Chapter 9 High Availability for FortiOS 5.0 : An introduction to the FGCP : HA and distributed clustering
  
HA and distributed clustering
The FGCP supports widely separated cluster units installed in different physical locations. Distributed clusters can have cluster units in different rooms in the same building, different buildings in the same location, or even different geographical sites such as different cities, countries or continents.
Just like any cluster, distributed clusters require heartbeat communication between cluster units. In a distributed cluster this heartbeat communication can take place over the Internet or over other transmission methods including satellite linkups.
Because of the possible distance it may take a relatively long time for heartbeat packets to be transmitted between cluster units. To support a distributed cluster you may need to increase the heartbeat interval so that the cluster expects extra time between heartbeat packets. For information about changing the heartbeat interval and other heartbeat related settings, see “Modifying heartbeat timing”.