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Catalyst 2960-S Switch Hardware Installation Guide
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Chapter 3 Troubleshooting
Diagnosing Problems

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.
A unidirectional link can cause loops. It occurs when 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 problems, 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 occurs when duplex and speed 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.
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.

Autonegotiation and Network Interface Cards

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 autonegotiation issues occur.
To troubleshoot autonegotiation problems, try manually setting both sides of the connection. If this does
not solve the problem, there could be a problem with the firmware or software on your NIC. You can
resolve this by upgrading the NIC driver to the latest version.