EIGRP Load Balancing.

One of the main features of Eigrp is, it support both Equal and Unequal Load Balancing, none of the other Routing Protocol have this feature.Load balancing is a Routers Capability to distribute the traffic of over multiple paths to a same destination.
By Default Eigrp Support four Equal Cost paths for IP. A Network Administrator can extend this UP to Sixteen Paths by using Maximum-Paths (No of Max.Path) IOS command.
[“When a packet is process –switched ,load balancing over equal-cost paths occurs on a per-packet basis, where as when a packet is fast-switched ,load balancing over equal-cost path occurs on a per-destination basis.”]


Eigrp can also balance traffic across multiple paths that have different metric; this is called unequal load balancing. EIGRP Perform Unequal load balancing by using Variance multiplier command.The Multiplier is a variance Value between 1 and 128, using for load- balancing. The Default is 1 which means equal load-balancing. Only paths that are FEASIBLE can be used for load balancing.


Two feasibility Conditions are:

A, The local Best metric (Current FD) must be greater than the best metric (AD) learned from the next router.
B, The Variance multiplied by the local Best Metric (Current FD) must be greater than the metric through the next router (alternative FD).


For Example, Router A has 4 paths to network F .


@ Path 1: FD-30 (via B)
@ Path 2: FD-20 (via C)
@ Path 3: FD-45 (via D)
@ Path 4: FD-55(via E)


Router A Choose Router C as the Successor ,because its FD is lowest (20).With Variance 2 command applied on Router A ,the Path through the Router B will meet the Criteria for Load balancing ,as the FD through the Router B will be less than the FD for the Current Successor(C)