Administrator Guide

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.
Figure 129. VLAN Stacking in a Service Provider Network
Important Points to Remember
● Interfaces that are members of the Default VLAN and are configured as VLAN-Stack access or trunk ports do not switch
untagged traffic. To switch traffic, add these interfaces to a non-default VLAN-stack-enabled VLAN.
● Dell Networking cautions against using the same MAC address on different customer VLANs, on the same VLAN-stack
VLAN.
888
Service Provider Bridging