Troubleshooting Tip: FortiSwitch Port showing down after the upgrade in an MC-LAG configuration

In the topology shown below, port53.1 on the second switch port appears down after the upgrade. A similar issue may also be observed on the first switch. Notably, the issue resolves upon downgrading the firmware to the previous version.

Root Cause:
The switches are configured in an MC-LAG (Multi-Chassis Link Aggregation) setup. This issue can occur when only one switch in the MC-LAG pair is upgraded. In such cases, the affected port may transition into LACP fallback mode and shut down to prevent network loops.

Example output:

diagnose switch mclag peer-consistency-check
G200FT920903808 MISMATCH UP MISMATCH port53.1 port53.1

diagnose switch physical-ports port-block status

Port Block Status
----------------------------------------------------------------------
port53.1 lacp-fallback

Resolution:
To resolve the issue, ensure that both switches in the MC-LAG pair are upgraded to the same firmware version. Once the upgrade is complete on both devices, recheck the port status.