6.2 EIGRP Features
6.2.2 Neighbor discovery and recovery
Remember that simple distance-vector routers do not establish any relationship with their neighbors. RIP and IGRP routers merely broadcast or multicast updates on configured interfaces. In contrast, EIGRP routers actively establish relationships with their neighbors, much the same way that OSPF routers do.

Figures - illustrate how EIGRP adjacencies are established. EIGRP routers establish adjacencies with neighbor routers by using small hello packets. Hellos are sent by default every 5 seconds. An EIGRP router assumes that, as long as it is receiving hello packets from known neighbors, those neighbors (and their routes) remain viable. By forming adjacencies, EIGRP routers do the following:

  • Dynamically learn of new routes that join their network
  • Identify routers that become either unreachable or inoperable
  • Rediscover routers that had previously been unreachable