Basic Layer 1 and 2 Troubleshooting

14-06+Basic+Layer+1+and+Layer+2+Troubleshooting.pdf

involves checking for layer 1 and 2 issues Copper and fibre cables are liable to break

Common layer 1 problems

The interface is administratively shut down The cable is disconnected on either of the ends The device on the other end of the cable is powered off Broken connectors which cause loose connections Bent or stretched cables which lead to broken wires or fibers. Electromagnetic interference (EMI) sources such as motors or microwaves which can cause errors in transmission (newer cable is less susceptible to this)

Layer 1 Troubleshooting commands

Switch# show ip interface brief

*Administratively down - issue "no shutdown" "down/down" This indicates a layer 1 issue. Check the interface is cabled at both ends and the device on the other side is powered on. *Up/down" - indicates a layer 2 issue or speed mismatch. Check the interface configuration matches on both sides of the link.

Switch# show interface

If the inteface is reporting an excessive amount of errors it could be either a layer 1 or layer 2 problem Check the integrity of the cable Check the configuration matches on both sides of the link.

Show interface fastEthernet 0/2 Full duplex 100mb/s

Speed and Duplex Mismatches

A possible error is speed and/or duplex mismatches. Incorrect speed settings can cause the interface to operate below its maximum speed Speed mismatches typically bring the interface down due to collisions or slow downs. The show interface command will report an excessively high number of errors. Both sides of the link should be the same

Cisco devices auto default to "Auto" Configure both sides of the link!

CDP should detect a duplex mismatch

CDP-4-DUPLEX_Mismatch: duplex mismatch discovered on FastEthernet0/0 (not half duplex)