About Active/Standby Failover

Active/Standby failover lets you use a standby Firewall Threat Defense device to take over the functionality of a failed unit. When the active unit fails, the standby unit becomes the active unit.

If the problem is temporary, the failed unit comes back online to become the standby unit. Otherwise, you need replace the unit. Set the standby unit to primary before the failed unit is replaced, in order to retain the configuration of the secondary unit.