Service Provider BridgingService provider bridging provides the ability to add a second VLAN ID tag in an Ethernet frame and is referred to as VLAN stacking in theDell Networking OS.VLAN StackingVLAN stacking, also called Q-in-Q, is defined in IEEE 802.1ad — Provider Bridges, which is an amendment to IEEE 802.1Q — VirtualBridged Local Area Networks. It enables service providers to use 802.1Q architecture to offer separate VLANs to customers with nocoordination 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 andthe service provider would have to coordinate to ensure that traffic mapped correctly across the provider network. Even under idealconditions, 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 thendifferentiate customers even if they use the same VLAN ID, and providers can map multiple customers to a single VLAN to overcome the4094 VLAN limitation. Forwarding decisions in the provider network are based on the provider VLAN tag only, so the provider can maptraffic 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. Fromthis point, the frame is double-tagged. The service provider uses the S-Tag, to forward the frame traffic across its network. At the egressedge, the provider removes the S-Tag, so that the customer receives the frame in its original condition, as shown in the followingillustration.45Service Provider Bridging 795
