9.3 Routing with the Load-Backup Feature
9.3.1 Load backup with OSPF
This section discusses the way load sharing and load balancing work with different routing protocols when the load-backup feature is on.

If the routing protocol used is Open Shortest Path First (OSPF), 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 cost assigned to the primary link and the backup link must be equal if both links are to be utilized. If one link has a lower cost than the other, all routing occurs over the link with the lower cost, even if both lines are up. The cost refers to the value assigned to a link in OSPF. This metric is based on the speed of the media.

OSPF does not support load balancing between the primary link and the backup connection. If load balancing is to occur in this environment, each connection must be able to support comparable bandwidths (a 56 Kbps serial backs up a 56 Kbps serial connection).