Load Balancing

Like all other protocols on a Cisco router, if EIGRP discovers up to size equal-cost paths (by default) to a given destination, it installs all routes in the routing table, and the router load balances (or traffic shares) over them. EIGRP, however, has the capability to install unequal-cost routes in the routing table, and the router shares traffic over them in proportion to their metrics. Use the variance command in router configuration mode to allow EIGRP to load balance over paths with unequal metrics.

The variance is a divider. If a route s distance, divided by the variance configured, is less than or equal to the best metric, the route is installed in the routing table. For example, if you have paths with metrics of 100, 200, 300, and 400 in the topology table, and the variance is set to the default value of 1, only the path with a metric of 100 is used.

If you set the variance to 2, both the best path (with a metric of 100) and the path that has a metric of 200 are installed in the routing table. Setting the variance to 3 includes the route that has a metric of 300, and so on. The router load balances over these multiple paths in proportion to their metrics.