Session failover
If you enable session failover (also called session pickup) for the cluster, during cluster operation the primary FortiGate-6000 informs the secondary FortiGate-6000 of changes to the primary FortiGate-6000 connection and state tables, keeping the secondary FortiGate-6000 up-to-date with the traffic currently being processed by the cluster.
After a failover the new primary FortiGate-6000 recognizes open sessions that were being handled by the cluster. The sessions continue to be processed by the new primary FortiGate-6000 and are handled according to their last known state.
If you leave session pickup disabled, the cluster does not keep track of sessions and after a failover, active sessions have to be restarted or resumed.