Reference Guide

Service Provider Bridging | 749

Service Provider Bridging

Service Provider Bridging is supported on platforms: e c s z

This chapter contains the following major sections:

• VLAN Stacking on page 749

• VLAN Stacking Packet Drop Precedence on page 760

• Dynamic Mode CoS for VLAN Stacking on page 762

• Layer 2 Protocol Tunneling on page 765

• Provider Backbone Bridging on page 769

VLAN Stacking

VLAN Stacking is supported on platforms: c e s z

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 need only coordinate at the provider edge.

In 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 (Figure 38-1).