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 or shutdown 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.
Is it in the right port? Verify against network diagrams or documentation that you are connected to the correct switch, VLAN, or patch panel port.

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.

Check for CRC errors. CRC errors almost always point back to a bad cable, loose connector, signal interference (EMI) or a problem with duplex/speed mismatch.
Check for runts and giants. This could indicate problems with the sending devices network interface card (NIC).
Check for dropped packets. If the interfaces input is full or output queue is full, it will drop packets. This is because of network congestion. It tells you that the interface cannot process traffic as fast as it is arriving.
Analyse network bandwidth. After seeing dropped packets, you must determine if the link is simply saturated with legitimate traffic. Use monitoring tools to see if the utilization is consistently at or near 100%. This may indicate a need for a higher speed link.

Check if you are using a lower cateogry cable. Using a cat5 cable for a 10 Gbps link will result in massive errors and performance degradation.
Check if the cable is in an area with high EMI.
Investigate signal strength problems. This applies mainly to long-distance fiber or wireless networks. For fiber, a signal that is too weak (due to distance or a failing transceiver) will cause errors. You may need a light meter to measure the signal in decibels.

Swap the Patch Cable: Replace the patch cable connecting to the wall, or the switch to the patch panel, with a brand new or known-good cable.
Swap the Switch Port: Move the connection to a different, known-working port on the same switch. Remember to configure the new port for the correct VLAN if necessary.
Test with a Known-Good Device: Plug a laptop you trust directly into the cable/port to see if it gets a link. This helps isolate if the problem is the cable infrastructure or the original end-device.