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VLAN Stacking (Q-in-Q) - 802.1ad
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Logical segregation of Ethernet frames into VLAN groups can provide several advantages as stated above. However, several large enterprises use VLANs extensively within their Local Area Networks. As a result, the Ethernet frames that enter the Service Provider domain often have their VLAN fields used up. IEEE 802.1ad defines a common way to implement VLAN 'stacking' in Ethernet switches. VLAN stacking is a technology that allows carriers to slap on a public VLAN tag onto packets, so that they can be transported over a metro network. The important thing about VLAN stacking is that it maintains the integrity of the traffic, and it keeps one customer's traffic separated from another's traffic. This allows the Service Provider to make use of the advantages that VLANs provide while at the same time ensuring complete transparency to their use in the customer domain.
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