vice VLANs resulted to be 4094, really too small even for a small competitive local exchange carriers (CLEX) that has generally hundred thousand customers. These problems were solved by IEEE with the so-called providers backbone bridges (PBB-IEEE 802-Iah) standard [31] that introduces three new fields in the Ethernet frame reserved to the carrier. Using these fields, the edge router that accepts the frame at the boundary between the private and the public network creates an envelope containing carriers' network informa- tion that in this way is completely separated from the customer network one. Customers MAC and VID are completely hidden for the carrier network and vice versa, carriers address and VLAN tag does not reach the customer LAN. The four fields are as follows: 1. A backbone destination address (B-DA, 48 bits) 2. A backbone source address (B-SA, 48 bits) 3. A backbone VLAN tag (B-VLAN, 12 bits) 4. A service instance tag (I-SID, 24 bits) Networks Fundamentals and Present Architectures The B-VLAN is used by the carrier to partition their PBB network in different broadcast domains that is useful not only for scalability but also in case broadcast applications like IP-TV have to be supported. Since the number of partitions is high (4096), the number of users collected in each partition is generally small (e.g., if the carrier has two millions customers, each partition collects only 500 customers). In this condition, the spanning tree protocol can be used for routing. The I-SID is used to tag the service instances that is substituted in practice by the ser- vice VID of the PBB in individuating carriers' virtual circuits. Differently from the service VID, the I-SID scale over more than 16 millions, assuring scalability up to a number of users typical of a large continental carrier. I-SIDs are seen only by edge switches that pro- vide, assign, and cancel it, and are ignored by all the intermediate switches of the carrier network. The last two fields, B-DA and B-SA, take the role of destination and origin of MA