Chapter 9 High Availability for FortiOS 5.0 : Configuring and connecting HA clusters : Example: HA and 802.3ad aggregated interfaces : HA interface monitoring, link failover, and 802.3ad aggregation
  
HA interface monitoring, link failover, and 802.3ad aggregation
When monitoring the aggregated interface, HA interface monitoring treats the aggregated link as a single interface and does not monitor the individual physical interfaces in the link. HA interface monitoring registers the link to have failed only if all the physical interfaces in the link have failed. If only some of the physical interfaces in the link fail or become disconnected, HA considers the link to be operating normally.