Chapter 9 High Availability for FortiOS 5.0 : HA and failover protection : HA heartbeat and communication between cluster units : Connecting HA heartbeat interfaces
  
Connecting HA heartbeat interfaces
For most FortiGate models if you do not change the heartbeat interface configuration, you can isolate the default heartbeat interfaces of all of the cluster units by connecting them all to the same switch. Use one switch per heartbeat interface. If the cluster consists of two units you can connect the heartbeat interfaces together using crossover cables. For an example of how to connect heartbeat interfaces, see “Connecting a FortiGate HA cluster”.
HA heartbeat and data traffic are supported on the same cluster interface. In NAT/Route mode, if you decide to use heartbeat interfaces for processing network traffic or for a management connection, you can assign the interface any IP address. This IP address does not affect HA heartbeat traffic.
In Transparent mode, you can connect the heartbeat interface to your network and enable management access. You would then establish a management connection to the interface using the Transparent mode management IP address. This configuration does not affect HA heartbeat traffic.