What are the common causes and solutions for native VLAN mismatches on a trunk port?

1 What is a native VLAN?

A native VLAN is the VLAN that is assigned to untagged traffic on a trunk port. Untagged traffic is traffic that does not have a VLAN identifier (VID) in its frame header. This can happen when a device that is not VLAN-aware is connected to a trunk port, or when a device sends traffic without specifying a VLAN. By default, the native VLAN is VLAN 1, but it can be changed to any other VLAN number. The native VLAN should be the same on both ends of a trunk link, otherwise a mismatch will occur.

3 How to detect a native VLAN mismatch?

One way to detect a native VLAN mismatch is to use the show interface trunk command on a Cisco switch. This command will display the trunk status, the allowed VLANs, and the native VLAN for each trunk port. If the native VLAN is different on both sides of a trunk link, you will see a warning message like this:

%CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on FastEthernet0/1 (1), with SwitchB FastEthernet0/1 (10).

Another way to detect a native VLAN mismatch is to use the show cdp neighbors detail command on a Cisco switch. This command will display the information about the neighboring devices that are discovered by the Cisco Discovery Protocol (CDP). If the native VLAN is different on both sides of a trunk link, you will see a mismatch indicator like this:

Native VLAN: 1 (Mismatch)

4 How to fix a native VLAN mismatch?

The simplest way to fix a native VLAN mismatch is to configure the same native VLAN on both ends of a trunk link. For example, if you want to change the native VLAN from 1 to 10 on both switches, you can use the following commands:

SwitchA(config)#interface fa0/1

SwitchA(config-if)#switchport trunk native vlan 10

SwitchB(config)#interface fa0/1

SwitchB(config-if)#switchport trunk native vlan 10

Another way to fix a native VLAN mismatch is to use VLAN tagging for all traffic on a trunk link, including the native VLAN. This can be done by enabling the IEEE 802.1Q encapsulation on both switches, and using the switchport trunk native vlan tag command on each trunk port. For example, if you want to tag the native VLAN 1 on both switches, you can use the following commands:

SwitchA(config)#interface fa0/1

SwitchA(config-if)#switchport trunk encapsulation dot1q

SwitchA(config-if)#switchport trunk native vlan tag

SwitchB(config)#interface fa0/1

SwitchB(config-if)#switchport trunk encapsulation dot1q

SwitchB(config-if)#switchport trunk native vlan tag

5 How to prevent a native VLAN mismatch?

In order to prevent native VLAN mismatches, it is important to adhere to certain best practices when designing and configuring your network. It is recommended to use a consistent naming and numbering scheme for your VLANs across the network, and avoid using VLAN 1 as the native VLAN. Additionally, it is important to use a dedicated VLAN for the native VLAN, and not assign any devices or services to it. Furthermore, trunk links should be configured with VLAN tagging for all traffic, and not rely on the native VLAN for untagged traffic. To verify the trunk status and the native VLAN on each trunk port, CDP or other discovery protocols should be used. Lastly, logging and monitoring tools should be utilized to detect and troubleshoot any native VLAN mismatch errors.