Simultaneous Support for Tagged and Untagged VLANs

Every interface is an untagged member of default VLAN or any user-specified VLAN, unless user explicitly removes this untagged interface from default VLAN. The interface will always remain in the default VLAN (or non-default VLAN in which the interface is configured as untagged member) when this interface is added as tagged member of non-default VLAN. Thus, all tagged interfaces are implicit dual-mode ports.

This implicit dual-mode feature simplifies the tagging concept by changing the system default behavior to allow both tagged and untagged frames simultaneously (as per configurations).

device(config)# vlan 100
device(config-vlan-100)# tagged ethernet 1/1/1
Added tagged port(s) ethernet 1/1/1 to port-vlan 100.
device(config-vlan-100)# vlan 1
device(config-vlan-1)# no untagged ethernet 1/1/1

Since you are allowed to remove ports from default VLAN (which completely removes the untagged VLAN membership of an interface), this will be saved to the running configuration to retain the VLAN membership upon reload. User VLANs will continue to display membership as per user configurations.

device(config)# vlan 100
device(config-vlan-100)# untagged ethernet 1/1/1
Added untagged port(s) ethernet 1/1/1 to port-vlan 100.
device(config-vlan-100)# vlan 101
device(config-vlan-1)# tagged ethernet 1/1/2 to 1/1/3
device(config-vlan-100)# vlan 1
device(config-vlan-1)# no untagged ethernet 1/1/2 to 1/1/3
device(config-vlan-1)# show run vlan
vlan 1 name DEFAULT-VLAN by port
no untagged ethe 1/1/2 to 1/1/3***
!
vlan 100 by port
 untagged ethe 1/1/1 
!

As you can see, only the interfaces that are deleted explicitly by the user are displayed. If an interface is moved internally as part of some other VLAN configurations (1/1/1, in the preceding example), it is not displayed under default-VLAN.