EIGRP: Unequal Cost Load Balancing

Unlike other routing protocols EIGRP allows for traffic to be distributed across multiple unequal cost paths.  This is enabled by the existence of Feasible Successors which are guaranteed loop free paths to a destination.  Enabling UCLB is done through the variance multiplier command.  Variance is configured in the topology section of named mode.

The multiplier says how many times worse than the best path a route can take for feasible successors.  For a route to be installed into the routing table the following condition must be met where V is the Variance configured.

CD via Successor < CD via FS < V x CD via Successor

If the CD of the FS met this condition then that FS path was installed into the routing table.

If there were multiple unequal cost paths to a destination installed in the routing table the router only forwards proportionally less traffic over the worse paths and vice versa.  Traffic flow paths are computed using the following formula:

Highest Installed Path Metric / Path Metric

The number of unequal cost paths installed into the routing table is also limited by the number of maximum-paths configured.

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