How QinQ works
As shown in Figure 59, a QinQ frame transmitted over the service provider network carries the following tags:
CVLAN tag—Identifies the VLAN to which the frame belongs when it is transmitted in the customer network.
SVLAN tag—Identifies the VLAN to which the QinQ frame belongs when it is transmitted in the service provider network. The service provider allocates the SVLAN tag to the customer.
The devices in the service provider network forward a tagged frame according to its SVLAN tag only. The CVLAN tag is transmitted as part of the frame's payload.
Figure 59: Single-tagged Ethernet frame header and double-tagged Ethernet frame header
As shown in Figure 60, customer A has remote sites CE 1 and CE 4. Customer B has remote sites CE 2 and CE 3. The CVLANs of the two customers overlap. The service provider assigns SVLANs 3 and 4 to customers A and B, respectively.
When a tagged frame from CE 1 arrives, PE 1 tags the frame with SVLAN 3. The double-tagged frame travels over the service provider network until it arrives at PE 2. PE 2 removes the SVLAN tag of the frame, and then sends the frame to CE 4.
Figure 60: Typical QinQ application scenario
