Trouble Shooting Networking Cables

Immediately Check the Physical & Status Links

Check the LED status indicators. Green light means healthy link. No light means no power or no physical link detected
Check the interface status. The port could be disabled on the switch, or the switch automatically disabled it due to duplex mismatch
Check physical cabling and connections. Are both ends of the cable terminated correctly? Is the fiber cable dirty? Make sure there are no transceiver mismatches. Check if interfaces are single mode or multimode.

Check for duplex and speed mismatches. If one side is set to full and the other to auto, this can cause massive collisions and CRC errors. Make sure the speed is the same on both ends as well.
Verify PoE status. Check if too much power is drawn from PoE ports. Make sure PoE devices are compatible with PoE standards (PoE, PoE+, PoE++).
Check for blocked ports. Due to STP putting a port in a blocking state to prevent network loops. It can also be caused by securtiy features like port-securtiy that shut down a port if an anauthorized device connects.