Troubleshooting Tip: An unexpected inter-switch link connection is causing Spanning Tree Protocol (STP) discarding in the Managed FortiSwitch

[sachitdas_FTNT]

Description	This article describes the steps to take when an inter-switch link shows STP discarding, but the expected behavior is the link should be in a 'forwarding state'.
Scope	FortiGate and Managed FortiSwitches version 7.x and above.
Solution	Refer to the diagram below where port53 of Core3 FSW shows STP discarding however port54 of Core3 should be in a discarding state because it is expected the Core1 and Core2 MCLAG-ICL pairs to act as Root Bridge, hence Core3 port53 should be the root port and forwarding. In such cases, collect the output of the 'diag stp instance list' on core1, core2, core3, and Third FortiSwitches to verify which FortiSwitch is acting as the root bridge. In the below example, the Third FortiSwitch is acting as the root Bridge which is incorrect. Third # diagnose stp instance list MST Instance Information, primary-Channel: Instance ID 0 (CST) Config Priority 16384 Bridge MAC 04dxxxxxad8, MD5 Digest 9999b43xxxxxx854f9991c4a487 Root MAC 04dxxxxxad8, Priority 16384, Path Cost 0, Remaining Hops 20 (This bridge is the root) Regional Root MAC 04dxxxxxad8, Priority 16384, Path Cost 0 (This bridge is the regional root) Instance ID 15 Config Priority 16384 , VLANs 4094 Bridge MAC 04dxxxxxad8 Regional Root MAC 04dxxxxxad8, Priority 16384, Path Cost 0 (This bridge is the regional root) Refer to the below KB article and configure lower & same STP priority on both Core1 and Core2 to make sure Core1 and Core2 act as STP root bridges. Technical Tip: Configure STP priority using 'custom-command 'on FortiGate