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Chapter 8: AI2524 Protocol Configuration Steps
August 1997 Page 8-13
2524UM
tions limit the number of cases in which load balancing can occur, but
ensure that the dynamics of the network will remain stable.
These general rules apply to IGRP unequal-cost load balancing:
z
IGRP will accept up to four paths for a given destination network.
z
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.
z
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 b
added to the routing table.
By default, the amount of variance is set to one (equal-cost load bal-
ancing). You can define how much worse an alternate path can be be-
fore that path is disallowed by defining the variance associated with a
particular path variance multiplier.
Note: By using the variance feature, the Cisco IOS software can
balance traffic across all feasible paths and can
immediately converge to a new path if one of the paths
fails.
Control Traffic Distribution
By default, if IGRP or Enhanced IGRP have multiple routes of un-
equal cost to the same destination, the Cisco IOS software will distrib-
ute traffic among the different routes by giving each route a share o
the traffic in inverse proportion to its metric. If you want to have faster
convergence to alternate routes, but you do not want to send traffic
across inferior routes in the normal case, you might prefer to have no
traffic flow along routes with higher metrics.
To control how traffic is distributed among multiple routes of unequal
cost, type this command in router configuration mode:
traffic-share {balanced | min}
Adjust the IGRP Metric Weights
You can alter the default behavior of IGRP routing and metric compu-
tations. Although IGRP metric defaults were carefully selected to pro-
vide excellent operation in most networks, you can adjust the IGRP
metric. Adjusting IGRP metric weights can dramatically affect net-