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.
- 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.