Interface Settings; Ping End Device; Spanning Tree Loops; Switch Performance - Cisco Catalyst 3850 Hardware Installation Manual

Diagnosing Problems
Note
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Interface Settings

Verify that the port or interface is not disabled or powered off. If a port or 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 shut down 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 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 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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Catalyst 3850 Switch Hardware Installation Guide
6-4
When ordering or using CX1 cables, ensure that the version identifier is 2 or higher.
For long wave SFP+ modules, a mode conditioning patch might improve performance over
maximum link distances with MMF connections.
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.
Chapter 6 Troubleshooting
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