Double VLAN tagging provides access to the infrastructure of Internet service providers (ISPs) in Metropolitan Area Networks (MANs). It allows the service provider to provide certain services, such as Internet access on specific VLANs for specific customers, and yet allow the service provider to provide other types of services for their other customers on other VLANs. VLAN double tagging enables service providers to use a single VLAN to support customers who have multiple VLANs.
Suppose an enterprise has multiple VLANs across multiple customer sites in a metropolitan area. The enterprise has a higher bandwidth requirement and needs more physical links for communicating with the different VLANs. In such a scenario, double VLAN tagging can be used as a solution.
Here, the enterprise can use ISPs in a public MAN to carry traffic between different customer sites. To identify the Ethernet frames/packets from different customers in the MAN, each frame needs to carry a customer identification tag. That is, a single identification label is needed to identify the VLAN traffic from a customer. With double tagging, another tag/identifier is added to the packets from the VLANs that belong to the same user.