3Com Switch 8800 Advanced Software V5 Configuration Guide

18
QINQ CONFIGURATION
Introduction to QinQ
Understanding QinQ In the VLAN tag field defined in IEEE 802.1Q, only 12 bits are used for VLAN IDs,
so a device can support a maximum of 4,094 VLANs. In actual applications,
however, a large number of VLAN are required to isolate users, especially in
metropolitan area networks (MANs), and 4,094 VLANs are far from satisfying such
requirements.
The port QinQ feature provided by the device tags a frame with double VLAN tags,
allowing for 4094 × 4094 VLANs, thus satisfying the demand for large amount of
VLANs in MANs.
The QinQ feature encapsulates the private network VLAN tag in the public
network VLAN tag, and enables the frame to be transmitted through the service
provider backbone network (the public network) with double VLAN tags. The
inner VLAN tag is the customer network VLAN tag while the outer one is the VLAN
tag assigned by the service provider to the customer. In the public network, frames
are forwarded based on the outer VLAN tag only, while the customer network
VLAN tag remains untouched.
n
■ To implement QinQ, configuration is required on devices in the service provider
network only.
■ The QinQ feature is implemented based on the 802.1q standard, so it is
necessary that all the devices along the tunnel support the 802.1q standard.
Figure 53 shows the structure of a single-tagged frame and a double-tagged
frame.
Figure 53 Single-tagged frame structure vs. double-tagged Ethernet frame structure
Advantages of QinQ:
Etype
DA SA
User
VLAN Tag
Etype DATA FCS
DA SA
Nested
VLAN Tag
DATA FCS
User
VLAN Tag
6 bytes 6 bytes
Single-tagged frame
structure
Double-tagged frame structure
4bytes
6 bytes 6 bytes 4 bytes 4 bytes
4bytes
4bytes
2bytes
2bytes
46 to 1500 bytes
46 to 1500 bytes
Outer
VLAN
tag
Inner
VLAN
tag