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Chapter 2

Routing Principles

R 172.16.40.0 [120/2] via 172.16.10.2,
FastEthernet0/0

R 172.16.30.0 [120/2] via 172.16.10.2,
FastEthernet0/0

R 172.16.20.0 [120/1] via 172.16.10.2,
FastEthernet0/0

C 172.16.10.0 is directly connected, FastEthernet0/0

2621A#

In the above output, notice that the R means that the networks were added

dynamically using RIP. The [120/3] is the administrative distance of the
route (120) along with the number of hops to that remote network (3).

IGRP

The Interior Gateway Routing Protocol (IGRP) is a Cisco proprietary dis-
tance-vector protocol. This means that all of your routers must be Cisco
routers in order to be able to use IGRP in your network. Cisco created this
routing protocol to overcome the problems associated with RIP.

IGRP has a maximum hop count of 255 with a default of 100. This is

helpful in larger networks and solves the problem of there being only 15
hops possible, as is the case in an RIP network. IGRP also uses a different
metric than RIP. IGRP uses bandwidth and delay of the line by default as a
metric for determining the best route to an internetwork. This is called a
composite metric. Reliability, load, and maximum transmission unit (MTU)
can also be used, although they are not used by default.

IGRP can load balance up to six unequal links to a remote network. RIP

networks must have the same hop count to be able to load balance, whereas
IGRP uses bandwidth to determine how to load balance. To load balance
over unequal-cost links, the variance command controls the load balancing
between the best metric and the worst acceptable metric.

IGRP Timers

To control performance, IGRP includes timers with default settings for each
of the following:

Update timers These specify how frequently routing update messages
should be sent. The default is 90 seconds.

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