transparent(1). Ignore VLAN tags on received frames. VLAN
tags are not added nor removed from the frames. If a frame
has a VLAN tag, then it is retained. If a frame does not have
a VLAN tag, then no tag is added.
customer(2). VLAN-handling in accordance with IEEE 802.1Q.
Frames are forwarded and filtered according to customer VLAN
tag. Frames that are not tagged are tagged with default VLAN
tag as configured on the interface.
provider(3). VLAN-handling in accordance with IEEE 802.1ad.
Frames are forwarded and filtered according to service VLAN
tag. Any customer VLAN is ignored. Frames that are not tagged
are tagged with default VLAN tag as configured on the
interface.