Unequal-Cost Load Balancing in IGRP

Unequal -Cost Load Balancing in IGRP

IGRP and RIP provide the capability to install up to six parallel equal-cost paths for load balancing. IGRP has an added feature that RIP does not have ‚ the capability to do unequal-cost load balancing, the capability to load-balance traffic over multiple paths that do not have the same metric to the destination. The advantage of this feature is that it offers the flexibility of load balancing, thus making more efficient use of the link. IGRP uses the variance command to perform unequal-cost load balancing.

Before unequal-cost load balancing can take place, however, two rules must apply:

10	The neighboring router utilized as an alternate pathway must be closer to the destination. (That is, the neighboring router's metric to the destination must be a smaller metric than that of the local router for a given destination.)
11	The metric advertised by the neighboring router must be less than the variance of the local router's metric to the destination.

Consider the network in Figure 4-5.

Figure 4-5. Unequal-Cost Load Balancing Example

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.