9.3 Routing with the Load-Backup Feature
9.3.2 Load backup with IGRP and EIGRP
If the routing protocol used is Interior Gateway Routing Protocol (IGRP) or Enhanced IGRP (EIGRP), or if you have a static route configured, the load backup feature will load share between the primary and backup links after the backup link is activated only if certain conditions are met. The metric assigned to the primary link and the backup link must be the same if both links are to be utilized, however. If one link has a lower metric than the other, all routing occurs over the link with the lower metric, even if both lines are up. If load balancing is to occur in this environment, each connection must be able to support comparable bandwidth environments (a 56 kbps serial backs up a 56 kbps serial connection).

Instead of relying on equal metrics to load share and load balance, the variance router configuration command can also be used to control load balancing in an IGRP/EIGRP environment. Use the variance multiplier command to configure unequal-cost load balancing by defining the difference between the best metric and the worst acceptable metric.

Figure shows a sample network and the configuration to create load balancing between primary and backup lines. Figure covers the variance command.

Setting this value lets the communication server determine the feasibility of a potential route. A route is feasible if the next communication server in the path is closer to the destination than the current communication server, and if the metric for the entire path is within the variance. Only paths that are feasible can be used for load balancing and included in the routing table.

If the following two conditions are met, the route is deemed feasible and can be added to the routing table:

  • The local best metric must be greater than the metric learned from the next communication server.
  • The multiplier times the local best metric for the destination must be greater than or equal to the metric through the next communication server.

In Figure , Router E will choose the second path, E-C-A with a metric of 20. This is because a metric of 20 is better than 30 and 45. To instruct EIGRP to select the path E-B-A as well, configure variance with a multiplier of 2:

router eigrp 1
network x.x.x.x
variance 2

This increases the minimum allowable metric to 40 (2 * 20 = 40). EIGRP includes all the routes that have a metric under 40, and are feasible successors. In the above configuration, EIGRP will now use two paths to get to Network X, E-C-A and E-B-A, because both paths have a metric under 40. EIGRP will not use path E-D-A because it has a metric of 45, and it is not a feasible successor.