2 forests, same domain name, same network, what i need to be careful ?

I would like to know the impact if 2 separate forest (2003 server)  in the same network have the same domain name?
The impact is it the same if i have 2 forest with the same domain name on 2 differant subnet and i route the two subnet to communicate each other?
WebHunter76Asked:
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RichardSlaterCommented:
You are going to run into some major problems with that, the two domains could quite happily coexist on the same physical network however you would need to make special arrangements with DHCP (i.e. reservations on each domain for all devices). Communication between the two domains is probably going to be a big problem because there is no way to differentiate between the two.
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LauraEHunterMVPCommented:
If by communications you are trying to establish any sort of a trust relationship so that users in one domain will be able to access resources in the other, this will not be difficult...it will be impossible.  (Trust relationships cannot be created between two domains that have the same name under any circumstances.)

Additionally, you will experience issues with any sort of NetBIOS traffic, since NetBIOS will detect a conflict between the NetBIOS records for the two separate domains. Even if you remove NetBIOS conflicts from the picture, you will need to procedurally enforce uniqueness of FQDNs across the two domains, since DNS resolution will obviously go haywire if you have two separate IP addresses configured for server1.domain.com.

Frankly I think it's asking for more trouble than it's worth - I certainly wouldn't deploy something like you're describing onto a production network.  My $.02, anyway.

Hope this helps.

Laura E. Hunter - Microsoft MVP: Windows Server - Networking

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WebHunter76Author Commented:
the main concern is that i want to recreate a new Active Directory forest with the same Name, so user will not be affected by the change (no local profile to copy) because the actual AD Structure is too bad i have to restart all from scratch ( actually  4 DC AD 2003 ( DHCP + DNS)+ 2 SQL all on the same class c subnet )  and i want to put only 2 DC with DHCP & DNS with windows 2003, SQL and exchange Server 2003 as member server and not at the same time DC as actually + a file server and finally a production server in a new class b subnet.  It is possible for me to have some maintenace periode but never more than 8 hours a week for the server.
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LauraEHunterMVPCommented:
I certainly understand the desire to have a transparent migration process for your users. However, I can promise you to a nearly 100% certainty that you won't be able to pull off that kind of migration by standing up two forests with the same name on the same network.  It just plain won't work.

It's also worth noting that, even if you were to move UserA from ForestA(1) to ForestA(2), a new profile would be generated because ForestA(1)\UserA and ForestA(2)\UserA would possess different SIDs, which would screw up the permissions on the local profile -anyway-.

If your organization's tolerance for downtime is that low for a migration of that level of complexity, I might recommend investing in a third-party migration tool from someone like Quest or NetPro.  A tool like that has a dollar-value pricetag associated with it, but it will drastically cut down the amount of time your migration process will take.


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LauraEHunterMVPCommented:
Additionally, any migration tool is going to require (or at least =strongly recommend=) that a trust relationship exist between the source and target domains in order to perform the migration.  As I mentioned above, this is not possible when 2 domains have the same name.
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