Network Ninja

EIGRP automatically load balances across equal-cost path links. You can also configure load balancing proportionally across unequal-cost paths using the variance command.

When variance  anything other than 1, the EIGRP process multiplies the metric of the best path is multiplied by the variance. All paths to the same destination that have metrics less than this products are now included in load balancing. The amount of traffic sent over each link is proportional to the metric for the path.  Alternatively paths with a feasible distance (FD) lower than that of the product (total) are used for load balancing.

The command is as follows:

Router(config)#router eigrp autonomous-system-number
Router(config-router)#network network-number
Router(config-router)#variance multiplier

The multiplier is a whole number between 1-128. The default is 1 which is equal cost path load balancing.

Take note:
-A Variance of two or three will do in most cases. Using higher values could cause EIGRP to start activating old 28.8 modems to load-balance with DS1s.
-Variance should be used with caution with delay-sensitive traffic. A DS1 link takes 8ms to transmit a 1500 B packet while a 256-kbps link takes 47ms. VoIP traffic, if shared over two links would see the difference as 39ms jitter.

To show this in an example for the EIGRP Process in the network from Router A to Router D

What you are looking at has been used before in my notes and comes from the CCNP Book referenced at the bottom of the post. The metric for the top half (Router A-B-C-D) is 4,869,120 and the bottom half (Router  A-E-D) is 6,024,000. If one configured unequal-cost path load balancing on Route B

Router(config)#router eigrp 1
Router(config-router)#network network-number
Router(config-router)#variance 2

The end result is as that because the clockwise metric is about 5 bar (5,000,000), a variance of 2 will balance with paths less than 10 bar (10,000,000). So the resulting unequal cost path load balancing will be that for every 6 packets sent Router A-B-C-D (clockwise), 5 packets will be sent Router A-E-D (anticlockwise).

Only paths that are in the topology table as FDs are eligible to be included in the variance command.

Software Study Resources:

The Command Memorizer was originally developed by a CCIE Candidate (David Bombal) for his own use and is now available to anyone who wants to use it.Command Memorizer helped him pass the CCIE Lab on the first attempt, and although I am not a CCIE candidate “officially” I have fiddling with it and finding it useful to test my command line retention and overall progress towards CCIE readiness as I do my current CCNP. The proof will be in the pudding as the Command Memorizer boasts 1000s of commands and hundreds of scenarios to test command line knowledge and retention. It has a section for EIGRP and I also like knowing where I am on my long road to Cisco. Like most study aids / study tools this tool / aid has a specific focus. The Command Memorizer only works when used in conjunction with theoretical backing because you need to know what a command does and how it relates to the technology area. IOW You need to make the connection before you can start drilling actual commands repetitively to get them to start flowing and become second nature. For a disclosure statement on my relationship with Configure Terminal.

Cisco Press Resources:

Stewart, B,D., Gough, C (2008). CCNP BSCI Official Exam Certification Guide, Fourth Edition. Indianapolis: Cisco Press.

Notes and Notices:

This is a part of my personal BSCI notes and research to assist myself in learning and understanding the concepts and theory for the BSCI exam. I learn by making notes reading and writing things down and wish to file them where I can’t lose them. These notes are not to be seen, judged or mistaken for replacements to Cisco recognized and authorized training which I personally support and attend and suggest you undertake if you are going for the BSCI Certification.