Data Center Interconnect with QinQ for vMotion
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.

Configuration on the Interconnect switch
SW1
!
vtp domain TEST
vtp mode tr ans
vtp password test
vtp version 2
!
vlan 666,777,888,999,4444
!
Sytem mtu 1504
sytem mtu routing 1500
!
vlan dot1q tag native
!
int g1/0/1
sw acc valn 666
sw mode dot1q-tunnel
l2protocol-tunnel cdp
l2protocol-tunnel stp
!
int g1/0/2
sw acc valn 777
sw mode dot1q-tunnel
l2protocol-tunnel cdp
l2protocol-tunnel stp
!
int g1/0/3
sw acc valn 888
sw mode dot1q-tunnel
l2protocol-tunnel cdp
l2protocol-tunnel stp
!
int g1/0/4
sw acc valn 999
sw mode dot1q-tunnel
l2protocol-tunnel cdp
l2protocol-tunnel stp
!
int g1/0/47
description to Dark Fiber
sw trunk encap do1q
sw mode trunk
sw trunk natve vlan 4444
sw trunk allowed vlan 666,777,888,999
switchport backup interface gigabitehernet1/0/48 preemption mode forced
!
int g1/0/48
description to VPLS
sw trunk encap do1q
sw mode trunk
sw trunk allowed vlan 666,777,888,999
sw trunk natve vlan 4444
!
=======================================================
SW2
!
vtp domain test
vtp mode trans
vtp password test
vtp version 2
!
vlan 666,777,888,999,4444
!
Sytem mtu 1504
sytem mtu routing 1500
!
vlan dot1q tag native
!
int g1/0/1
sw acc valn 666
sw mode dot1q-tunnel
l2protocol-tunnel cdp
l2protocol-tunnel stp
!
int g1/0/2
sw acc valn 777
sw mode dot1q-tunnel
l2protocol-tunnel cdp
l2protocol-tunnel stp
!
int g1/0/3
sw acc valn 888
sw mode dot1q-tunnel
l2protocol-tunnel cdp
l2protocol-tunnel stp
!
int g1/0/4
sw acc valn 999
sw mode dot1q-tunnel
l2protocol-tunnel cdp
l2protocol-tunnel stp
!
int g1/0/47
sw trunk encap do1q
sw mode trunk
sw trunk allowed vlan 666,777,888,999
sw trunk natve vlan 4444
switchport backup interface gigabitehernet1/0/48 preemption mode forced
!
int g1/0/48
sw trunk encap do1q
sw mode trunk
sw trunk allowed vlan 666,777,888,999
sw trunk natve vlan 4444
!