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Multi Domain (different forest) Acquisition Setup

I'll try to keep this to the point and provide enough information, as I have my own thoughts on how to achieve what I am looking to do but have not done this before.

We are in the process of acquiring two new firms.  The first firm has 3 office locations, site to site vpn between them, exchange server at one location, database server and then the rest is just data and license servers.  The second firm is 1 office location with exchange, database and license server as well.

First Firm Layout:

Site 1 - Single Server that has Exchange/Data/License servers
Site 2 - 1 Server with Data/License, 1 Server with Databases
Site 3 - Single Server with only Data

They are using a 10.x.x.x ip scheme, share files between offices, etc.

Second Firm Layout:

Site 1 - 1 Server with Data, 1 Server with Exchange

We need to bring these sites into our domain but keep them up and running with their current email until a later date, as well as keeping their current file shares between their offices and join into ours.  

We are on a 192.x.x.x scheme.

We are getting 2 new servers for Site 1, which I am planning on setting up to join into our current main domain.

Is the best option to just VPN site to site (have the hardware already capable of this) between all of our offices and then create domain trusts between the three different domains?

Down the line when we switch them over to our email server, should their servers be demoted and brought back up into our main domain?

Is it a possibility, although I imagine much more work, to bring their current hardware up in our domain?

My main concern is that these sites are all being done one weekend after the next, and need to maintain connectivity to everything they currently have.  So if we were to switch Site 1 at Firm 1 to our domain, we'd still have to then create that domain trust to Site 2 at Firm 1 so they have Exchange and file level access.

I'll stop rambling and wait for a solution!  I know it's easier than I am making it out to be.  The one issue I could see is if Firm 1 has their functionality level set to 2008, and we need to stay with 2003 for now, which I am not sure of yet.
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Joseph Daly
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Just some more info.

Down the line when we switch them over to our email server, should their servers be demoted and brought back up into our main

You will probably want to do this after an AD migration and then use powershell to move their mail. Assuming all email servers are exchange 2007 or higher. What you do with the email servers depends on your exchange infrastructure but once they are on your exchange server presumably in your corporate office there is no need for them to have their own any longer.

Is it a possibility, although I imagine much more work, to bring their current hardware up in our domain?

This again is something that will be done as part of an AD migration. The ADMT tool has the ability to automatically migrate computer accounts over to your domain and run security translations on them so that user accounts etc are maintained and useable in the new domain. You would do this on servers and workstations
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I was looking into the domain trust way due to the time constraints we are on.  Essentially we have 6 weeks to bring up 4 sites and only can take them down on weekends.

Is there a feasible way that we could migrate their exchange cross domain to either another server locally or our main exchange server and have that handle their domain?  Considering a 2 man crew, weekends only and 14 users, 11 users and 3 users across the 3 sites?

I appreciate all of your information, and they will eventually have to be part of our main domain.  The main hold up is that they have to keep their old email address and exchange server up for awhile.
Im not sure about having your exchange server handle their emails without having their user accounts in your domain. There may be a way but its not something I ever have done so Im going to refrain from making guesses.  

For one of the acquisitions we did let them keep/run their exchange server for about 2 months while we were preparing to migrate their mail. If i recall we did this by setting up a send connector between our exchange and theirs.
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Is there a way, with DNS, to create sub zones which would enable name/ip resolution to any pc across the 3 domains?
Yes there is a way to do this. Its done by using forwarders. Below is the MS link to details.

Basically say  your domain is and they're On your DNS server you would put an entry for and use a forwarder to their DNS server IPS.

On their end they would create an entry for and put your DNS server IP as forwarders