QinQ

QinQ, short for Q-in-Q or VLAN stacking, is an Ethernet networking technique defined by IEEE 802.1ad that allows an Ethernet frame carrying a customer VLAN to be encapsulated with an additional outer VLAN tag by a service provider edge device. This creates a stacked, or double-tagged, frame that can traverse a provider’s backbone while preserving the customer’s original VLAN identification.

In a typical QinQ deployment, the customer edge or access switch tags frames with a customer VLAN ID (C-VLAN). When such a frame enters the service provider network, a second tag—the provider VLAN ID (S-VLAN)—is added as an outer tag. The frame then travels across the provider network with both tags. At the destination edge, the outer provider tag is removed, and the frame exits with the original customer tag intact. This arrangement allows a provider to carry many customer VLANs over a single network backbone.

QinQ is used to improve VLAN scalability and isolation in multi-tenant environments. It enables service providers to aggregate traffic from numerous customers onto shared infrastructure while keeping each customer’s VLAN space separate. It is also used in data center interconnects and large enterprise networks that require simplified VLAN management across wide areas.

Standards and compatibility: QinQ is standardized as IEEE 802.1ad and is widely supported by modern switches and routers. While it offers scalability, correct tagging configuration and adherence to VLAN ID ranges are essential to maintain isolation and prevent VLAN misconfigurations or security risks.