AIS AI2524 user manual Control Traffic Distribution, Adjust the Igrp Metric Weights

Chapter 8: AI2524 Protocol Configuration Steps

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:

zIGRP will accept up to four paths for a given destination network.

zThe 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.

zThe 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-

August 1997	Page 8-13
2524UM