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Concept
IEEE 802.1ad
Résumé
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. 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 (double-tagged).
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 idea is to provide, for example, the possibility for customers to run their own VLANs inside a 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:
802.1Q has a 12-bit VLAN ID field, which has a limit of 212=4096 tags. With the growth of networks, this limitation has become more acute. A double-tagged frame has a limitation of 4096×4096=16777216 tags, more sufficient to accommodate network growth.
The addition of a second tag allows operations that would not have been available had the VLAN ID field simply been expanded from 12 bits to 24 bits (or any other large value). Having multiple tags—a tag stack—allows switches to more easily modify frames.
Source officielle
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