Troubleshooting Tip: 25Gbps DAC cable does not link between different FortiSwitch models

•  Set fec-state to the value cl91 on both sides of the link:

config switch physical-port
    edit "portxx"
        set fec-state cl91
end

Explanation:
On high-speed links such as 25 Gbps, especially when using DAC cables, both devices must use the same 'error correction mechanism' at the physical layer.

This mechanism is called FEC (Forward Error Correction).

FEC helps ensure a clean signal by correcting errors caused by:

• Cable distance.
• Electrical interference.
• Differences in the PHY chips of each switch model.

There are different types of FEC, and the most commonly used in 25/100G links are:

• CL74: Basic FEC.
• CL91: Advanced FEC (Reed-Solomon).

1. Corrects many more errors than CL74.
2. Required when switches use different ASICs/PHYs or have different tolerances.
3. It is the standard for 100G and may also be necessary for certain 25G links.