Neighbor Discovery and Recovery

The neighbor discovery/recovery mechanism enables routers to dynamically learn the status of other routers on directly attached networks, such as up, down, or unreachable (router is up, but connection cannot be established). This neighbor discovery process is achieved by periodically sending small Hello packets. As long as an EIGRP router receives Hello packets from a neighboring router, the router presumes that the neighbor is functioning, and the two can exchange routing information. EIGRP does not rely solely on the use of timers for routing table maintenance; instead, EIGRP uses periodic communication to perform the following tasks:

• Dynamically learn of new routers that may join the network

• Identify routers that become either unreachable or inoperable

• Rediscover routers that had previously been unreachable

The initial exchange of routing tables is accomplished with the EIGRP Hello packet between neighbors. Hello packets are discussed later in this appendix in the section "EIGRP Packet Types."