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