FortiSwitches are status not stable appearing onlin/offline frequentl. High CPU usage on Fortilink.

i am afraid this is too complex to check it using forum tools. Seems your networks is some kind complex.

there is some usefull comands on FGT side

execute switch-controller diagnose-connection

execute switch-controller get-conn-status

(take note of the fortilink interface/stack name)

execute switch-controller get-physical-conn standard FortiSwitch-Stack-FortiLink

Please check with this last command the name of the fortilink stack and change if it is necessary

diagnose switch-controller switch-info mclag peer-consistency-check

on FSW side you can try to check for non planed ports on STP blocked or backup status

diagnose stp instance list

also check if your intended network topology is matching LLDP outputs.

get switch lldp neighbors-summ

this command is assuming all relevant devices have LLDP enabled. Per default it is enabled on all FSW devices, but be aware some other 3rd party devices may not have it enabled, so doble check with STP diagnose command if these ports are not blocked.

i encourage you if this is an urgent matter, get in contact with TAC support.