ASAs use keepalive packets between eachother that are sent over the failover link. By using the keepalive packets, the standby ASA monitors the health status of the Active ASA. If the standby ASA stops recieving keepalive packets from the active ASA it will send out 3 test packets, out the monitored interfaces. that is to say it will send test packets out the actual interfaces that will trigger a failover if one of them fails. If the standby ASA still does not recieve a reply from the active ASA it will now assume that the active ASA is dead and will take over the role as active ASA.
The failover link is also used to replicate the configuration between the active and standby ASAs.
The state link is used to replicate the state table and other relevant active connection information.
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