What should I do if the ports of two optical modules are not lit (Link Down) after being connected?

Troubleshoot the problem by following these steps:

Confirm that both ports are not shut down.

Check that the optical module model, wavelength, speed, and distance match.

Check that the fiber type (single-mode/multi-mode) is correct and that the patch cord is intact.

Clean the optical port and fiber connector.

Cross-connect the optical fibers or modules to locate the fault.