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Chapter 6
IGRP and EIGRP
Updates can follow two paths. If a route update contains a better metric
or a new route, the routers simply exchange the information. If the update
contains information that a network is unavailable or that the metric is
worse than before, an alternate path must be found. When a new path must
be found, the router first searches the topology database for feasible succes-
sors. If no feasible successors are found, a multicast request is sent to all adja-
cent routers. Each router will then respond to the query. Depending on how
the router answers, different paths will be taken. After the intermediate steps
are taken, two final actions can occur:
1.
If route information is eventually found, the route is added to the rout-
ing table, and an update is sent.
2.
If the responses from the adjacent routers do not contain any route
information, the route is removed from the topology and routing
tables.
After the routing table has been updated, the new information is sent to
all adjacent routers via a multicast.
EIGRP Metrics
EIGRP utilizes several databases or tables of information to calculate routes.
These databases are as follows:
The route database (routing table) where the best routes are stored
The topology database (topology table) where all route information
resides
A neighbor table that is used to house information concerning other
EIGRP neighbors
Each of these databases exists separately for each routed protocol config-
ured for EIGRP. The following characteristics identify each session of
EIGRP:
The IP session is called IP-EIGRP.
The IPX session is called IPX-EIGRP.
The AppleTalk session is called AT-EIGRP.
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