Hello Giovanni
That’s the definition of QinQ. The provider offers an access port on a single VLAN, while the customer uses a trunk port, sending multiple VLANs. The key command in all of this is the third command in the following sequence on the provider’s interface to the customer:
SW1(config)#interface fastEthernet 0/1
SW1(config-if)#switchport access vlan 123
SW1(config-if)#switchport mode dot1q-tunnel
This third command tells the switch to accept all tagged frames and tunnel them through a single VLAN (123) providing a second tag within the provider’s network.
No, BPDUs will not affect the customer. This is because we have created a tunnel. The internal (customer) traffic does not know of or conceive of the external (provider’s) traffic. If you were to do a CDP neighbor lookup on the customer’s device, they will not see any of the provider’s equipment, even if it is Cisco, because the CDP traffic is tunnelled through the QinQ tunnel. So from the point of view of the customer devices, they are directly connected to each other, and there is no provider network.
I hope this has been helpful!
Laz