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IEEE 802.1ad〔Standard approved 8 December 2005 and published May 26, 2006.〕 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). ==Background== 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 was a trunk. IEEE 802.1ad was created for the following reasons: # 802.1Q has a 12-bit VLAN ID field, which has a theoretical limit of 212=4096 tags. With the growth of networks, this limitation has become more acute. A double-tagged frame has a theoretical limitation of 4096×4096=16777216, sufficient to accommodate network growth for the next several years. # 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—the tag stack—allows switches to more easily modify frames. In a tag stack scheme, switches can add ("push"), remove ("pop") or modify single or multiple tags. # A multi-tagged frame not only has multiple VLAN IDs, but has multiple VLAN header bit fields. # A tag stack creates a mechanism for Internet Service Providers to encapsulate customer single-tagged 802.1Q traffic with a single tag, the final frame being a QinQ frame. The outer tag is used to identify and segregate traffic from different customers; the inner tag is preserved from the original frame. # QinQ frames are convenient means of constructing Layer 2 tunnels, or applying Quality of service (QoS) policies, etc. # 802.1ad is upward compatible with 802.1Q. Although 802.1ad is limited to two tags, there is no ceiling on the standard limiting a single frame to more than two tags, allowing for growth in the protocol. In practice Service Provider topologies often anticipate and utilize frames having more than two tags. # It is easier for networking equipment makers to modify their existing equipment by creating multiple 802.1Q headers than to modify their equipment to implement some hypothetical new non-802.1Q extended VLAN ID field header. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「IEEE 802.1ad」の詳細全文を読む スポンサード リンク
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