EIGRP – the variance command
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.
Simple explanation but easily understandable
If we are using the real metric, then do we need to add the total real metric from source to destination?
In that case how to find the “n” value foe varience command between different paths?
As touched on by spidernik84, the route must be a feasible successor before it can be considered for load balancing.
In this example, configuring a variance of 3 (3 x 20 = 60) would not allow E-D-Z to become a load balancing path, as it does not meet the conditions to become a feasible successor.
In calculation, RD of D-Z, or 25, is greater than E-C-Z FD of 20.
Thanks for this, was having difficulty understanding ‘variance’ in EIGRP
Great explanation but I’d love to see it done with the real numbers. EIGRP is confusing with it’s metrics value and it uses massive 7 and 8 digit numbers to represent metric values.
Exactly. To make thing easier you could put dots every three numbers, from right!
ie: 40398230 > 40.398.230
This helps, since sometimes it is possible to get confused between the number of digits and compare a 4M metric with a 40M one!
One doubt on the example, not concerning variance: is the path E->B->Z chosen as a Feasible Successor in normal conditions? I guess so, since B->Z (10, reported distance to Z sent from B) is less than E->C->Z (20, feasible distance to Z through C).
Thanks for the article
simple expln,
thankqu