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Catalyst 2350 Switch Hardware Installation Guide
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Chapter 4 Troubleshooting
Diagnosing Problems
SFP Module 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.
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 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.
Loops can be caused by a unidirectional link. A unidirectional link occurs whenever the traffic sent by
the switch is received by its neighbor, but the traffic from the neighbor is not received by the switch. A
broken fiber-optic cable, other cabling, or a port issue could cause this one-way communication.
You can enable UniDirectional Link Detection (UDLD) on the switch to help identify unidirectional link
problems. For information about enabling UDLD on the switch, see the “Understanding UDLD” section
in the switch software configuration guide on Cisco.com.
Switch Performance
Speed, Duplex, and Autonegotiation
If the port statistics show a large amount of alignment errors, frame check sequence (FCS), or
late-collisions errors, this might mean a speed or duplex mismatch.
A common issue with speed and duplex is when the duplex settings are mismatched between two
switches, between a switch and a router, or between the switch and a workstation or server. Mismatches
can happen when manually setting the speed and duplex or from autonegotiation issues between the two
devices.
