Chapter 9 High Availability : An introduction to the FGCP : FGCP HA terminology : Interface monitoring
  
Interface monitoring
You can configure interface monitoring (also called port monitoring) to monitor FortiGate interfaces to verify that the monitored interfaces are functioning properly and connected to their networks. If a monitored interface fails or is disconnected from its network the interface leaves the cluster and a link failover occurs. For more information about interface monitoring, see “Link failover (port monitoring or interface monitoring)”.