Case Study Unequal Cost Load Balancing

Given up to six parallel routes of equal cost,[5] IGRP will do equal-cost load balancing under the same fast/process switching constraints as RIP. Unlike RIP, IGRP can also perform unequal-cost load balancing. An additional serial link has been added between Acheson and Kennan in Figure 6.12, with a configured bandwidth of 256K. The goal is to have Acheson perform unequal-cost load balancing across these two links-spreading the traffic load inversely proportional to the metrics of the link.

[5] The default is four paths. See the case study on setting maximum paths for further details.

Figure 6.12. IGRP can be configured to perform unequal-cost load balancing across links such as the two between Acheson and Kennan.

Figure 6.12. IGRP can be configured to perform unequal-cost load balancing across links such as the two between Acheson and Kennan.

Examining the route from Acheson's S0 interface to network 10.0.0.0, the minimum bandwidth is 1544K (assuming Kennan's Ethernet interface is using the default 10000K bandwidth). Referring to Table 6.1, DLYIGRP(sum) for the serial interface and the Ethernet interface is 2000 + 100 = 2100. BWIGRP(min) is 107/1544 = 6476, so the composite metric of the route is 6476 + 2100 = 8576.

The minimum bandwidth on the route via Acheson's S1 to 10.0.0.0 is 256K; DLYIGRP(sum) is the same as on the first route. Therefore, the composite metric for this route is 107/256 + 2100 = 41162. Without further configuration, IGRP will simply select the path with the lowest metric cost. Figure 6.13 shows that Acheson is using only the path with a metric of 8576.

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