Service Provider BridgingService provider bridging provides the ability to add a second VLAN ID tag in an Ethernet frame and is referredto as VLAN stacking in the Dell Networking OS.VLAN StackingVLAN stacking, also called Q-in-Q, is defined in IEEE 802.1ad — Provider Bridges, which is an amendment toIEEE 802.1Q — Virtual Bridged Local Area Networks. It enables service providers to use 802.1Q architecture tooffer separate VLANs to customers with no coordination between customers, and minimal coordinationbetween customers and the provider.Using only 802.1Q VLAN tagging all customers would have to use unique VLAN IDs to ensure that traffic issegregated, and customers and the service provider would have to coordinate to ensure that traffic mappedcorrectly across the provider network. Even under ideal conditions, customers and the provider would stillshare 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 providers canmap multiple customers to a single VLAN to overcome the 4094 VLAN limitation. Forwarding decisions in theprovider network are based on the provider VLAN tag only, so the provider can map traffic through the coreindependently; 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 framebefore the 802.1Q tag. From this point, the frame is double-tagged. The service provider uses the S-Tag, to50Service Provider Bridging 1013
