Only 12 bits in the VLAN Tag field defined in IEEE802.1Q are used to represent the VLAN ID, so the device can support up to 4094 VLANs. In practical applications, especially in metropolitan area networks, a large number of VLANs are needed to isolate users, and 4094 VLANs are far from meeting the demand. QinQ technology [also known as Stacked VLAN or Double VLAN]. The standard comes from IEEE 802.1ad, which implements the encapsulation of the user’s private network VLAN tags in the public network VLAN tags, so that the packets carry two layers of VLAN tags through the operator’s backbone network.
In the public network, the device only forwards the packet according to the outer VLAN tag, and learns the source MAC address entry of the packet into the MAC address table of the VLAN where the outer tag is located, while the user’s private network VLAN tag is transmitting. In the process, it will be transmitted as the data part of the message
QinQ technology effectively expands the number of VLANs by stacking two 802.1Q headers in an Ethernet frame, so that the number of VLANs can be up to 4094×4094. At the same time, multiple VLANs can be multiplexed into a core VLAN. MSP usually establishes a VLAN model for each customer, automatically monitors the VLAN of the entire backbone network using the General Attribute Registration Protocol / General VLAN Registration Protocol (GARP / GVRP), and accelerates the network convergence speed by extending the Spanning Tree Protocol (STP) Provide flexibility to the network. SVLAN technology is good as an initial solution, but as the number of users increases, the SVLAN model also brings scalability issues. Because some users may wish to carry their own VLAN ID when transferring data between branches, this makes MSP using QinQ technology face the following two problems: First, the VLAN ID of the first customer may conflict with other customers; Second, service providers will be severely limited by the number of signs that customers can use. If users are allowed to use their own VLAN ID space in their own way, then the core network still has a limit of 4096 VLANs.
The QinQ feature enables the network to provide up to 4094X4094 VLANs to meet the demand for the number of VLANs in the metropolitan area network. It mainly solves the following problems:
1. Alleviate the problem of increasingly scarce public network VLAN ID resources.
2. Users can plan their own private network VLAN ID without conflicting with public network VLAN ID.
3. Provide a relatively simple Layer 2 VPN solution for small metropolitan area networks or enterprise networks.
QinQ technology is divided into basic QinQ and flexible QinQ.
(1) Basic QinQ
Basic QinQ is implemented based on the port method. After the basic QinQ function of the port is enabled, when the port receives a packet, the device will tag the packet with the VLAN tag of the default VLAN of the port. If a packet with a VLAN tag is received, the packet becomes a double-tag packet; if a packet without a VLAN tag is received, the packet becomes a default VLAN tag with a port Message.
(2) Flexible QinQ
Flexible QinQ is a more flexible implementation of QinQ, which is based on the combination of ports and VLANs. In addition to all the basic QinQ functions, packets received on the same port can also perform different actions according to different VLANs, and can achieve the following functions:
1. Add different outer VLAN tags to packets with different inner VLAN IDs.
2. Mark the 802.1p priority of the outer VLAN packet according to the 802.1p priority of the original inner VLAN of the packet.
3. You can modify the inner user VLAN ID while adding the outer VLAN Tag.
The TPID value of VLAN TAG is adjustable:
TPID (Tag Protocol Identifier, Tag Protocol Identifier) is a field in VLAN Tag used to indicate the protocol type of VLAN Tag. The IEEE 802.1Q protocol specifies that the value of this field is 0x8100.
The device can identify whether the packet carries the corresponding VLAN tag according to the TPID value: when the port receives the packet, it is compared with the corresponding field in the packet according to the configured TPID value. Carry the corresponding VLAN Tag. In addition, devices of different manufacturers may set the TPID field of the outer VLAN Tag of QinQ packets to different values. In order to be compatible with these devices, users can configure the TPID value by themselves, so that the QinQ message sent to the public network carries the same TPID value as other manufacturers, and can communicate with other manufacturers’ devices.
Technical advantages:
1. QinQ can be simply considered as a packet carrying two layers of 802.1Q Tag.
2. The emergence of QinQ technology allows operators to provide customers with Layer 2 VPNs at a lower cost. QinQ is fully implemented on the operator’s network, and users are not aware of QinQ.
3. For the packets in the operator’s network, the inner tag is the customer’s private VLAN ID, and the outer tag is the VLAN assigned by the operator to the customer. Customers can plan their own VLAN ID independently, and changes in the operator’s network will not affect the customer’s network.
4. QinQ does not require a separate signaling protocol, only static configuration, simple and stable.
5. QinQ expands VLAN resources and provides operators with the possibility to differentiate access users by VLAN.
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