Background and benefits
The IEEE 802.1Q VLAN tag uses 12 bits for VLAN IDs. A device supports a maximum of 4094 VLANs. This is far from enough for isolating users in actual networks, especially in metropolitan area networks (MANs).
By tagging tagged frames, QinQ expands the available VLAN space from 4094 to 4094 × 4094. QinQ delivers the following benefits:
Releases the stress on the SVLAN resource.
Enables customers to plan their CVLANs without conflicting with SVLANs.
Provides an easy-to-implement Layer 2 VPN solution for small-sized MANs or intranets.
Enables the customers to keep their VLAN assignment schemes unchanged when the service provider upgrades the service provider network.