Technical Tip: Unable to select interface when configuring HA Reserved Management Interface

1. For the example below, it is impossible to select port3 under the ha-mgmt-interface.
                                        

2. Check that port3 has been assigned to the DHCP server, remove the port3 if it is assigned:
                      

3. Check if the static route has been configured, remove the port3, or assign the static route to the other interface:
                                                                         

4. Check if the port3 is assigned to address remove the address or assigned to a different interface:
                                                                        

5. Alternatively, a grep command can be used to check whether the port3 is in use and verify the related configuration to it:

show | grep -f port3

More ideas about the usage of the grep command can be found in Technical Tip: The usage of 'grep' filter command on the FortiGate CLI.

6. The port3 will be shown under the ha-mgmt-interface after removing from the DHCP server, static route, and the address:

Alternatively, check if the interface is used as a reference via the CLI:

diagnose sys cmdb refcnt show system.interface.name port3

Related article:

Technical Tip: How to Check Referenced Objects