© 2002, CiscoSystems, Inc. All rights reserved. Cisco CCIE Prep v1.0—Module 6-60
EIGRP also has the capability to use unequal-cost load balancing in the same manner as IGRP. The router uses variance as a multiplier in choosing the upper boundary of path with the greatest metric.
Configuring EIGRP unequal-cost load balancing is a three-step process:
Step 1 Configure the bandwidth on both sides of all the interfaces involved in the load-sharing group. Use the bandwidth xxkbps command to accomplish this.
Step 2 Define the lowest cost metric and the highest cost metric. From these values, compute the variance multiplier and add it to the EIGRP routing process. The composite metric EIGRP is using can be viewed with the show ip eigrp topology command, as discussed in previous sections.
Step 3 (Optional) Set the maximum-paths or the traffic-share variables.
The following example takes you through the calculation of a fictional variance. EIGRP has a route and the metric of that route is 100. The router also has two more routes to that same destination, and the metric for those routes is 200 and 300. To allow EIGRP to use all three paths in sharing data, set the variance to 3.
Another way to view it is the (lowestmetric) = largest metric of path to load share over, in this case 300. To properly set the variance in a real network use the following formula:
Variance = 1 + [[[metric of highest cost route] / [metric of the lowest cost route]] rounded up to the nearest 1s decimal place.
The metric of the lowest cost and highest cost routes can be discovered with the show ip eigrp topology command. Be sure to change variance and any other variables, such as bandwidth, on both ends of the link. The bandwidth should be set on all serial links. The following are the syntax for the commands used in configuring load balancing:
router(config-router)# variance [metric_multiplier 1-128] router(config-router)# maximum-paths [1-6]
router(config-router)# traffic-share [balanced | min across-interface] router(config-if)# bandwidth xx kbps
The variance command defines the metric multiplier of which routes to use in unequal-cost load balancing. The default variance is 1, which is equal cost load balancing.
By default the router will load balance across four equal cost paths. To modify this number use the maximum-paths command. The maximum setting for this command is six equal cost paths. The minimum setting of one disables load balancing. EIGRP can perform unequal cost load balancing in addition to equal cost load balancing.
The multiple paths that make up a single hop transport to a common destination are called a load-sharing group. The default value is 4.
With the traffic-share command, if there are multiple minimum-cost paths and traffic-share-min is configured, EIGRP will use equal-cost load balancing. By default, the command is set to balanced, where traffic will be distributed proportionally to the ratio of the metrics. For example, if variance is set to 3, and traffic-share is set to balanced, then the best route will transport traffic three times that of the worst route.
For a route to be included in unequal-cost load sharing, three other conditions must be met.
■ The maximum-paths limit must not be exceeded as a result of adding this route to the load-sharing group.
The downstream router must be metrically closer to the destination.
■ The metric of the lowest-cost route, multiplied by the variance, must be greater than the metric of the route to be added to the load-sharing group.
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