Case Study Setting Maximum Paths

The maximum number of routes over which IGRP can load balance is set with the maximum-paths paths command. Paths may be any number from one to six in IOS 11.0 and later and any number from one to four in earlier versions. The default for all versions is four.

Figure 6.16 shows three parallel paths of varying costs from McCloy to network 172.18.0.0. The network administrator wants to load balance over a maximum of only two of these routes while ensuring that if either of these paths should fail, the third route will replace it.

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Figure 6.16. The maximum-paths and variance commands can be used together to configure load balancing over only two of the three links between McCloy and Bohlen. If either link fails, the third will take its place.

Figure 6.16. The maximum-paths and variance commands can be used together to configure load balancing over only two of the three links between McCloy and Bohlen. If either link fails, the third will take its place.

The metrics from McCloy are:

The metric of the S2 route is 6.76 times as large as the lowest-cost metric, so the variance is seven. McCloy's IGRP configuration is router igrp 10 variance 7 network 172.20.0.0 network 192.168.1.0 network 192.168.2.0 maximum-paths 2

The variance command ensures that any of the three routes to 172.18.0.0 is feasible; the maximum-pathscommand limits the load-sharing group to only the two best routes. The results of this configuration can be seen in Figure 6.17. The first routing table shows that McCloy was load balancing over the two links with the lowest of the three metrics, S0 and S1. After a failure of the S1 link, the second routing table shows that the router is now load balancing over the S0 and S2 links. In each instance, the router will load balance inversely proportional to the metrics of the two paths.

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