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Chapter 6
IGRP and EIGRP
System
Routes advertised by other IGRP neighbors within the same
autonomous system (AS). The AS number (ASN) identifies the IGRP ses-
sion, because it's possible for a router to have multiple IGRP sessions.
Exterior
Routes learned via IGRP from a different ASN, which provide
information used by the router to set the
gateway of last resort
. The gate-
way of last resort is the path a packet will take if a specific route isn't
found on the router.
When we talked about the scalability of distance-vector protocols, we
told you that they don't establish a formal neighbor relationship with
directly connected routers and that routing updates are sent at designated
intervals. IGRP's interval is 90 seconds, which means that every 90 seconds
IGRP will broadcast its entire routing table to all directly connected IGRP
neighbors.
IGRP Metrics
Metrics are the mathematics used to select a route. The higher the metric
associated with a route, the less desirable it is. The overall metric assigned to
a route is created by the Bellman-Ford algorithm, using the following
equation:
metric = [K1
Ч
Bw + (K2
Ч
Bw) / (256 ­ Load) + K3
Ч
Delay]
Ч
[K5 /
(Rel + K4)]
By default: K1 = 1, K2 = 0, K3 = 1, K4 = 0, K5 = 0.
Delay is the sum of all the delays of the links along the paths.
Delay = [Delay in 10s of microseconds]
Ч
256.
BW is the lowest bandwidth of the links along the paths.
BW = [10000000 / (bandwidth in Kbps)]
Ч
256.
By default, metric = bandwidth + delay.
The formula above is used for the non-default setting, when K5 does not equal
0. If K5 equals the default value of 0, then this formula is used: metric = K1
Ч
bandwidth
+
(K2
Ч
bandwidth) / (256
-