Troubleshooting Tip: FortiGate GUI fails to fully load allowed VLANs in FortiSwitch ports

After upgrading to FortiOS 7.4.9 and later 7.4.x builds (including 7.4.11), administrators may experience an issue when configuring Allowed VLANs on FortiSwitch ports via the FortiGate GUI.

When editing a FortiSwitch port, some VLAN entries fail to fully load in the GUI. These VLANs appear unchecked, even though they are correctly configured on the port.

In FortiOS 7.4.9-7.4.11, when an administrator performs the following steps, several VLANs that were previously selected will appear unchecked:

1. Logs in to the FortiGate GUI.

2. Navigates to WiFi & Switch Controller -> FortiSwitch Ports.

3. Edits a switch port under Allowed VLANs.

4. If Apply is selected:

• Only the VLANs currently visible and checked in the GUI are saved.

• Missing VLANs are removed from the port configuration, which can cause a traffic disruption.

5. If the administrator edits the allowed Allowed VLANs a second time, all previously selected VLANs will be visible as expected. 

Recommended actions:

• This behavior is not reproducible in 7.6.6, which is the current recommended release. See Technical Tip: Recommended release for FortiOS.

• If the issue remains on 7.4.x:

  • Use the CLI where possible.

  • If the GUI is used, double-check selected VLANs before applying changes.