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Chapter 6
that come online. Then when the originating router receives information from
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.
Route Calculation
EIGRP uses multicasts instead of broadcasts. Therefore, only identified sta-
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.
EIGRP is called an advanced distance-vector protocol although it con-
tains properties of both distance-vector and link-state routing protocols
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.
This last statement may be confusing. If a change occurs to a network that
is directly connected to a router, all of the relevant information is used to cal-
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.
Now that we understand the difference between a route update and a
route calculation, we can summarize the steps that a router takes to calcu-
late, learn, and propagate route update information.
Redundant Link Calculation
The topology database stores all known routes to a destination and the met-
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.
The path-cost calculation decisions are made from information contained
in the routing table using the bandwidth and delay from both the local and
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