IEEE 802.1ad

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IEEE 802.1ad is an amendment to the IEEE 802.1Q-1998 networking standard which adds support for provider bridges. It was incorporated into the base 802.1Q standard in 2011.^[1] The technique specified by the standard is known informally as stacked VLANs or QinQ.

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.

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 two VLAN 802.1Q headers (i.e. it is double-tagged).