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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
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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
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