How QinQ works

Under QinQ, the provider network operates on a different VLAN space, independent of the VLANs that are used in the customer network.
VLANs in a QinQ configuration

Customer VLANs (referred to as C-VLANs by the IEEE 802.1ad specification) are not used to make any forwarding decisions inside the provider network where customer frames get assigned to service VLANs (S-VLANs). Inside the provider cloud, frames are forwarded based on the S-VLAN tag only, while the C-VLAN tag remains shielded during data transmission. The S-VLAN tag is removed when the frame exits the provider network, restoring the original customer frame.