Admin uploads new FortiOS image via GUI to the Active member.

Active Fortigate verifies validity of the image (tampered/broken image will be rejected).

Active member asks admin whether to Back up configuration and upgrade or just Upgrade. In any case, the current configuration will be stored in the partition of the harddisk, together with the current FortiOS image.

On confirmation, Active member saves new image into the secondary partition of the harddisk, then pushes the image to the Standby member and starts upgrade of the Standby member by uploading the new image to the secondary partition,…