Chapter 9 High Availability for FortiOS 5.0 : An introduction to the FGCP : HA override : Points to remember about primary unit selection when override is enabled
  
Points to remember about primary unit selection when override is enabled
Some points to remember about primary unit selection when override is enabled:
The FGCP compares primary unit selection criteria in the following order: Failed Monitored Interfaces > Device Priority > Age > Serial number. The selection process stops at the first criteria that selects one cluster unit.
Negotiation and primary unit selection is triggered whenever an event occurs which may affect primary unit selection. For example negotiation occurs, when you change the device priority, when you add a new unit to a cluster, if a cluster unit fails, or if a monitored interface fails.
Device priority is considered before age. Otherwise age is handled the same when override is enabled.