What is a Native VLAN?

Native VLAN is used to carry untagged traffic on a 802.1Q trunk port.This makes it possible for your VLAN to support legacy devices or devices that don't tag their traffic like some wireless access points. When a Switch sees untagged traffic on a trunk port, it understands that the traffic belongs to a Native VLAN. We have to make sure that the Native VLAN matches on both trunk ports when two Switches are connected via a trunk line.

How to identify the current  Native VLAN being used? Show interface trunk command can be used to identify the current Native VLAN being used on a trunk port. Initially we have to make sure that both the ports are in trunk mode. As per the below topology, fas0/24 on both the switches are in trunk mode.

Switch#show interface trunk

Port      Mode      Encapsulation      Status      Native vlan

Fa0/24    on        802.1q             trunking     1

Native VLAN Mismatch
If the Native VLAN configured on the trunk ports connecting both switches are different, you will get a native VLAN mismatch message.

%CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on FastEthernet0/24 (10), with Switch FastEthernet0/24 (1).

According to the message, on one end the Native VLAN is 10 and on the other end it is 1.
To avoid this, make sure that both ports have same Native VLAN configured.

How to change the Native VLAN ?
To change the Native VLAN, Go to the respective port number and issue the command as shown below in the interface configuration mode.

Switch#conf t

Switch(config)#int fas0/24

Switch(config-if)#switchport trunk native vlan 1