3Com 4210G Series, Configuration Manual, Page: 172 / 1133

10-2
Figure 10-1
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 10-1 , 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 does not
become mixed.
By tagging tagged frames, QinQ expands the available VLAN space from 4094 to 4094 × 4094 and
thus satisfies the requirement for VLAN space in MAN. It mainly addresses the following issues:
z
Releases the stress on the SVLAN resource.
z
Enables customers to plan their CVLANs without conflicting with SVLANs.
z
Provides an easy-to-implement Layer 2 VPN solution for small-sized MANs or intranets.
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 10-2 shows the structure of single-tagged and double-tagged Ethernet frames.