3-3
Catalyst 3750 Switch Hardware Installation Guide
OL-6336-07
Chapter 3 Troubleshooting
Diagnosing Problems
• For copper connections, determine if a crossover cable was used when a straight-through cable was
required or the reverse. Enable auto-MDIX on the switch, or replace the cable.
Link Status
Verify that both sides have link. A single broken wire or one shutdown port can cause one side to show
link, but the other side does not have link.
A link LED does not guarantee that the cable is fully functional. The cable might have encountered
physical stress that causes it to function at a marginal level. If the link light for the port does not come on:
• Connect the cable from the switch to a known, good device.
• Make sure that both ends of the cable are connected to the correct ports.
• Verify that both devices have power.
• Verify that you are using the correct cable type. See Appendix A, “Connector and Cable
Specifications,” for more information.
• Look for loose connections. Sometimes a cable appears to be seated, but is not. Disconnect and then
reconnect the cable.
PoE Connections
When a powered device is connected to PoE port, but no power is received, you should:
• Use the Mode button to show the PoE status for all ports. See Table 1-5 on page 1-18 for a
description of the LEDs and their meanings.
• Monitor the port status by using the show interfaces privileged EXEC command to check the port
error-disabled, disabled, or shutdown status. Re-enable the port if necessary.
• Verify that the power supply installed in the switch meets the power requirements of your connected
devices. See the “Power Connectors” section on page 1-25 for more information.
• Check the cable type. Many legacy powered devices, including older Cisco IP phones and access
points that do not fully support IEEE 802.3af, might not support PoE when connected to the switch
by a crossover cable. Replace the crossover cable with a straight-through cable.
Caution PoE faults are caused when noncompliant cabling or powered devices are connected to a
PoE port. Only standard-compliant cabling can be used to connect Cisco pre-standard
IP phones and wireless access points or IEEE 802.3af-compliant devices to PoE ports.
(You must remove a cable or device that causes a PoE fault from the network.)
Transceiver Module Port Issues
Use only Cisco SFP modules on the switch. Each Cisco module has an internal serial EEPROM that is
encoded with security information. This encoding provides a way for Cisco to identify and validate that
the module meets the requirements for the switch. Check these items:
• Bad or incorrect SFP module. Exchange the suspect module with a known, good module. Verify that
this module supports this platform. See the “SFP Modules” section on page 1-6 for a list of
supported SFP modules.
