1-2
How QinQ Works
The devices in the public network forward a frame only according to its outer VLAN tag and learn its
source MAC address into the MAC address table of the outer VLAN. The inner VLAN tag of the frame is
transmitted as the payload.
Figure 1-1
Schematic diagram of the QinQ feature
Network
Service provider network
VLAN 1~10
VLAN 1~10
VLAN 1~20
VLAN 1~20
VLAN 3
VLAN 3
VLAN 4
VLAN 4
Customer network A
Customer network A
Customer network B
Customer network B
As shown in
Figure 1-1
, customer network A has CVLANs 1 through 10, while customer network B has
CVLANs 1 through 20. The SVLAN allocated by the service provider for customer network A is SVLAN
3, and that for customer network B is SVLAN 4. When a tagged Ethernet frame of customer network A
enters the service provider network, it is tagged with outer VLAN 3; when a tagged Ethernet frame of
customer 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 identified
separately.
QinQ Frame Structure
A QinQ frame is transmitted double-tagged over the service provider network. The inner VLAN tag is the
CVLAN 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.
