Cisco Systems CGS 2520 Switch Performance

4-4

Cisco CGS 2520 Hardware Installation Guide

OL-31444-01

Chapter 4 Troubleshooting

Diagnosing Problems

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 re-enable 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, re-enable 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.

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 the

neighbor, but the traffic from the neighbor is not received by the switch. A broken 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 Auto-Negotiation, page 4-4

• Auto-Negotiation and Network Interface Cards, page 4-5

• Cabling Distance, page 4-5

Port statistics that show a large amount of alignment errors, Frame Check Sequence (FCS), or late

collisions errors, might mean a speed or duplex mismatch.

A common issue 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 auto-negotiation 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.