Troubleshooting Tip: FortiGate is forwarding Rapid STP frames in a software switch even it’s disabled.

Diagram:

Rapid STP frames are forwarded in a software switch even though it is disabled on v7.4.3 and below.

Lab_FGT1 # diagnose sniffer packet port1

interfaces=[port1]

filters=[none]

17.548280 stp 802.1s, rapid stp, cist flags [forward, agreement]

19.688234 stp 802.1s, rapid stp, cist flags [forward, agreement]

21.798232 stp 802.1s, rapid stp, cist flags [forward, agreement]

Upgrade the firmware to v7.4.4 or higher to fix this behavior. This is related to bug 985928.

When the members of software switch (port1 and port2) are connected to the same switch network, enable stpforward first before upgrading the firmware to v7.4.4 or higher, as it will cause a switch loop due to correcting the misbehavior of rapid STP forward.

config system interface

    edit port1

        set stpforward enable

    next

        edit port2

            set stpforward enable

        next

    end