Chapter 9 High Availability for FortiOS 5.0 : An introduction to the FGCP : Primary unit selection : Points to remember about primary unit selection
  
Points to remember about primary unit selection
Some points to remember about primary unit selection:
The FGCP compares primary unit selection criteria in the following order: Failed Monitored interfaces > Age > Device Priority > Serial number. The selection process stops at the first criteria that selects one cluster unit.
Negotiation and primary unit selection is triggered if a cluster unit fails or if a monitored interface fails.
If the HA age difference is more than 5 minutes (300 seconds), the cluster unit that is operating longer becomes the primary unit.
If HA age difference is less than 5 minutes (300 seconds), the device priority and FortiGate serial number selects the cluster unit to become the primary unit.
Every time a monitored interface fails the HA age of the cluster unit is reset to 0.
Every time a cluster unit restarts the HA age of the cluster unit is reset to 0.