Exchange 2003 and 2010 Coexisting

We have a single Ex2003 32-bit server (no more than 500 mailboxes) in an isolated site with no OWA/Autodiscover requirement.
I am replacing this server with a 5 server Ex2010 platform consisting of 2 x HubCAS servers, 3 Mailbox Servers, and 2 Network Load Balancers.
The EX2003 server is on an old network (10.) and I am building the new 5 server platform on a new network (172.), these networks do not current talk to each other.
I have built the 5 EX2010 servers to OS level (Server 2012)and have the following questions.

Why can I not install EX2010 server roles onto Windows Server 2012 R2 servers as I have been told they will have to be Server 2012 Standard boxes? Not sure if there was a 32-bit - 64-bit OS conflict between Server 2003 and Server 2012?

Also, is it OK for me to install the Exchange 2010 roles onto the 5 servers whilst the 2 networks are separate, and then when bridging the two networks together for coexistence and mailbox migration will the 2 environments automatically start talking, or will I have do initiate the process?

In advance thank you for the support.
CTCRMInfrastructure EngineerAsked:
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Simon Butler (Sembee)ConsultantCommented:
Exchange 2010 is not supported on Windows 2012 R2. Simple as that.
Windows 2008 was the last version of Windows Server that came in 32 bit version. Windows 2008 R2 and later is 64 bit only.

It sounds like you have two separate forests. If so, then you are doing a cross forest migration.
Cross forest is much easier between two Exchange 2010 servers. Therefore I would be looking to install an Exchange 2010 server in the existing domain where the Exchange 2003 server is, migrate everything to that server, then cross forest.

I would also cast some doubt on your design.
Splitting the roles up is no longer considered best practise. Why are you deploying separate CAS role servers if you are also deploying load balancers? I would drop the CAS completely and consider a three server design - three servers hold all three roles, in a three node DAG, active, active, passive.
The two load balancers can use all three servers for client access functionality.

Jian An LimSolutions ArchitectCommented:
another thought is why bother to install exchange 2010 when you can just install exchange 2013? (given it is cross forest)

it only make sense if you are doing on the same forest.
CTCRMInfrastructure EngineerAuthor Commented:
Hi Guys
The topology here is currently a single site, single domain, but two separate networks (current and new) so no Cross-Forest element here.

The new network has been created on a much faster back bone and all new VM servers being built are placed on the new network. The single Ex2003 sits on the current (slow) network which does not currently talk to the new. However, 'Networks' will bridge the two networks together once the new Exchange platform is built to allow the migration of mailboxes whilst in a coexistence state.

I have built the 5 Server 2012 boxes for Exchange 2010 but not yet installed Ex2010 and wanted to know if 'Networks' need to bridge the two networks before I install Ex2010, or can I complete the installation of Ex2010 on the isolated network and then configure the Ex roles after the networks are temporarily bridged?

Hope that makes sense..
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Simon Butler (Sembee)ConsultantCommented:
"However, 'Networks' will bridge the two networks together once the new Exchange platform is built to allow the migration of mailboxes whilst in a coexistence state."

That isn't going to work.
If this is a single forest then you need to have all servers able to talk to the domain.
Exchange is not a standalone application - it is heavily integrated to AD. The installation simply will not work on your plan.

If you have two networks but they are all part of the same domain, then as long as the networking is configured correctly you will have no problems.

@ limjianan - "another thought is why bother to install exchange 2010 when you can just install exchange 2013? (given it is cross forest)"

As it isn't cross forest, that isn't going to work because you cannot have Exchange 2003 and 2013 in the same forest.


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CTCRMInfrastructure EngineerAuthor Commented:
Simon, you're correct. This is a single domain whereby Ex2003 (10.x.x.x) is seen, and the 5 new Ex2010 servers (172.x.x.x) have now been joined to the same domain following Networks configuring the firewall that splits the environments. This isn't cross forest and is one of the reasons why we have gone to Ex2010, we will migrate the mailbox data across during coexistence and then once the Ex2003 server has been decommissioned we can plan for a EX2013 upgrade in the future.
Jian An LimSolutions ArchitectCommented:
i think simon comment make well.
CTCRMInfrastructure EngineerAuthor Commented:
Thanks for your support
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Windows Server 2003

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