Editing Enhanced Interior Gateway Routing Protocol (section)

== Overview ==
Almost all [[router (computing)|routers]] contain a [[routing table]] that contains rules by which traffic is forwarded in a network. If the router does not contain a valid path to the destination, the traffic is discarded. EIGRP is a [[dynamic routing]] protocol by which routers automatically share route information. This eases the workload on a [[network administrator]] who does not have to configure changes to the [[routing table]] manually.

In addition to the [[routing table]], EIGRP uses the following tables to store information:
*Neighbor Table: The neighbor table keeps a record of the [[IP addresses]] of [[router (computing)|routers]] that have a direct physical connection with this router. Routers that are connected to this router indirectly, through another router, are not recorded in this table as they are not considered neighbors.
*Topology Table: The topology table stores routes that it has learned from neighbor routing tables. Unlike a routing table, the topology table does not store all routes, but only routes that have been determined by EIGRP. The topology table also records the metrics for each of the listed EIGRP routes, the feasible successor and the successors. Routes in the topology table are marked as "passive" or "active". Passive indicates that EIGRP has determined the path for the specific route and has finished processing. Active indicates that EIGRP is still trying to calculate the best path for the specific route. Routes in the topology table are not usable by the router until they are inserted into the routing table. The topology table is never used by the [[router (computing)|router]] to forward traffic. Routes in the topology table will not be inserted into the routing table if they are active, are a feasible successor, or have a higher [[administrative distance]] than an equivalent path.<ref name=admin />

Information in the topology table may be inserted into the router's [[routing table]] and can then be used to forward traffic. If the network changes (for example, a physical link fails or is disconnected), the path will become unavailable. EIGRP is designed to detect these changes and will attempt to find a new path to the destination. The old path that is no longer available is removed from the routing table. Unlike most distance vector routing protocols, EIGRP does not transmit all the data in the router's [[routing table]] when a change is made, but will only transmit the changes that have been made since the routing table was last updated. EIGRP does not send its routing table periodically, but will only send routing table data when an actual change has occurred. This behavior is more inline with [[link-state routing protocol]]s, thus EIGRP is mostly considered a hybrid protocol.

When a [[router (computing)|router]] running EIGRP is connected to another router also running EIGRP, information is exchanged between the two routers. They form a relationship, known as an ''adjacency''. The entire routing table is exchanged between both routers at this time. After the exchange has completed, only differential changes are sent.

EIGRP is often considered a hybrid protocol because it also sends link state updates when link states change.