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Cisco ME 3400E Ethernet Access Switch Hardware Installation Guide
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Appendix 4 Troubleshooting
Diagnosing Problems
• Verify that you are using the correct cable type. See Appendix B, “Connector and Cable
Specifications,” for more information.
• Look for loose connections. Sometimes a cable appears to be seated, but is not. Disconnect the cable
and then reconnect it.
SFP 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 wrong SFP module. Exchange the suspect module with known good module. Verify that the
module is supported on this platform. (The switch release notes on Cisco.com list the SFP modules
that the switch supports.)
• Use the show interfaces privileged EXEC command to see if the port or module is error-disabled,
disabled, or shutdown. Re-enable the port if needed.
• Make sure that all fiber connections are properly cleaned and securely connected.
Port and Interface Settings
An obvious but sometimes overlooked cause of port connectivity failure is a disabled interface. Verify
that the interface is not disabled or powered off for some reason. If an interface is manually shut down
on one side of the link or the other side, the link does not come up until you re-enable the interface. Use
the show interfaces privileged EXEC command to see if the interface is error-disabled, disabled, or
shutdown on either side of the connection. If needed, re-enable the interface.
Ping the End Device
Ping from the directly connected switch first, and then work your way back port by port, interface by
interface, trunk by trunk, until you find the source of the connectivity issue. Make sure that each switch
can identify the end device MAC address in its Content-Addressable Memory (CAM) table.
Spanning Tree Loops
STP loops can cause serious performance issues that look like port or interface problems.
Unidirectional links can cause spanning-tree loops. A unidirectional link occurs when the traffic sent by
the switch is received by its neighbor, but does not receive traffic sent by the neighbor. A broken
fiber-optic cable, other cabling, or a port issue could cause this one-way communication.
The UniDirectional Link Detection (UDLD) protocol helps identify unidirectional link problems. For
more information, see the “Understanding UDLD” section in the switch software configuration guide on
Cisco.com.