I must say this is one of the technology's I love and some times also love to hate as it can make you very confused on where to start and what to do to make it work, but once you do make it work you fill like Houdini.
why like Houdini because basically the technology make your middle switch go away like it never was there and sowing to the person you wanted to show like he is directly connected to a switch when he is not.
now that I made every one a little confused, i will start to explain, QinQ allow you to take a middle switch and turn it to transparent while connecting 2 other components and maid them fill like they are directly connected.
why do you need that, well the CCIE lab love it as it is confusing setup but there are more reason then that, with QinQ you can connect topology's while you migrating, you can also connect between clients that have there own topology that you do not want to encounter in your topology by transferring all of them with a single tag between there point of presents you do not need to create a trunk between the 2 branches and pass all the vlan id inside while limiting your self to 1 or 2 clients.
you can see a much more elaborate details in cisco site
here i will give you few steps to create QinQ simple scenario
you can see that R1 is connected to SW1 and SW1 connected to SW2, if you will do a show cdp nei on R1 you will see that you are connected on F0/0 to SW1 F0/1, I want you to make you see like you are connected to SW2 port F0/10, how to do that magic?!
Go into SW1
! this is to allow another tag to pass the metro tag as it also
! called an additional of 4 byte
switch(config)#system mtu 1504
! i will create a vlan for access control between the ports
!now I will go under int F0/1 and F0/10 and type the same commands
switch(config-if)#sw mode dot1q-tunnel
switch(config-if)#sw acc vlan 200
now when you will go to R1 and do show cdp nei you will see like magic that you are directly connected to SW2 F0/10 instead of SW1 and I didn't switched the cable!!!
Read more on cisco site you will love it.
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