EIGRP Neighbor Adjacencies

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Before exchanging any EIGRP update packets between routers, EIGRP must first discover its neighbor. EIGRP neighbor is 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 neighbours, For example, the same autonomous system number and EIGRP metric must be required for establishing neighbor adjacencies.

Each EIGRP enabled routers maintain a list of a router that has 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 neighbours. The figure below illustrates the process of exchanging two EIGRP routers initial EIGRP Hello packets and discovering neighbor process.

EIGRP Neighbor Adjacencies

The EIGRP enabled router send EIGRP hello packet. When other EIGRP enabled router receives that Hello packet, it adds that router to its neighbour table. For example

  1. Router R1 has just powered up or just enabled EIGRP and it sends an EIGRP Hello packet through all of its EIGRP-configured interfaces.
  2. Router R2 receives the Hello packet from router R1 on an EIGRP-enabled interface. It replies with an EIGRP update packet. The update packet contains all the routes containing 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.
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