Refer to the exhibits.

When loop guard is enabled on port1 and port2 configured with the same native VLAN (VLAN 10), there are specific scenarios under which port1 can be shut down due to loop guard operation:

A.port1 was shut down by loop guard protection.Loop guard is a specific feature used in network environments to prevent alternative or redundant loops. When loop guard is active, it can shut down a port if it stops receiving BPDU (Bridge Protocol Data Units) on a port that is expected to receive them, assuming a loop or link failure and putting the port into an inconsistent state to prevent potential loops.

B.STP triggered a loop and applied loop guard protection on port1.If the Spanning Tree Protocol (STP) detects a loop or loss of BPDU transmissions while loop guard is enabled, it will proactively shut down the port to prevent network instability or a broadcast storm. This is an essential function of loop guard within the context of STP, providing additional protection against topology changes that could introduce loops.

References:

Additional details about loop guard functionality and STP interaction can be found in the FortiSwitch administration guides, accessible viaFortinet Documentation.