QinQ tunneling is mainly used by telcos to allow for the trunking of multiple customer VLANs over one telco VLAN. It is rare to actually use QinQ tunneling just within a customer’s network.
I can give you an example of the use of QinQ within a customer environment based on my experience. I am administrator of a Municipal Fibre Optic (MAN) network which is owned by the municipality that I partner with. There are over 80 km of fibre optic cabling that terminates at 24 different nodes with Cisco equipment in each. The fibre optic network serves many of the municipality’s buildings but also provides network connectivity for the city’s hospitals, schools and other public services. Each service that is connected is provided with one VLAN.
The municipality itself is a special case, because we have 18 VLANs that serve the internal networks of the municipality itself. So, I created VLAN 35 on the Fibre Optic network (propagated to all switches via VTP) as a QinQ VLAN and pass all of the municipality’s internal VLANs over that VLAN. So because the MAN belongs to the municipality, strictly speaking, I created a QinQ VLAN within the customer’s network.
In order to warrant the use of QinQ on a customer’s network, it must be a very large network with special VLAN requirements. You will not come across such a network very often.
I hope this has been helpful!