Native VLAN

Implementing Cisco IP Switched Networks (300-115):
Native VLAN:
A VLAN that travel Without tag it assigned to an 802.1Q trunk port. By default, the Native VLAN is 1. Packet without tagged on a dot1q link belongs to Native VLAN. Best practice to change Native VLAN on all switches. Native VLAN ID must match on both end of the trunk. The VLAN dot1q tag native command will tag VLAN on all trunks.
SW(config-if) # switchport trunk native vlan vlan-id .