Some FortiGate models with NP6 processors include the following option that can help your FortiGate recover from an internal link failure:
config system npu
set recover-np6-link (disable | enable}
This command is available on several FortiGate models, including the 1500D, 1500DT, 3000D, 3100D, and 3200D.
In some configurations with aggregate interfaces, an internal link failure can occur on some FortiGate models. This failure can cause one of the aggregate interface members to transmit irregular LACP packets. You can recover from this failure by enabling
recover-np6-link and restarting the FortiGate. Every time the FortiGate restarts, this command checks for and recovers from any internal link failures that it finds.
Enabling this option may cause the FortiGate to take slightly longer to start up but should not affect performance.