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Failover: Everything needs to match between the ASA.
You can achieve two types of failover configuration with the ASA: active/active and active/standby. 
Active/Active
 Allows for a much greater percentage of available resources for deployment. However, active/active configuration does not provide any support for any type of VPN deployment because the ASA needs to run in Multiple Context mode. So, no further time is spent looking at this option, although it is good to know it exists. 
Active/Standby
 ASA device is active and passing/inspecting traffic while
 the other is in standby, monitoring the state of the other until the time comes when it must take an active role. You have two configuration options when using active/standby failover: 
Stateful
 Stateful configuration allows existing VPN sessions and tunnels to stay
 up even when a failover has occurred and the connecting clients and sites are now
entering through the previous standby device. The current connection “states” are
 synchronized between devices across a dedicated stateful connection between the 
  two ASAs or by using the existing failover interfaces. The following clientless SSL
 VPN objects are not supported with stateful failover:
 ■ Smart tunnels
 ■ Port forwarding
 ■ Plug-ins
 ■ Java applets
 ■ IPv6 clientless or AnyConnect sessions
 ■ Citrix authentication (Citrix users are required to authentication after a failover.)
Stateless
 Stateless configuration supports HA in as much as during a failover the
standby device assumes the active role. It does not support any stateful behavior,
meaning all sessions and connections have to be reestablished after a failover has
Occurred.
 
				
