 A first proposal, called Q-in-Q, also known as Provider Bridge, VLAN stacking or tag stacking, allows service providers to insert an additional VLAN tag (referred to as provider VLAN) in the Ethernet frame in order to identify the service, resulting in a unique 24-bit length label.While this solution enables one, in theory, to identify up to 16 million services (4094 * 4094), in reality, one provider VLAN is dedicated to one customer, and therefore the number of supported customers is still limited to 4094.Q-in-Q also introduces a scalability issue within the core of the carrier network, where every core switch needs to learn and maintain forwarding entries for every customer MAC address.