Chapter 9 High Availability for FortiOS 5.0 : Virtual clusters : Virtual clustering overview
  
Virtual clustering overview
Virtual clustering is an extension of the FGCP for a cluster of 2 FortiGate units operating with multiple VDOMS enabled. Virtual clustering operates in active-passive mode to provide failover protection between two instances of a VDOM operating on two different cluster units. You can also operate virtual clustering in active-active mode to use HA load balancing to load balance sessions between cluster units. Alternatively, by distributing VDOM processing between the two cluster units you can also configure virtual clustering to provide load balancing by distributing sessions for different VDOMs to each cluster unit.
Figure  shows an example virtual cluster configuration consisting of two FortiGate-620B units. The virtual cluster has two virtual domains, root and Eng_vdm.
The root virtual domain includes the port1 and port2 interfaces. The Eng_vdm virtual domain includes the port5 and port6 interfaces. The port3 and port4 interfaces (not shown in the diagram) are the HA heartbeat interfaces.
 
FortiGate virtual clustering is limited to a cluster of 2 FortiGate units with multiple VDOMs enabled. If you want to create a cluster of more than 2 FortiGate units operating with multiple VDOMS you could consider other solutions that either do not include multiple VDOMs in one cluster or employ a feature such as standalone session synchronization. See “FortiGate Session Life Support Protocol (FGSP)”.