A high-availability (HA) pair configuration uses only the hardwire serial cable connection to determine device state. A power outage occurs to the PDU powering the active unit. The standby unit takes over the active role as expected.
How is the peer unit able to determine the active unit is unavailable?
In a high-availability (HA) pair configuration using a hardwire serial cable connection, the peer unit determines the active unit is unavailable by detecting a voltage loss on the serial cable. When there is a power outage, the voltage level drops, signalling to the standby unit that the active unit is down, prompting it to take over the active role.
A - Correct Hardwired failover When BIG-IP systems are configured to use hardwired failover, a failover cable monitors the voltage levels of the systems and assures that the peer unit is available. If the standby BIG-IP system detects a voltage level drop in the active system, the standby system initiates the failover process and becomes the active device. No data is sent or received over a failover cable https://my.f5.com/manage/s/article/K95002127