Define Unequal-Cost Load Balancing

IGRP can simultaneously use an asymmetric set of paths for a given destination. This feature is
known as unequal-cost load balancing. Unequal-cost load balancing allows traffic to be distributed
among multiple (up to four) unequal-cost paths to provide greater overall throughput and reliability.
Alternate path variance (that is, the difference in desirability between the primary and alternate
paths) is used to determine the feasibility of a potential route. An alternate route is feasible if the next
router in the path is closer to the destination (has a lower metric value) than the current router and if
the metric for the entire alternate path is within the variance. Only paths that are feasible can be used
for load balancing and included in the routing table. These conditions limit the number of cases in
which load balancing can occur, but ensure that the dynamics of the network will remain stable.
The following general rules apply to IGRP unequal-cost load balancing:
• IGRP will accept up to four paths for a given destination network.
• The local best metric must be greater than the metric learned from the next router; that is, the
next-hop router must be closer (have a smaller metric value) to the destination than the local best
metric.
• The alternative path metric must be within the specified variance of the local best metric. The
multiplier times the local best metric for the destination must be greater than or equal to the
metric through the next router.
If these conditions are met, the route is deemed feasible and can be added to the routing table.
By default, the amount of variance is set to one (equal-cost load balancing). You can define how
much worse an alternate path can be before that path is disallowed by performing the following task

in router configuration mode: