RouterGeek

When would you need this: When you are using a dynamic routing protocol, and have more than one path to destination networks.

Special Requirements: None

The first fact to be set is that ALL router platforms support load balancing. And in a short description, load balancing is the operation in which the router forwards packets in different routes to the same destination. This happens when there is more than one entry in the routing table for the same destination network.

There are two types of load balancing:

1. Multiple entries to the same destination with equal metrics.

In this situation, protocols like RIP, RIPv2, IGRP, EIGRP, and OSPF automatically does the operation and no configuration is needed.

2. Multiple entries to the same destination with different metrics.

With a complex metric calculation, like IGRP and EIGRP, it is rare to get metrics to be equal to the same destination for different paths. In this case, configuration is needed.

You can configure something called ‘variance’. The variance value, determines the percentage that you are willing tolerate in choosing a secondary path. If the value of the variance is chosen to be 1, this means that only the paths with equal best metric will be used. And a value of 1.2, for example, means that the best path as well as the paths with a metric up to 1.2 of best path’s metric will be used.

A numerical example is,

For a variance of 1.3

If the best path’s metric is 1000, paths of metric in the range of 1000 to 1300 will be used. And remember that we are talking about multiple paths to the same destination.

And one more important note, we are talking about paths derived from the same routing protocol, i.e, paths with the same administrative distance.

The configuration of unequal path load balancing for IGRP and EIGRP is done with a single command:

Router(config-router)#variance X

Where X represent the value of the variance that you want to use.

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Read the rest of the article in the Router Geek Book: Guide to Cisco Routers Configuration