its neighbors, the route calculation process begins. Let's now take a look at
how EIGRP uses metrics to calculate the best routes through the network.
tions are affected by routing updates or queries. Where IGRP updates use a
24-bit format, EIGRP uses a 32-bit format for granularity. Only changes in
the network topology are advertised instead of the entire topology table.
when calculating routes. DUAL is much faster and calculates new routes
only when updates or Hello messages cause a change in the routing table.
And then recalculation occurs only when the changes directly affect the
routes contained in the routing table.
culate a new metric and route entry for it. If a link between two EIGRP peers
becomes congested, both routers would have to calculate a new route metric,
then advertise the change to any other directly connected routers.
late, learn, and propagate route update information.
rics used to calculate the least-cost path. Once the best routes have been cal-
culated, they are moved to the routing table. The topology table can store up
to six routes to a destination network, meaning that EIGRP can calculate the
best path for up to six redundant paths. Using the known metrics to the des-
tination, the router must make a decision as to which path to make its pri-
mary path and which path to use as a standby or secondary path to a
destination network. Once the decision is made, the primary route will be
added to the routing table as the active route, or successor, and the standby
will be listed as a passive route, or the feasible successor, to the destination.