Is there any way to limit number of C(ustomer)-VLANs on QinQ tunnel port?
For example, only VLANS 10-20 are double tagged, all other traffic is dropped.
as far as I know this is not possible: you cannot choice what vlan-ids have to be tunneled and what should be filtered/stopped.
all vlan-tags presented to the dot1q tunnel interface are externally tagged with the service provider tag for this customer.
This is for forwarding efficiency.
see for example this chapter from config guide of C3750
Hope to help