tection route. In case of unidirectional failure, the terminals first enter in contact with the remaining working route, then switch on the protection one, and the process goes on as described in the bidirectional case. During the failure, the terminals go on with a continuous probing of the failed link via control messages; when the failure is restored, the terminals can conclude a handshake on the working line and, after that, they can switch the traffic on the working path and recover the low-priority flux. These protection mechanisms achieving protection times smaller than 50 ms can be applied both to rings and to point-to-point architectures. 3.2.4.3 Scalability We have seen that in Carrier Class Ethernet, the traditional broadcast of frames on a shared medium supported by the CDMA algorithm to manage collisions is substituted by a vir- tual circuit frames management, where frames belonging to a connection are all routed along the same VLAN whose route represents the virtual circuit. In order to gain efficiency and scalability, Carrier Class Ethernet also foresees the pos- sibility of disabling the spanning tree to introduce more efficient frame routing. When this is done, generally dynamic MAC learning is also disabled in favor of a static MAC address configuration and recognition, more suitable for a network of great dimensions. At this point, only the Ethernet frame and the concept of switching of the LAN-oriented Ethernet remain. Nevertheless, Carrier Class Ethernet revealed scalability problems due to its origin that had to be solved. The first problem to face was linked to the use of VLAN itself. As a matter of fact, if carriers can use the VLAN concept to establish virtual circuits, customers using Ethernet Telecommunication Networks LANs and asking for public network Ethernet services want to use VLANs for their own original task. If the customer frames are tagged with a VID, such VID cannot be confused with VIDs used from the carrier, requiring coordination not only between the carrier and the cus- 