Throughout this document, customer network VLANs (CVLANs), also called "
VLANs that a customer uses on the private network; service provider network VLANs (SVLANs), also
Introduction to QinQ
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 switch 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 switch supports a maximum of 4094 VLANs.
This is far from enough for isolating users in actual networks, especially in metropolitan area networks
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.
Enables the customers to keep their VLAN assignment schemes unchanged when the service
provider upgrades the service provider network.
How QinQ works
The switches in the public network forward a frame only according to its outer VLAN tag and obtain 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.
", refer to the VLANs that a service provider uses to carry VLAN tagged traffic for
, refer to the