3Com 4800G Series, Руководство по настройке

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IN
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ONFIGURATION
When configuring QinQ, go to these sections for information you are interested
in:
■
“Introduction to QinQ” on page 133
■
“Configuring Basic QinQ” on page 135
■
“Configuring Selective QinQ” on page 136
■
“Configuring the TPID Value to Be Carried in VLAN Tags” on page 137
■
“QinQ Configuration Example” on page 137
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 switch can support a maximum of 4,094 VLANs. In actual applications,
however, a large number of VLANs 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 is a flexible, easy-to-implement Layer 2 VPN technique,
which enables the access point to encapsulate an outer VLAN tag in Ethernet
frames from customer networks (private networks), so that the Ethernet frames
will travel across the service provider’s backbone network (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, with
the source MAC address learned as a MAC address table entry for the VLAN
indicated by the outer tag, while the customer network VLAN tag is transmitted as
part of the data in the frames.
Figure 39 shows the structure of 802.1Q-tagged and double-tagged Ethernet
frames. The QinQ feature enables a device to support up to 4,094 x 4,094 VLANs
to satisfy the requirement for the amount of VLANs in the MAN.