Interface Settings
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 shutdown on either side of the connection. If
needed, reenable 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.
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.
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.
Catalyst 3560-CX and 2960-CX Switch Hardware Installation Guide
52
Troubleshooting
Switch Performance
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