Unequal-Cost Load Balancing in IGRP

Consider the network in Figure 4-5.

When Router 1 calculates its IGRP 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, IGRP will simply select the 1544 kbps link to forward packets to Router 3, as shown in the output in Example 4-1.

Example 4-1 Output of Routing Table in Router 1 Without Unequal-Cost Load Balancing

Router_1#

show ip route 133.33.0.0

Routing entry for 133.33.0.0/16

  Known via "igrp 1", distance 100, metric 8576

  Redistributing via igrp 1

  Advertised by igrp 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 is 8576, 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 IGRP, you use the variance command. Variance is a multiplier in which a metric might 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 4-1, 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 igrp command. The output from the show ip route command in Example 4-2 displays that Router 1 is installing both links in its routing table.

Example 4-2 Output of show ip route in Router 1 After Implementing Unequal-Cost Load Balancing by Adding the variance Command

Router_1#

show ip route 133.33.0.0

Routing entry for 133.33.0.0/16

  Known via "igrp 1", distance 100, metric 8576

  Redistributing via igrp 1

  Advertised by igrp 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 is 8576, 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 is 41162, 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

Notice in Example 4-2 that 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.