Data Centers can be connected with Overly Transport Virtulaiztion OTV, FabricPath or MPLS, In addition QinQ can be used to connect two Data Centers. Below is a diagram shows you how two Data Centers connected via 1G and 10G Dark fiber. In this case QinQ is used to secure the different vlans by encapsulating inside a Metro vlan.
QinQ uses two dot1q tags, the outer tag, which is the metro vlan used to transport the inner vlan, it is like MPLS VPN with two labels, the IGP label used to transport the VPN label through the MPLS provider/Provider Edge and the VPN label used to secure the different customers traffics. MPLS works over IP layer and it creates AToM or VPLS to achieve a layer two connectivity, but QinQ works only over the Data link layer.
I have also added the two DCI switches configuration with this diagram. This design has been tested on real world application to migrate a big multitenant Data Center with VMware vMotion.