This article describes how to reboot only the Slave firewall in HA cluster without interrupting services in Master device.
Login to the Slave FortiGate via SSH/Console on Master FortiGate.
# execute ha manage
After login to the Slave FortiGate run execute reboot.
In this case, there will be no interruption in traffic since all the traffic will be flowing from Master FortiGate and only the Slave FortiGate will be rebooted.
Master # execute ha manage 0 admin <---- login to the slave firewall via ssh
Warning: Permanently added '169.254.0.1' (ED25519) to the list of known hosts.
Slave # execute reboot >>> Now you are logged in slave firewall and you can run the reboot command
This operation will reboot the system !
Do you want to continue? (y/n)y
System is rebooting...
System Event logs in Slave Firewall: