2.1.d [ii] dot1Q
802.1Q tunneling enables service providers to use a single VLAN to support customers with multiple VLANs, while preserving customer VLAN IDs and keeping traffic in different customer VLANs segregated. A port configured to support 802.1Q tunneling is called a tunnel port. When you configure tunneling, you assign a tunnel port to a VLAN that you dedicate to tunneling, which then becomes a tunnel VLAN. To keep customer traffic segregated, each customer requires a separate tunnel VLAN, but that one tunnel VLAN supports all of the customer’s VLANs.
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 1622-1626). . Kindle Edition.