n for this is that broadcast networks are ultimately not scalable. What we require are solutions that retain the fundamental behaviors of Ethernet for customers while over- coming the limitations of broadcast services in the carrier environment. Attempts to limit the impact of the broadcast service model use mechanisms to partition or segment the network into multiple "loop-free" domains. Why loop free? The broadcast model floods frames throughout the network because end-system locations are not explic- itly, known. If there are loops present, a single broadcast frame is duplicated exponentially, resulting in catastrophic behavior. To this end considerable effort has been spent on the development of spanning tree protocols to compute a spanning tree-forwarding topology with no loops. We will return to the subject of spanning tree protocols when we discuss resiliency and protection. Initially the segmentation into multiple domains to limit the size of broadcast domains was achieved by means of Virtual LANs, or VLANs, and as the name suggests it was primar- ily intended for the enterprise environment (IEEE 802.1D, 2004; IEEE 802.1Q, 2005b). This was done by adding a short tag to the Ethernet frame to identify VLAN instances. The number of instances that could be supported was limited to 4094, with two other instances reserved for particular applications. To improve scalability, support for a second VLAN tag within the frame was specified as part of the provider bridging activity (IEEE 802.1ad, in progress). This allowed providers to impose their own partitioning without disturbing customer partitioning by allocating the inner tag to the customer and the outer tag to the provider. The inner tag is referred to as the Customer VLAN (C-VLAN) and the outer tag as the Service VLAN (S-VLAN) tag. However, this was done via stacking identifiers that were designed for single-Enterprise-scale requirements. The S-VLAN limits the number of service instances in a network to 4094. For the C-VLAN, stacking means that the service t