Administrator Guide

Service Provider Bridging
VLAN Stacking
VLAN stacking, also called Q-in-Q, is defined in IEEE 802.1ad — Provider Bridges, which is an amendment to IEEE 802.1Q — Virtual
Bridged Local Area Networks. It enables service providers to use 802.1Q architecture to offer separate VLANs to customers with no
coordination between customers, and minimal coordination between customers and the provider.
Using only 802.1Q VLAN tagging all customers would have to use unique VLAN IDs to ensure that traffic is segregated, and customers and
the service provider would have to coordinate to ensure that traffic mapped correctly across the provider network. Even under ideal
conditions, customers and the provider would still share the 4094 available VLANs.
Instead, 802.1ad allows service providers to add their own VLAN tag to frames traversing the provider network. The provider can then
differentiate customers even if they use the same VLAN ID, and providers can map multiple customers to a single VLAN to overcome the
4094 VLAN limitation. Forwarding decisions in the provider network are based on the provider VLAN tag only, so the provider can map
traffic through the core independently; the customer and provider only coordinate at the provider edge.
At the access point of a VLAN-stacking network, service providers add a VLAN tag, the S-Tag, to each frame before the 802.1Q tag.
From this point, the frame is double-tagged. The service provider uses the S-Tag, to forward the frame traffic across its network. At the
egress edge, the provider removes the S-Tag, so that the customer receives the frame in its original condition, as shown in the following
illustration.
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