f service instances. The limitation of a 12-bit VLAN-ID (VID) is that it only supports 4094 service instances. The development of 802. 1ah represents a significant step toward making Ethernet suit- able for carriers. It does this by mapping customer frames into provider frames (hence it is also known as MAC-in-MAC) and in doing SO provides the benefits of hierarchy by isolating customer and provider concerns. Hierarchy is of particular benefit when the customer base consists of a large number of relatively small communities of interest that can be mapped onto a common transport network. It reduces the amount of provisioning and forwarding state in the network core and consequently reduces the load and ongoing cost of performing service assurance and fault management. To address the scalability of service instances a new 24-bit Service Instance Identifier (I-SID) has been created that provides support for approximately 17 million service instances, thereby removing the limitations imposed by the 12-bit VID. Furthermore, to provide backward compatibility with existing bridges, a 12-bit VID referred to as the Backbone-VID, or B-VID, is used as the outer VID, which is used as part of the forward- ing decision. The evolution of Ethernet as specified by the IEEE, including the 802.1 1ah frame format, is illustrated in Figure 14.4. In contrast to 802.1Q and the subsequent 802.1ad, which simply partitioned the for- warding of a single-layer network, 802.1a implies complete recursion such that customer domains are completely encapsulated and isolated from the provider Ethernet domain. Yet 802.1ah still uses the same original service model, with spanning tree protocols used to prevent loops. As such it still inherits the limitations of the existing service model. What is needed is a change in the service model inside the carrier network. In what follows we describe a method for how this can be achieved (Allan, 2006). The Evolution of Ethernet 802.1 802.1q 802. 1ad 802. 1ah Payload Payload Payload Payload C-VID C-VID S-VID 