If the optical module malfunctions, such as aging or damage due to overheating, even if the optical fiber cable is in good condition, the device cannot correctly receive and process the optical signal, resulting in the optical signal indicator light staying red. If compatibility issues persist, replace the optical module with one certified for the switch brand. You can test whether there is light from the TX end of the module and whether the transmitted light power is within the normal range, or check DDM information.