This explanation cover how variance EIGRP Routing protocol work. Some case there is 2 routing protocol with different bandwidth, EIGRP make decision based on this logic.

Cisco EIGRP variance load balancing

This is my first blog for not writing about photo of food and travelling, this CCIE blog actually dedicated for who want to learn CCIE and special mnemonic purpose for me.

This explanation here may not very detail because i’m talk about concept only, you still should find from Cisco Doc for further explanation. How Cisco EIGRP over equal or not equal cost works ?

Equal-cost load balancing is the ability of a router to distribute traffic over all  its network ports that are the same metric from the destination address. Cisco IOS software by default will install up to four equal-cost paths in the routing table for most routing protocols

COMMAND : router(config-router)# variance multiplier

a value from 1 to 128, used for load balancing. The default is 1, which  indicates equal-cost load balancing. The multiplier defines the range of metric  value that are accepted for load balancing by eigrp process.

Load balancing can use only feasible paths, and the routing table includes only  these paths.

From Router 1 perspective to reach Network X :
B, D, and F is an administrative distance
A+B, C+D. and E+F is feasible distance
Through router C is best router so current FD is C+D

Load balancing can use only feasible paths.

2 feasibility condition are :
- The current FD (is the smallest FD) is C+D must be greater than AD  learned from the other router (in this case is Router 2). This is to prevent loops.
- The variance multiplied by the current FD [ 2 x (C+D) ] must greater than  the metric through alternative FD is (A+B).
note : Router 4 never considered to load balance whatever variance value, because F > C+D

This entry was posted on Saturday, May 2nd, 2009 at 7:40 and is filed under CCIE. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.