One of my colleagues got me to look at an issue he was having today with a FortiGate reverting to Standalone mode when configuring HA (High Availability) clustering in the GUI. He had checked the usual requirements – same firmware, same VDOM mode etc.
First thing I did was try to configure this via the CLI using:
config system ha
set mode a-p
When I noticed the set mode command was missing in the CLI.
It was then that I recalled this normally happens when an interface is configured for PPPoE or DHCP. In order to allow a FortiGate router to participate in HA all interfaces must be manually configured with IP Addressing. Sure enough, dmz2, an unused interface was configured for DHCP. Setting this to manual and leaving the ip of 0.0.0.0 on the interface then allowed us to finalise the HA configuration in the GUI.