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Catalyst 3750 Switch Hardware Installation Guide
OL-6336-07
Chapter3 Troubleshooting
Diagnosing Problems
Verify Switch POST Results
As the switch powers on, it begins the POST, a series of tests that runs automatically to ensure that the
switch functions properly. It might take several minutes for the switch to complete POST.
When the switch begins POST, the system LED slowly blinks green. When POST completes, the system
LED blinks amber. If POST fails, the system LED remains amber. If POST completes successfully, the
system LED rapidly blinks green.
Note POST failures are usually fatal. Contact your Cisco technical support representative if your switch does
not pass POST.
Monitor Switch LEDs
You must have physical access to the switch to do this. Look at the port LEDs for troubleshooting
information about the switch. See the “LEDs” section on page 1-7 for a description of the LED colors
and their meanings.
Verify Switch Connections
Review this section when troubleshooting switch connectivity problems.
Bad or Damaged Cable
Always look at the cable for marginal damage or failure. A cable might connect at the physical layer but
then cause packet corruption because of subtle damage to its wiring or connectors. You can identify this
situation because the port will have many packet errors, or the port constantly loses and regains link. In
these situations:
• Change the copper or fiber-optic cable with a known, good cable if necessary.
• Look for broken or missing pins on cable connectors.
• Rule out any insufficient patch panel connections or media convertors between the source and the
destination. If possible, bypass the patch panel or eliminate faulty media convertors, such as
fiber-optic-to-copper convertors.
• Try using the cable in another port or interface to see if the problem also exists there.
Ethernet and Fiber Cables
Make sure that you have the correct cable type for the connection:
• For Ethernet, use Category 3 copper cable for 10 Mb/s unshielded twisted pair (UTP) connections.
Use either Category 5, Category 5e, or Category 6 UTP for 10/100 or 10/100/1000 Mb/s
connections.
• For fiber-optic connectors, verify that you have the correct cable for the distance and port type.
Make sure that the ports on the connected device match and that they use the same type of encoding,
optical frequency, and fiber type. For more information about cabling, see Appendix A, “Connector
and Cable Specifications.”