Chapter 9 High Availability for FortiOS 5.0 : HA and failover protection : Link failover (port monitoring or interface monitoring) : Testing link failover
  
Testing link failover
You can test link failure by disconnecting the network cable from a monitored interface of a cluster unit. If you disconnect a cable from a primary unit monitored interface the cluster should renegotiate and select one of the other cluster units as the primary unit. You can also verify that traffic received by the disconnected interface continues to be processed by the cluster after the failover.
If you disconnect a cable from a subordinate unit interface the cluster will not renegotiate.