First of all I would like to say I absolutely love your site, the lessons on networking/linux is better than a lot of the online resources out there and you guys are all doing it for free!!
Before the advent of MPLS, how does the implementation of VLAN based IP-VPN services offered by carriers work? Does it use Dot1Q VLAN trunking protocol separating users of different geographical locations? How is Dot1Q protocol implemented across the carrier networks? does it traverse L3 Switch/router? HOw does Q-in-Q work?
How you could shed some like on this topic, it's very hard to find out how it works online.
Don't have answers to all the history questions.. But I've recently gotten much more experience with QinQ.
QinQ just adds another Qtag on the outside of any incoming frame, and while in side the "provider network" just looks at that Qtag the "outside" Qtag, for its management. Once it exits the provider network, the outside tag is stripped, and it appears just as it was on the other side.
Thats kinda it.. Less you want more technical info... which you'll want google for anyway.