Troubleshooting a VLAN Mismatch

Switch Configuration, Access Management, Troubleshooting

Contents

Scenario

In the Packet Tracer lab, I will be troubleshooting a VLAN mismatch between two switches. A VLAN mismatch occurs when there is a discrepancy between the VLAN configurations on two connected network devices, typically switches, that are communicating over a trunk link. This mismatch can lead to various network issues, including connectivity problems, security vulnerabilities, and degraded network performance.

This is the network topology I will be using:

Objectives

  1. Create a VLAN mismatch in Switch0
  2. Verify the native VLAN on Switch0
  3. Configure both switches to use the same VLAN

Results

📄 Task 1: Create a VLAN mismatch in Switch0

VLAN mismatches occur when two switch communicating over a trunk link utilize different VLANs on either end of the link. Trunk-linked switches must use the same VLAN. To create a VLAN mismatch, I will configure Switch0 to use VLAN 10 instead of the default VLAN 1 as its native VLAN.

CDP generates an error message stating that it detects a VLAN mismatch on Switch0’s F0/1 interface.


📄 Task 2: Verify the native VLAN on Switch0

I will issue two CiscoIOS commands that will help me find out what native VLAN is assigned to port F0/1 on Swithc0:

show interfaces trunk

show interfaces g0/1 switchport

The output shows that VLAN 10 is assigned to interface F0/1.


📄 Task 3: Configure both switches to use the same VLAN

To resolve the VLAN mismatch, I will configure interface F0/1 of Switch1 to VLAN 10, matching the VLAN used on the interface of Switch0.

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