A Network Tunneling Protocol used to make multiple layers of VLANs.
It is also known as double tagging or 802.1Q tunneling as it extends the IEEE 802.1Q standard.
In a standard 802.1Q VLAN setup a single VLAN tag is added to an Ethernet Frame, to identify which VLAN it belongs to. Q-in-Q adds a second VLAN tag to the frame, thus creating a hierarchy of VLANs where the inner VLAN is identified by the initial VLAN and the outer VLAN is identified by the second VLAN.
This outer VLAN tag is added and removed by the access Switch between the inner and outer VLAN.