S-VID Provider I-SID DA: Destination MAC Address SA: Source MAC Address Bridges B-VID VID: VLAN ID C-VID: Customer VID S-VID: Service VID I-SID: Service ID B-VID: Backbone VID Provider B-DA: Backbone DA Backbone B-SA: Backbone SA Bridges 14.4 Evolution of Ethernet (the frame formats have been simplified). The introduction of hierarchy isolates customers from the operations of the network provider and allows the provider to implement different forwarding modes with the objec- tive of engineering the network by providing determinism and predictability. This is a sig- nificant benefit to the provider because it allows for deterministic traffic, fault management, and performance management of customer services. New forwarding modes are required to overcome the limitations of Ethernet networks, which are limited to trivial physically loop-free topologies or constraints imposed by spanning trees. Fortunately there are options based on alternative provisioning and control/management planes that can be employed to permit the Ethernet infrastructure to be engineered. The isolation provided by hierarchy means that a provider now has complete control of its MAC address and VLAN space. This separation allows new forwarding modes to be exploited. One feature that is of interest and not supported by Ethernet is the capability set up and pin connection-oriented paths across an Ethernet network in a scalable manner. One approach is to create cross-connect entries using VLAN tags, where forwarding is based purely on the outer VLAN tag regardless of the MAC addresses associated with the incoming frames. If the tags are used in a global context, then, as described earlier, the number of connections in the network is limited to 4094. To overcome this it is possible to carry out per-port translation (as described in 802.1ad) of the VLAN tag in a manner similar to the use of the Virtual Path Identifier (VPI) and Virtual Channel Identifier (VCI) in ATM, thereby allowing the VID to have local rather than global significance. This is ill