Cisco trunks native vlans

interface Ethernet 0/2
 switchport
 switchport mode access
 switchport access vlan 10
 spanning-tree portfast


interface Ethernet 0/2
 switchport
 switchport mode trunk
 switchport trunk native vlan 10
 switchport trunk allowed vlan 10
# switchport trunk allowed vlan 10,20
 spanning-tree portfast trunk

Both configurations will result in the same behaviour.  Only VLAN 10 traffic will be forwarded on each port, and it will be forwarded untagged.  Any untagged traffic arriving at the port will be classified into VLAN 10.


ref:
http://rednectar.net/2012/03/11/the-access-vlan-is-dead-long-live-the-native-vlan/