About high availability

Table 19: Comparison of HA modes
Active-passive HA	Config-only HA
2 FortiMail units in the HA group	2-25 FortiMail units in the HA group
Typically deployed behind a switch	Typically deployed behind a load balancer
Both configuration* and data synchronized	Only configuration* synchronized
Only primary unit processes email	All units process email
No data loss when hardware fails	Data loss when hardware fails
Failover protection, but no increased processing capacity	Increased processing capacity, but no failover protection

* For exceptions to synchronized configuration items, see “Configuration settings that are not synchronized”.


	If the config-only HA group is installed behind a load balancer, the load balancer stops sending email to failed FortiMail units. All sessions being processed by the failed FortiMail unit must be restarted and will be re-directed by the load balancer to other FortiMail units in the config-only HA group.

You can mix different FortiMail models in the same HA group. However, all units in the HA group must have the same firmware version.


	When mixing FortiMail models, the HA group is limited by the capacity and configuration limits of the least powerful model.

Communications between HA cluster members occur through the heartbeat and synchronization connection. For details, see “About the heartbeat and synchronization”.

To configure FortiMail units operating in HA mode, you usually connect only to the primary unit (master). The primary unit’s configuration is almost entirely synchronized to secondary units (slave), so that changes made to the primary unit are propagated to the secondary units.

Exceptions to this rule include connecting to a secondary unit in order to view log messages recorded about the secondary unit itself on its own hard disk, and connecting to a secondary unit to configure settings that are not synchronized. For details, see “Configuration settings that are not synchronized”.


	To use FortiGuard Antivirus or FortiGuard Antispam with HA, license all FortiMail units in the cluster. If you license only the primary unit in an active-passive HA group, after a failover, the secondary unit cannot connect to the FortiGuard Antispam service. For FortiMail units in a config-only HA group, only the licensed unit can use the subscription services.