Chapter 9 High Availability for FortiOS 5.0 : Operating a cluster : Monitoring cluster units for failover
  
Monitoring cluster units for failover
If the primary unit in the cluster fails, the units in the cluster renegotiate to select a new primary unit. Failure of the primary unit results in the following:
If SNMP is enabled, the new primary unit sends HA trap messages. The messages indicate a cluster status change, HA heartbeat failure, and HA member down. For more info about HA and SNMP, see “Clusters and SNMP”.
If event logging is enabled and HA activity event is selected, the new primary unit records log messages that show that the unit has become the primary unit.
If alert email is configured to send email for HA activity events, the new primary unit sends an alert email containing the log message recorded by the event log.
The cluster contains fewer FortiGate units. The failed primary unit no longer appears on the Cluster Members list.
The host name and serial number of the primary unit changes. You can see these changes when you log into the web‑based manager or CLI.
The cluster info displayed on the dashboard, cluster members list or from the get system ha status command changes.
If a subordinate unit fails, the cluster continues to function normally. Failure of a subordinate unit results in the following:
If event logging is enabled and HA activity event is selected, the primary unit records log messages that show that a subordinate has been removed from the cluster.
If alert email is configured to send email for HA activity events, the new primary unit sends an alert email containing the log message recorded by the event log.
The cluster contains fewer FortiGate units. The failed unit no longer appears on the Cluster Members list.