Spanning Tree Loops; Switch Performance; Speed, Duplex, And Autonegotiation; Autonegotiation And Network Interface Cards - Cisco Catalyst 3560-C Hardware Installation Manual

Diagnosing Problems

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 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 Autonegotiation

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 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.
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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 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 the NIC. You can
resolve this by upgrading the NIC driver to the latest version.

Cabling Distance

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. See the
Adapters" section on page
Catalyst 3560-C and 2960-C Switch Hardware Installation Guide
3-4
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.
B-3.
Chapter 3 Troubleshooting
"Cables and
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