Dot1q Tunneling: 802.1q, QinQ Tunneling

When a TUNNEL port receives Customers Traffic - INGRESS PORT adds 2 Byte EtherType field 0x8100 + 2 Bytes for CoS and VLAN
Eggress tunnel port STRIPS THESE 4 BYTES
 (config-if)#switchport access vlan 100
 (config-if)#switchport mode dot1q-tunnel

You can also configure L2 TUNNELING (CDP, STP and VTP can be tunneled)
(config-if)#l2protocol-tunnel [cdp | stp | vtp]

#show l2protocol-tunnel summary

*Take SPECIAL CARE about the MTU SIZE on Swithches (might need to increase to 1504 due to the ADDED 4 BYTES IN THE TUNNEL)