Troubleshooting Tip: MCLAG-ICL interface in STP discarding state

[sachitdas_FTNT]

Description	This article describes steps to take when the MCLAG-ICL interface shows a 'discarding' STP state.
Scope	Managed FortiSwitch version 7.2.x and above.
Solution	Refer to the following topology: Both the core1 and core2 MCLAG-ICL FortiSwitches should act as root bridge. Execute the following command on both Core FortiSwitches to identify if both core FortiSwitches are acting as root bridge: diag stp instance list Instance ID 0 (CST) Config Priority 20480 Bridge MAC xxxxxc6, MD5 Digest 9999b4991c4a487 Root MAC xxxxxc6, Priority 20480, Path Cost 0, Remaining Hops 20 (This bridge is the root) Regional Root MAC xxxxxc6, Priority 20480, Path Cost 0 (This bridge is the regional root) Instance ID 15 Config Priority 20480 , VLANs 4094 Bridge MAC xxxxxc6 Regional Root MAC xxxxxc6, Priority 20480, Path Cost 0 (This bridge is the regional root) The root MAC address will be the MCLAG mac address: diagnose switch mclag icl _FlInK1_ICL0_ icl-ports 25-26 egress-block-ports 1,5,13 interface-mac 78:18:ec:xx local-serial-number FS1E24TFxxxxxxxx peer-mac 78:18:ec:yy peer-serial-number FS1E24TFyyyyyyyy Local uptime 0 days 20h: 2m: 0s Peer uptime 0 days 19h:30m: 0s MCLAG-STP-mac xxxxxc6 keepalive interval 1 keepalive timeout 60 dormant candidate Peer split-brain Disabled But if only one of the core FortiSwitches acts as a root Bridge, then the MCLAG-ICL interface will show the STP discarding state on both Core FortiSwitches. For example: diagnose stp instance list _FlInK1_ICL0_ 200G 1 128 DESIGNATED LEARNING 2 EN ED >>> port25 STP state FORWARDING mismatches trunk `_FlInK1_ICL0_`: Active members in HW: port25 port26 >>> port26 STP state FORWARDING mismatches trunk `_FlInK1_ICL0_`: Active members in HW: port25 port26 Solution: Refer to the article and change STP priority to 4096 on both Core FortiSwitches MSTP instance 0 and 15. In this way both Core FortiSwitches will act as root Bridge and MCLAG-ICL interface STP state will change to forwarding. See this article.