Troubleshooting Switch Interfaces

February 22, 2025

Troubleshooting Cisco switch interface errors requires a structured approach to diagnose and resolve issues efficiently. Here’s a step-by-step guide:

1. Check Interface Status

Run:

Switch#show interfaces status

Connected → Interface is up and functioning
Notconnect → No device is connected or a cable issue
Err-disabled → Error condition detected (see step 5)
Disabled → Interface is administratively down

If the interface is down, enable it:

Switch#conf t
interface <interface_id>
no shutdown
exit

2. Verify Cable and Physical Connection

Ensure the cable is properly plugged in.
Test with a different cable.
Check port LED status (blinking green = active, amber = issue).

3. Check Interface Errors

Run:

Switch#show interfaces <interface_id>

Look for:

Input errors: CRC errors, frame errors → Cable or duplex mismatch.
Output errors: Congestion or hardware failure.
CRC errors: Bad cables, interference, or duplex mismatch.
Collisions: High count may indicate a duplex mismatch.

4. Check Duplex and Speed Settings

Run:

Switch#show interfaces <interface_id> status

If there is a duplex mismatch, manually configure it:

Switch#conf t
interface <interface_id>
duplex full
speed 1000
exit

5. Check for Err-Disabled State

Run:

Switch#show interfaces status | include err-disabled

Common causes:

Port security violation
BPDU guard violation
Link flap detection
UDLD failure

Fix:

Switch#conf t
interface <interface_id>
shutdown
no shutdown
exit

To find the specific reason:

Switch#show interfaces <interface_id> | include error

If it’s due to port security:

Switch#show port-security interface <interface_id>
Switch#clear port-security sticky interface <interface_id>

If BPDU guard triggered:

Switch#spanning-tree bpduguard disable

To automatically recover from err-disabled state:

Switch#conf t
errdisable recovery cause all
errdisable recovery interval 30
exit

6. Check VLAN and Trunk Settings

Run:

Switch#show vlan brief

Ensure the port is in the correct VLAN.

For trunk ports:

Switch#show interfaces trunk

If VLANs are missing, add them:

Switch#conf t
interface <interface_id>
switchport mode trunk
switchport trunk allowed vlan add <vlan_id>
exit

7. Check Spanning Tree (STP) Issues

Run:

Switch#show spanning-tree interface <interface_id>

If the port is in Blocking state, STP is preventing loops.
If the port is flapping, check STP settings.

To disable STP (use cautiously):

Switch#conf t
interface <interface_id>
spanning-tree portfast
exit

8. Check MAC Address Table

Run:

Switch#show mac address-table interface <interface_id>

If there are no MAC addresses, the device might not be communicating.

9. Debugging Further

To see real-time logs:

Switch#terminal monitor
debug spanning-tree events
debug interface <interface_id>

Disable debugging after use:

Switch#undebug all

10. Reload Interface or Switch

If all else fails:

Switch#reload

or shut/no shut the interface.

The Lazy Networker