Troubleshooting Tip: FortiLink ISL timing out for trunk after upgrading FortiSwitch to v7.6.4

After upgrading a FortiSwitch configured in FortiLink Layer 3 mode, the following log messages may appear continuously every second:

1: 2025-10-10 11:01:49 log_id=XXXXXXXXXX tz=+0200 type=event subtype=switch_controller pri=notice vd=root msg="FortiLink: ISL timing-out for trunk(fortilinkLayer3) member port-num(1) port1 did not receive ISL pkt for(30) sec"

2: 2025-10-10 11:01:48 log_id=XXXXXXXXXXX tz=+0200 type=event subtype=switch_controller pri=notice vd=root msg="FortiLink: ISL timing-out for trunk(fortilinkLayer3) member port-num(1) port1 did not receive ISL pkt for(30) sec"

Resolution:

To address the FortiLink ISL timeout issue, perform the following steps directly on the FortiSwitch CLI:

Step 1: Disable LLDP on the FortiSwitch uplink port (the FortiLink port). In this example, port1 is the uplink experiencing the ISL timeout.

config switch physical-port
    edit port1
        set lldp-status disable
    next
end

Step 2: After about one minute, re-enable LLDP in tx-rx mode on the same port:

config switch physical-port
    edit port1
        set lldp-status tx-rx
    next
end

Note:
Ensure these commands are executed directly on the FortiSwitch and not from the FortiGate.

Alternatively, the LLDP profile can be set to default on the uplink FortiSwitch port that connects to the router.
This configuration should only be applied on the top FortiSwitch connected to the router and not on ports used for FortiSwitch-to-FortiSwitch connections.

config switch physical-port
    edit "port1"
        set lldp-profile "default"
    next

end

For more info, see Enabling LLDP on a port and Configuring LLDP-MED settings.