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Refer to the above topology, where the FortiLink interface is configured as an aggregate (using ports x1 and x2).
Note: STP is disabled by default on the FortiLink trunk (G200E4Q16900196) connecting to the FortiGate.
In some cases, the STP state is observed to be randomly disabled on FortiLink trunks or trunks configured with MCLAG.
For example, consider the following trunk configuration on FortiSwitch S448DF3XXXXXX305:
config switch trunk
edit "G200E46XXXXX196"
set mode lacp-active
set auto-isl 1
set fortilink 1
set members "port45"
next
edit "_FlInK1_ICL0_"
set mode lacp-active
set auto-isl 1
set mclag-icl enable
set members "port42"
next
edit "8EFTFXXXXX075-0"
set mode lacp-active
set auto-isl 1
set mclag enable <------------------
set static-isl enable
set members "port44"
next
edit "PO3"
set mode lacp-active
set mclag enable <---------------------
set members "port41"
next
end
To verify the config regarding STP state for MCLAG-enabled trunks, use the following command on the FortiSwitch:
show full-configuration switch interface <trunk-name> | grep -B 12 stp-state
config switch interface
edit "8EFTFXXXXX075-0"
...
set stp-state enabled
config switch interface
edit "PO3"
...
set stp-state enabled
Despite STP being enabled in the configuration, logs indicate that the STP state was disabled unexpectedly:
execute log display
2025-06-19 02:09:13 log_id=0105008251 type=event subtype=spanning_tree pri=notice
vd=root user="admin" userfrom="ssh(1.1.1.1)" unit="primary" switch.interface="PO3"
event="stp disabled on interface" msg="user admin disabled STP on primary interface PO3"
2025-06-19 02:09:13 log_id=0105008251 type=event subtype=spanning_tree pri=notice
vd=root user="admin" userfrom="ssh(1.1.1.1)" unit="primary" switch.interface="8EFTFXXXXX075-0"
event="stp disabled on interface" msg="user admin disabled STP on primary interface 8EFTF18000075-0"
Note: The same can be verified by FortiGate: Viewing event logs.
Root Cause:
This behavior occurs when mclag-stp-aware is disabled on the MCLAG-ICL peer FortiSwitches. By default, this setting is enabled on FortiSwitches forming the MCLAG-ICL pair.
To verify the current setting:
show full-configuration switch global
config switch global
set mclag-stp-aware disable
end
Resolution:
Ensure mclag-stp-aware is enabled on the MCLAG-ICL peer switches.
config switch global
set mclag-stp-aware enable
end
Additionally, verify whether this setting is being overridden via configuration pushed from the FortiGate (for example, through custom-commands) or modified by external sources. If this was disabled unintentionally, re-enable the setting as shown above.
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