Chapter 9 High Availability for FortiOS 5.0 : An introduction to the FGCP : FGCP high availability best practices : Heartbeat interfaces
  
Heartbeat interfaces
Fortinet suggests the following practices related to heartbeat interfaces:
 
Do not use a FortiGate switch port for the HA heartbeat traffic. This configuration is not supported.
Isolate heartbeat interfaces from user networks. Heartbeat packets contain sensitive cluster configuration information and can consume a considerable amount of network bandwidth. If the cluster consists of two FortiGate units, connect the heartbeat interfaces directly using a crossover cable or a regular Ethernet cable. For clusters with more than two units, connect heartbeat interfaces to a separate switch that is not connected to any network.
If heartbeat traffic cannot be isolated from user networks, enable heartbeat message encryption and authentication to protect cluster information. See “Enabling or disabling HA heartbeat encryption and authentication”.
Configure and connect redundant heartbeat interfaces so that if one heartbeat interface fails or becomes disconnected, HA heartbeat traffic can continue to be transmitted using the backup heartbeat interface. If heartbeat communication fails, all cluster members will think they are the primary unit resulting in multiple devices on the network with the same IP addresses and MAC addresses (condition referred to as Split Brain) and communication will be disrupted until heartbeat communication can be reestablished.
Do not monitor dedicated heartbeat interfaces; monitor those interfaces whose failure should trigger a device failover.