Neighbor Discovery and Recovery

EIGRP, unlike virtually every other distance-vector routing protocol, does not rely exclusively and rigidly on the use of timers for maintaining its routing table. Instead, the basis for maintaining routing

Enhanced IGRP

tables is a periodic communication between EIGRP routers. They use this process to

• Dynamically learn of new routers that may join their network

• Identify routers that become either unreachable or inoperable

• Rediscover routers that had previously been unreachable

The basic neighbor discovery/recovery process consists of periodi- cally transmitting a small hello packet to neighbors. The hello packet establishes the relationship between immediate neighbors (known as an adjacency). This relationship is used to exchange routing metrics and information.

An EIGRP router can safely assume that, as long as it is receiving hello packets from known neighbors, those neighbors (and their routes) remain viable. If an EIGRP router ceases to receive such greetings from a neighbor, however, it can assume that something is amiss. That router will enter the DUAL process for those routes. These processes are examined in more detail in the section titled "Hello Packets

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