP process. The topology table is built immediately after a neighbor relationship begins and hellos have been exchanged; session establishment is followed by an immediate exchange of update packets. This is the table that the DUAL algorithm is performed upon to determine successor routes for insertion into the routing table.

Note that EIGRP updates from one router to another only advertise successor routes. The other routes in the router’s topology table are not advertised to its neighbors.

Routing Table – This is the table that is ultimately responsible for making forwarding decisions when a packet arrives at the router intended for a remote destination. It contains route codes (such as D for EIGRP or D EX for External EIGRP), successor destination routes, the metrics of a given route, and the next hop IP addresses.