H3C Technologies H3C S12500 Series Switches - Manual (Page 151)

138
Configuring QinQ
Throughout this document, customer network VLANs (CVLANs), also called inner VLANs, refer to the
VLANs that a customer uses on the private network; and service provider network VLANs (SVLANs), also
called outer VLANs, refer to the VLANs that a service provider uses to carry VLAN tagged traffic for
customers.
Overview
QinQ stands for 802.1Q in 802.1Q. QinQ is a flexible, easy-to-implement Layer 2 VPN technology
based on IEEE 802.1Q. QinQ enables the edge device on a service provider network to insert an outer
VLAN tag in the Ethernet frames from customer networks, so that the Ethernet frames travel across the
service provider network (public network) with double VLAN tags. QinQ enables a service provider to
use a single SVLAN to serve customers who have multiple CVLANs.
Background and benefits
The IEEE 802.1Q VLAN tag uses 12 bits for VLAN IDs. A device supports a maximum of 4094 VLANs.
This is far from enough for isolating users in actual networks, especially in metropolitan area networks
(MANs).
By tagging tagged frames, QinQ expands the available VLAN space from 4094 to 4094 × 4094. QinQ
delivers the following benefits:
• Releases the stress on the SVLAN resource.
• Enables customers to plan their CVLANs without conflicting with SVLANs.
• Provides an easy-to-implement Layer 2 VPN solution for small-sized MANs or intranets.
• Allows the customers to keep their VLAN assignment schemes unchanged when the service provider
upgrades the service provider network.
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.