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| 3.5 |  | Isolating Physical Layer Problems |
| 3.5.1 |  | Methodology |

To isolate networking problems that occur at the physical layer an effective and systematic technique must be used to reach a successful outcome. To isolate problems at the physical layers do the following:

- Check for bad cables or connections.
- Verify that the cable from the source interface is properly connected and is in good condition. When doubting the integrity of a cable, swap suspect cables with a known working cable.
- If in doubt that the connection is good, remove the cable, do a physical inspection of both the cable and the interface, and then reseat the cable. Use a cable tester with suspect wall jacks to ensure that the jack is properly wired. A glowing link light will also be an indicator of a successful connection.
- Check that the correct cabling standard is adhered to throughout the network.
- Verify that the proper cable is being used. A crossover cable may be required for direct connections between some devices.
- Ensure the cable is correctly wired.
- Check to make sure that all cables are connected to their correct ports or interfaces. Make sure that any cross-connects are properly patched to the correct location.
- Verify proper interface configurations.
- Check that all switch or hub ports are set in the correct VLAN or collision domain, and that Spanning Tree, speed, and duplex settings are correctly configured. Confirm that any active ports or interfaces are not shut down.
- Check operational statistics and data error rates.
- Use Cisco show commands to check for statistics such as collisions, input, and output errors. The characteristics of these statistics will vary depending on the protocols used on the network.