1-2How QinQ WorksThe devices in the public network forward a frame only according to its outer VLAN tag and learn itssource MAC address into the MAC address table of the outer VLAN. The inner VLAN tag of the frame istransmitted as the payload.Figure 1-1 Schematic diagram of the QinQ featureNetworkService provider networkVLAN 1~10VLAN 1~10VLAN 1~20 VLAN 1~20VLAN 3 VLAN 3VLAN 4 VLAN 4Customer network ACustomer network ACustomer network BCustomer network BAs shown in Figure 1-1, customer network A has CVLANs 1 through 10, while customer network B hasCVLANs 1 through 20. The SVLAN allocated by the service provider for customer network A is SVLAN3, and that for customer network B is SVLAN 4. When a tagged Ethernet frame of customer network Aenters the service provider network, it is tagged with outer VLAN 3; when a tagged Ethernet frame ofcustomer network B enters the service provider network, it is tagged with outer VLAN 4. In this way,there is no overlap of VLAN IDs among customers, and traffic from different customers can be identifiedseparately.QinQ Frame StructureA QinQ frame is transmitted double-tagged over the service provider network. The inner VLAN tag is theCVLAN tag while the outer one is the SVLAN tag that the service provider has allocated to the customer.Figure 1-2 shows the structure of single-tagged and double-tagged Ethernet frames.
