Reference Guide

45
Service Provider Bridging
Service provider bridging is supported on the S4820T platform.
VLAN Stacking
Virtual local area network (VLAN) stacking is supported on the S4820T platform.
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.
715