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Variance

How does variance help in EIGRP load balancing ? How do we decide on the variance value & where do we configure this ? why would we require "trafic share min" since it's equivalent to having no variance configured ?

6 REPLIES
Hall of Fame Super Blue

Re: Variance

Variance is used when you have more than one route to a destination but they are not equal cost eg.

Link 1 = 128Kbps

Link 2 - 256Kbps

Under default settings EIGRP would only use Link 2. if you configure a variance eg.

router eigrp 1

variance 2

then EIGRP will now use both links and for every 2 packets sent on Link 2 it will send one on link 1.

The traffic-share min command if applied to the above example would ensure that EIGRP only used Link 2.

Jon

Re: Variance

Thanks for your quick reply Jon!

So, this would help in load balancing right ?

How would work on a scenario with 3 paths between router A and B with metric / bandwidth of 1.4Mb, 2.48Mb & 4.09Mb? How do we implement Variance here ?

Hall of Fame Super Blue

Re: Variance

You would need a variance value of 3 to allow EIGRP to use all 3 links in it's forwarding.

Jon

Re: Variance

what does the variance value depend on? On the number of links that participate in the load sharing / balancing?

Hall of Fame Super Blue

Re: Variance

No its worked out from the bandwidths, so from your example

1.4Mb, 2.48Mb & 4.09Mb

With a variance of 2 EIGRP would only use the 2.48Mb and 4.09Mb links - why

2.48 x 2 = 4.96 which is greater than 4.09

1.4 x 2 = 2.8 which is less then 4.09

so the 1.4Mb link would not be used but

3 x 1.4 = 4.2 which is greater than 4.09.

Jon

Re: Variance

Thanks a lot Jon. I gotthe concept.

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