Same VLAN between router

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Jul 6th, 2009
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We would like to add a server for our Level 5 clients to be located in our Data Centre but would like to use the same IP Address on the Data server (red box) as what is currently use at Level 5 data network (red box). Attached diagram depicts traffic between the 2 endpoints.

Is there any other way this could be setup? PC will communication with Domain Controller...

From my understanding, answer is no but would like to get other network engineers opinion.



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marccourt1975 Mon, 07/06/2009 - 21:37
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This is an interesting problem. I think you are right, you can't add a server in your data centre with the same IP range as your PCs on Level 5. The main reason for this is because the two locations are separated by a layer 3 device (router). You might, however, be able to "simulate" this using static NAT. You might want to test this first as I could only simulate the result using GNS3.

Basically, you add your server in the Data centre and give it an IP address from the data centre range (, in my example). Then on your intermediate router add a static NAT pointing -> From the LANs point of view when the PC try to communicate with the "local server" using the ip address, the router will pick it up and translate the IP address to the servers Data Centre IP address ( and forward it to the server. The server response (from the IP) will then be translated back to its local IP ( on the way back in.

The result is the PCs will think the server is local even though it is on a different IP range and location.

I also had to add a static route (return path) from the server.

I'm not sure if this is what you're after as you didn't give much detail but I hope it helps put things on the right path. I've attach a picture of the test I did and the configuration is embedded in the picture.

Good luck


marccourt1975 Mon, 07/06/2009 - 21:57
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After looking at your diagram a bit more closely, the solution I recommended above won't work as you Server is placed 2 hops away from the router.. you could do multiple translations but that is getting very messy and I wouldn't recommend it. Perhaps the best idea is to question why you need the server to be on the same VLAN as your PCs... can't you simply route to it?



Leo Laohoo Mon, 07/06/2009 - 21:42
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Try VRF.

Peter Valdes Mon, 07/06/2009 - 21:56
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Hi Guys,

Thanks for the reply. The NAT makes sense and I will test this in our LAB environment.

The server has to be hosted in the comms room due to cooling and UPS. Server will be running as domain controller for PCs at Level 5 and needs to be on the same network. We could put this on a switch under the 3825, the switch will be part of the VTP domain for Level 5 but this will waste switch ports as we only need 5 ports to connect 2 servers and the trunk to the Cisco 3825.

The other server can be on a different IP Address, I just need some ideas on how to setup the DC server.

I will also read up on NAT VRF and see if this is applicable.




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