Below a simple topology using QinQ tagging (officially known as IEEE 802.1ad or unofficially IEEE 802.1 QinQ ) on Cisco Catalyst C3550 switches.
C-VLANS 10 and 20 come from customer side and are encapsulated in the S-VLAN 1800.
This feature is also called VLAN double tagging or Vlan stacking, or Nested VLANs (Allied Telesis) or vMAN technology ( Extreme Networks).
This feature is also called VLAN double tagging or Vlan stacking, or Nested VLANs (Allied Telesis) or vMAN technology ( Extreme Networks).