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Extending multiple VLAN's over a single hosted provider VLAN

I'm currently in the process of extending 3 seperate networks to another building.  (Bldg 100 to Bldg 200)
There is a network in place between the two buildings and the people that own the network are giving us a VLAN to ride over.  They have assigned us the VLAN 1050.

My question is, if I have 3 separate switched networks and they were all built using Native VLAN 1, and I want to have the three network connect to a single switch, get transported over the local provided VLAN 1050, then separated back out at Bldg 200, how should I setup the switches.  

So in reference to the drawing below, I want Switch A1 to be able to talk to Switch A2 over the provided VLAN 1050, the same as Switch B1 to B2 and C1 to C2, but all the network must remain logically seperated.

So far I have connected Switch D1 to the local provided switch and made the uplink port a trunk port and set the Native Trunk VLAN to 1050.
I did the same on Switch D2 and I can see the switches when I do a sh cdp nei.

BTW A1, B1, C1, D1, A2, B2, C2, and D2 are all Cisco switches.
The switches that are providing transport over VLAN 1050 are Foundry switches that I do not have access to their configuration.
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2 Solutions
So first question, are you saying you want three different instances of VLAN 1 that don't interact with each other?  I'm not sure there's a good way of doing that, at least not without virtualizing the D switches.  If they need to be separate, my suggestion is to use separate VLANs for each.

As far as transport, have you considered using an L2TPv3 tunnel?  That's the only way I can think of to transport the VLANs over the provider VLAN and maintain your VLAN numbers.  Take a look at http://www.cisco.com/en/US/docs/switches/lan/catalyst3550/software/release/12.2_25_see/configuration/guide/swtunnel.html#wp1006657 and see if that might help.
ingersoeAuthor Commented:
Yes, I have 3 different instances of VLAN 1 that are from 3 different networks that I need to extend and I don't want them to interact with each other.

I would like to use separate VLANs for each, but I don't actually administer all 3 networks.
Also behind switches A1, B1, and C1 are many more switches that are all using the default VLAN 1.

What I was hoping to do was have switch D1 tag the incoming traffic from each of the networks, say into VLAN 100, 200, and 300, have the provider encapsulate those in VLAN 1050, then it will remove the 1050 tag when it leaves the trunk on the other side, so the 100, 200, and 300 tags will still be there.
Then once the traffic gets to Switch D2, it will strip off the VLAN tags of 100, 200, and 300 when it forward it through the trunk ports to each of the switches A2, B2, and C2.

Would this scenario be possible, or is the double tagging something that cannot be done and I'll have to go with something like L2TPv3 tunneling?
Marius GunnerudSenior Systems EngineerCommented:
you can do this with QinQ but that would require a total of 3 VLANS that is provided by the network you are transiting through.

Another option would be to go with a tunneling protocol. L2TP you have mentioned or you could use VRF or even MPLS VPN. But the last two would require involvement from the network traffic is travelling across.
ingersoeAuthor Commented:
Thanks.  It's not the easy answer I was hoping for, but I think I can get it working nonetheless.

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