44Service Provider BridgingService provider bridging is supported on Dell Networking OS.VLAN StackingVLAN stacking, also called Q-in-Q, is defined in IEEE 802.1ad — Provider Bridges, which is an amendmentto IEEE 802.1Q — Virtual Bridged Local Area Networks. It enables service providers to use 802.1Qarchitecture to offer separate VLANs to customers with no coordination between customers, andminimal coordination between customers and the provider.Using only 802.1Q VLAN tagging all customers would have to use unique VLAN IDs to ensure that trafficis segregated, and customers and the service provider would have to coordinate to ensure that trafficmapped correctly across the provider network. Even under ideal conditions, customers and the providerwould still share the 4094 available VLANs.Instead, 802.1ad allows service providers to add their own VLAN tag to frames traversing the providernetwork. The provider can then differentiate customers even if they use the same VLAN ID, and providerscan map multiple customers to a single VLAN to overcome the 4094 VLAN limitation. Forwardingdecisions 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 eachframe before the 802.1Q tag. From this point, the frame is double-tagged. The service provider uses theS-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 followingillustration.754 Service Provider Bridging
