VLAN stacking is a Layer 2 technology that encapsulates different outer VLAN tags for different user VLANs.
On a carrier's access network, user packets need to be differentiated according to users' applications, access points, or access devices. VLAN stacking is introduced to differentiate users by adding outer VLAN tags to user packets based on user packets' inner tags or IP or MAC addresses.
A VLAN stacking interface adds different outer VLAN tags to its received packets and strips the outer VLAN tags from the packets to be sent.