The Big show and Big D

EIGRP Equal- and Unequal -Cost Load Balancing

By default, EIGRP load-shares over four equal-cost paths. For load sharing to happen, the routes to load-share over must show up in the IP forwarding table or with the show ip route command. Only when a route shows up in the forwarding table with multiple paths to it will load sharing occur. Use the bandwidth interface command on serial links to ensure that EIGRP has a consistent perspective of the metrics of the network. This also might aid in making the route show up in the IP forwarding table.

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 the 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 xx_kbps 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 that 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 walks through the calculation of a fictional variance. EIGRP has a route whose metric is 100. The router also has two more routes to that same destination whose metrics are 200 and 300. To allow EIGRP to use all three paths in sharing data, you would set the variance to 3:

Another way to view it is as variance x lowest_metric = 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:

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 is 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-router)  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.

With the maximum-paths command, the router uses up to six paths to share traffic across; to limit this number, use the maximum-paths command. 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, 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.

Chapter 10 provides a detailed example of load sharing over IGRP, which is syntactically identical to configuring EIGRP traffic load balancing.