Chapter 9 High Availability for FortiOS 5.0 : Configuring and connecting HA clusters : Example: HA and redundant interfaces : HA interface monitoring, link failover, and redundant interfaces
  
HA interface monitoring, link failover, and redundant interfaces
HA interface monitoring monitors the redundant interface as a single interface and does not monitor the individual physical interfaces in the redundant interface. HA interface monitoring registers the redundant interface to have failed only if all the physical interfaces in the redundant interface have failed. If only some of the physical interfaces in the redundant interface fail or become disconnected, HA considers the redundant interface to be operating normally.