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.
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.
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.
Clearing the Switch IP Address and Configuration
If you have configured a new switch with a wrong IP address, or if all of the switch LEDs start blinking when
you are trying to enter Express Setup mode, you can clear the configured IP address. The switch returns to
the factory default settings.
This procedure clears the IP address and all configuration information stored on the switch. Do not follow
this procedure unless you want to completely reconfigure the switch.
Note
Catalyst 3850 Switch Hardware Installation Guide
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Troubleshooting
Switch Performance
