QinQ

IEEE 802.1ad is an Ethernet networking standard informally known as QinQ and is an amendment to IEEE standard IEEE 802.1Q-1998. The technique is also known as provider bridging, Stacked VLANs, or simply QinQ or Q-in-Q. "Q-in-Q" can for supported devices apply to C-tag stacking on C-tag (Ethernet Type = 0x8100) but this has limited application in the modern methodology of network routing.

The original 802.1Q specification allows a single Virtual Local Area Network (VLAN) header to be inserted into an Ethernet frame. QinQ allows multiple VLAN tags to be inserted into a single frame, an essential capability for implementing Metro Ethernet network topologies. Just as QinQ extends 802.1Q, QinQ itself is extended by other Metro Ethernet protocols.

In a multiple VLAN header context, out of convenience the term "VLAN tag" or just "tag" for short is often used in place of "802.1Q VLAN header". QinQ allows multiple VLAN tags in an Ethernet frame; together these tags constitute a tag stack. When used in the context of an Ethernet frame, a QinQ frame is a frame that has 2 VLAN 802.1Q headers (double-tagged).

802.1ad specifies architecture and bridge protocols to provide separate instances of the MAC services to multiple independent users of a Bridged Local Area Network in a manner that does not require cooperation among the users, and requires a minimum amount of cooperation between the users and the provider of the MAC service.

The idea is to provide, for example, the possibility for customers to run their own VLANs inside service provider's provided VLAN. This way the service provider can just configure one VLAN for the customer and customer can then treat that VLAN as if it were a trunk.

IEEE 802.1ad was created for the following reasons:

These examples are for an Ethernet II framing (length/ethertype field => ethertype) frame. This could also be applied to 802.3 frames (length/ethertype field => length), with or without an LLC (i.e. Logical Link Control), LLC+SNAP header. The top frame is a simple Ethernet II frame. The middle frame has a .1Q (shorthand for 802.1q) tag added to it. The bottom frame has yet another .1Q tag added to it.

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