ag is not only a transport separator function; it is also a customer identification function. An 802. 1ad S-VID therefore represents both a community of interest in the form of an individual service instance and a specific topology (a spanning tree instance) within the provider network. Chapter 14 Next Generation Ethernet The double-tagging mechanism of 802.1 1ad only gets us SO far. It does not address the fundamental scaling issues associated with broadcast domains and flat addressing (increased MAC table sizes), nor does it offer any real solution to the virtualization of services over a common Ethernet infrastructure. This has scaling limitations when compared with a hierarchical relationship, where the different levels of hierarchy are functionally decoupled. Nor does simple partitioning provide any real control over how a carrier's facilities are utilized, and the use of multiple spanning trees only increases the degree of indeterminism in network operations. To operate such networks, the operator has until now had little option but to severely limit the scale and scope of applicability of Ethernet networks in a service context or to rely on it as a means of interconnect for other networks, such as IP. To get to the next step of supporting millions of service instances in Ethernet, a new approach is required. To enable true hierarchical scaling, virtualization, and full isolation of the provider infrastructure from customer broadcast, domains, the IEEE has embarked on the develop- ment of Provider Backbone Bridges (PBBs) (IEEE 802.1ah, in progress). It addresses two primary problems: The scalability of MAC address tables. Without PBBs the service provider equipment needs to learn all the customer MAC addresses. Where the customer edge device is a router, the learning is minimized, but if the edge device is a bridge, it is also necessary to learn all of the customer end-station addresses. Limits on the size of bridge MAC tables offer limited scalability as the number of customers increases. The scalability o