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Chapter 6

Troubleshooting TCP/IP Connectivity

IGRP and EIGRP

Each protocol has its own method of route redistribution. You must be
familiar with each protocol's implementation of route redistribution and
default-metric settings.

IGRP and EIGRP use the same command to adjust metrics. It is done

through the default-metric command. Here is an example:

default-metric bandwidth delay reliability load MTU

This command takes the metrics for the protocol being injected into IGRP

or EIGRP, and converts them directly to values that IGRP or EIGRP can use.
The bandwidth is the capacity of the link; delay is the time in microseconds;
reliability and load are values from 1 to 255; and MTU is the maximum
transmission unit in bytes.

Finally, you can also change the distance values that are assigned to

EIGRP (90 internal; 170 external). The distance value tells the router which
protocol to believe. The lower the distance value, the more believable the
protocol. The administrative distance values for EIGRP are changed with the
following command from within the EIGRP session:

distance eigrp internal-distance external-distance

Internal distance and external distance have a range of values from 1 to 255.

Remember that a value of 255 tells the router to ignore the route. So,

unless you want the routes from the protocol to be ignored, never use the
value of 255.

This can be a source of problems when troubleshooting routing problems.

If multiple protocols advertise the same routes, it is possible that differences
in the administrative distance may cause the route to be learned by the wrong
protocol, and thus it is not propagated correctly throughout the network.

Metrics used by EIGRP are the same as those used by IGRP. As with

IGRP, metrics decide how the routes are selected. The higher the metric asso-
ciated with a route, the less desirable the route is. The overall metric assigned
to a route is created by the Bellman-Ford algorithm, using the following
equation:

In this equation,

CM is the composite metric assigned to the route.

and K

are equation constants.

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