variance.. eigrp

the idea of variance is to calculate routes as near good enough to make it into the routing table to balance the load...

this from http://itdualism.wordpress.com/2010/02/27/eigrp-variance-command/

February 27, 2010 Rofi Neron 

I was reading the variance concept again as I didn’t feel comfortable with my level of understanding. It is now much clearer to me and I want to share my notes.
The variance command is used to balance traffic across multiple routes that have different metric values. If we have one link with lower metric and as a result it is being over utilized while other connections are idle, we can load balance using the variance command.

Router(config-router)# variance multiplier

The multiplier value, from 1 to 128 is used for load balancing. The default is 1 and it indicates equal-cost for load balancing, as a result path is chosen by metric values.
Multiplier define the range of metric values that the EIGRP process allow for load balancing.

variance command - example

In this example these are the FD values:
B -> 30
C -> 20
D -> 45
By default, when variance=1 the only path used is via router C, because the lowest FD is 20 and only this path have this low metric.
By configuring variance 2 the required FD is 2*20 (=40) and the path via router B is now less than the requirement and can be used to load balance traffic from router E to network Z.
This is an easy to configure load balancing tweak and a good point to remember for the exam.