Chapter 9 High Availability for FortiOS 5.0 : Operating a cluster : Clusters and FortiGuard services : FortiGuard and active-passive clusters
  
FortiGuard and active-passive clusters
For an active-passive cluster, only the primary unit processes traffic. Even so, all cluster units communicate with the FDN. Only the primary unit sends FortiGuard Web Filtering and Antispam requests to the FDN. All cluster units receive FortiGuard Antivirus, IPS, and application control updates from the FDN.
In an active-passive cluster the FortiGuard Web Filter and Email Filter caches are located on the primary unit in the same way as for a standalone FortiGate unit. The caches are not shared among cluster units so after a failover the new primary unit must build up new caches.
In an active-passive cluster all cluster units also communicate with the FortiGuard Analysis and Management Service (FAMS).