Unequal-Cost Load Balancing in EIGRP

Unequal -Cost Load Balancing in EIGRP

EIGRP and IGRP use the same equation to calculate their metrics, and they share the same behavior when it comes to unequal-cost load balancing. EIGRP also can install up to six parallel equal-cost paths for load balancing, like IGRP can, and EIGRP also uses the same variance command as IGRP to do unequal-cost path load balancing.

Consider the network in Figure 6-12.

Figure 6-12. Unequal-Cost Load Balancing Example

Remember the rules for multipath operation:

• The neighboring router utilized as an alternate pathway must be closer to the destination (that is, it must be advertising a smaller metric than that of the local router for a given destination). It's not possible to go back to go forward.

• The metric advertised by the neighbor must be less than the variance of the local router's metric. Variance = Variance Factor 3 Local Metric.

When Router 1 calculates its EIGRP metrics to Router 3, the metric going through the 1544 kbps link is as follows :

The metric going through the 256 kbps link is as follows:

Without unequal-cost load balancing, EIGRP will simply select the 1544 kbps link to forward packets to Router 3, as shown in the output in Example 6-3.

Example 6-3 show ip route Output Shows Router 1 Choosing a Suboptimal Route Without Unequal-Cost Load Balancing

 Router_1#  show ip route 133.33.0.0  Routing entry for 133.33.0.0/16   Known via "eigrp 1", distance 90, metric 2195456   Redistributing via eigrp 1   Advertised by eigrp 1 (self originated)   Last update from 192.168.6.2 on Serial0, 00:00:20 ago   Routing Descriptor Blocks: * 192.168.6.2, from 192.168.6.2, 00:00:20 ago, via Serial0       Route metric is2195456, traffic share count is 1 Total delay is 21000 microseconds, minimum bandwidth is 1544 Kbit Reliability 255/255, minimum MTU 1500 bytes       Loading 1/255, Hops 0 

To use the unequal-cost load-balancing feature of EIGRP, you use the variance command. Variance is a multiplier in which a metric may be different from the lowest metric to a route. The variance value must be of integer value; the default variance value is 1, meaning that the metrics of multiple routes must be equal to load-balance.

In Example 6-3, the metric through the 256 kbps link is 4.8 times larger than the metric through the 1544 kbps link. Therefore, for the 256 kbps link to be considered in the routing table, a variance of 5 must be configured in Router 1. The configuration in Router 1 is simply variance 5 under the router eigrp command. The output from the show ip route command in Example 6-4 displays that Router 1 is installing both links in its routing table.

Example 6-4 Example Output of Unequal-Cost Load Balancing in EIGRP

 Router_1#  show ip route 133.33.0.0  Routing entry for 133.33.0.0/16   Known via "eigrp 1", distance90, metric 2195456   Redistributing via eigrp 1   Advertised by eigrp 1 (self originated)   Last update from 10.1.1.2 on Serial1, 00:01:02 ago   Routing Descriptor Blocks:   * 192.168.6.2, from 192.168.6.2, 00:01:02 ago, via Serial0       Route metric is2195456, traffic share count is 5      Total delay is 21000 microseconds, minimum bandwidth is 1544 Kbit Reliability 255/255, minimum MTU 1500 bytes       Loading 1/255, Hops 0     10.1.1.2, from 10.1.1.2, 00:01:02 ago, via Serial1       Route metric is10537472, traffic share count is 1       Total delay is 21000 microseconds, minimum bandwidth is 256Kbit       Reliability 255/255, minimum MTU 1500 bytes       Loading 1/255, Hops 0 

In Example 6-4, the route through Serial 0 has a traffic share count of 5, compared to a traffic share count of 1 through Serial 1. This indicates that the router will send five packets over Serial 0 for every packet sent over Serial 1.