 delivered using this technology. If the frame belongs to a multicast service, more than one output port is calcu- lated as frame destination. In this case, the input interface creates replicas of the frame and repeats the sending operation toward all the selected output ports [84]. The first important observation is that a switch with multiple B-VLANs cannot pass Ethernet frames from a B-VLAN to the other. If this is needed, either the switch incorpo- rates also a layer 3 engine, that naturally provides full connectivity between input and output ports, or an external router has to be connected to the switch. From a switching point of view, a carrier class Ethernet switch is a configurable set of independent Ethernet forwarding planes, one for each B-VLAN, that are defined at a cer- tain moment. This structure is configurable since B-VLAN can be set up and tiered down, but once a B-VLAN is defined it is a completely independent MAC addressing plane. Another important observation is that, differently from IP forwarding, MAC addresses do not grow in number SO much while the network grows, due to the segmentation in B-VLANs. MAC address-based routing can thus be performed searching a MAC address database, without the complication of storing only prefixes that is present in IP forwarding. Finally, a carrier class Ethernet switch has to manage all LAN Ethernet features. For example, it has to be able to recognize a standard VLAN and treat it as a LAN Ethernet switch, isolating it from the other traffic belonging to the same B-VLAN. In this sense, it has to manage a structure with two stacked VLAN layers. If the Ethernet switch is deployed on top of an SDH/SONET layer, for example inside an MSSP, the physical layer provided by SDH/SONET is sufficient to solve all the physi- cal layer issues, comprising integration into the new network of legacy SDH/SONET equipment. A problem arises when, leveraging on the fact that the great majority of the traffic in the metro network is originating from Ethernet equipment, carrier cl