eigrp neighbor adjacency process

Before exchanging any EIGRP update packets between routers, EIGRP must first discover its neighbor. EIGRP neighbor is an adjacent router running EIGRP on directly connected networks. EIGRP Hello packets are used to establish and maintain neighbor adjacencies.

Several parameters between the two routers must match to become neighbors; for example, the same autonomous system number and EIGRP metric must be required for establishing neighbor adjacencies.

Each EIGRP-enabled router maintains a list of routers that have EIGRP neighbor adjacencies with this router. This list is known as the neighbor table.

The router uses the neighbor table to track the status of EIGRP neighbors. The figure below illustrates exchanging two EIGRP routers’ initial EIGRP Hello packets and discovering the neighbor process.

The EIGRP-enabled router sends an EIGRP hello packet. When another EIGRP-enabled router receives that Hello packet, it adds that router to its neighbor table. For example

1. Router R1 has powered up or enabled EIGRP, sending an EIGRP Hello packet through its EIGRP-configured interfaces.
2. Router R2 receives the Hello packet from router R1 on an EIGRP-enabled interface and replies with an EIGRP update packet. The update packet contains all the routes in the R2 routing table, excluding the routes learned through that interface.
3. The neighbour adjacency is still not established until R2 sends an EIGRP Hello packet to R1. Now R2 sends hello packet to R1, the neighbour adjacency is now established. R1 and R2 update their neighbour tables adding the adjacent router as a neighbour.