Examine the cable for marginal damage or failure. A cable might be just good enough to connect at the physical layer, but it could corrupt packets as a result of subtle damage to the wiring or connectors. You can identify this problem because the port has many packet errors or it constantly flaps (loses and regains link).

• Exchange the cable with a known good cable.

• Look for broken or missing pins on cable connectors.

• Rule out any bad patch panel connections or media converters between the source and the destination. If possible, bypass the patch panel.

• Try the cable in another port to see if the problem follows the cable.

Verify that both sides have a link. A broken wire or a shutdown port can cause one side to show a link even though the other side does not have a link.

A port LED that is on does not guarantee that the cable is functional. It might have encountered physical stress, causing it to function at a marginal level. If the port LED does not turn 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.

• Look for loose connections. Sometimes a cable appears to be seated but is not. Disconnect the cable, and then reconnect it.

If a port appears to malfunction:

• Verify the status of all ports. See Port status LEDs for descriptions of the LEDs and their meanings.

• Use the show interfaces privileged EXEC command to see if the port is error-disabled, disabled, or shut down. Reenable the port if necessary.

• Verify the cable type.

Verify that the interface is not disabled or powered off. If an interface is manually shut down on either side of the link, it does not come up until you reenable the interface. Use the show interfaces privileged EXEC command to see if the interface is error-disabled, disabled, or shut down on either side of the connection. If needed, reenable the interface.

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.

Port statistics that show a large amount of alignment errors, frame check sequence (FCS), or late-collision errors, a common issue when duplex and speed settings are mismatched between two devices.

To maximize switch performance and to ensure a link, follow one of these guidelines when changing the duplex or the speed settings.

• Let both ports autonegotiate both speed and duplex.

• Manually set the speed and duplex parameters for the interfaces on both ends of the connection.

• If a remote device does not autonegotiate, use the same duplex settings on the two ports. The speed parameter adjusts itself even if the connected port does not autonegotiate.

Problems sometimes occur between the switch and third-party network interface cards (NICs). By default, the switch ports and interfaces autonegotiate. Laptops or other devices are commonly set to autonegotiate, yet sometimes issues occur.

To troubleshoot autonegotiation problems, try manually setting both sides of the connection to the same speed and duplex mode. If this does not solve the problem, there could be a problem with the firmware or software on the NIC. You might resolve this by upgrading the NIC driver to the latest version.

If the port statistics show excessive FCS, late-collision, or alignment errors, verify that the cable distance from the switch to the connected device meets the recommended guidelines.