Cisco ASA Active-Standby
With Active Standby, one ASA is active and the other ASA is in standby mode. The configuration of the two ASAs is identical.
The two ASAs communicate with keepalive packets. When the standby ASA doesn't receive keepalive packets within a certain time (3x 5 seconds) then the standby ASA becomes active. When this happens, the standby ASA acquires the active IP address and MAC address.
When the original active ASA returns, the standby ASA will become the standby ASA again. This is explained in the following note: