Chapter 9 High Availability for FortiOS 5.0 : Configuring and connecting HA clusters : Example: HA and redundant interfaces : Connecting multiple redundant interfaces to one switch while operating in active-active HA mode
  
Connecting multiple redundant interfaces to one switch while operating in active-active HA mode
In an active-active cluster, all cluster units send and receive packets. To operate a cluster with redundant interfaces in active-active mode, with multiple redundant interfaces connected to the same switch, you must separate the redundant interfaces of each cluster unit into different redundant interfaces on the connecting switch.