Stateful failover means that the connection state information is replicated from the active ASA to the standby ASA, so that in the event of a failover, the connections are preserved and do not need to be reestablished. Stateless failover means that the connection state information is not replicated, so that in the event of a failover, the connections are dropped and need to be reestablished by the end hosts. Stateful failover provides a smoother transition and less disruption to the network traffic, while stateless failover is simpler and less resource-intensive. References: * Failover for High Availability - Cisco * Failover Types / ASA Failover Addresses / Failover Requirements | Cisco ...