IEEE 802.1ad

[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.

802.1ad specifies architecture and bridge protocols to provide separate instances of the medium access control (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 conventions for 802.1ad terminology typically are as follows: In IEEE 802.1ad, the single-bit Canonical Format Indicator (CFI) is replaced by a Drop Eligibility Indicator (DEI), increasing the functionality of the PCP field.

The diagram shows switches as hexagons, and a service provider (SP) network encompassing all items within the dotted oval.

The items on the periphery of the oval are networks belonging to SP customers.

Different physical locations appear in the shaded rectangle and include both customer and SP network components.

A service provider (SP) offers L2 connectivity to customers in the cities of Seattle and Tacoma.

An experienced network engineer will immediately recognize the shortcomings of the above example.

This is the reason why 802.1ad is more of a definition for a method of adding multiple tags to a frame than it is an end-to-end self-contained solution.