Implementing Sites for a new Single Domain Environment and effects on Exchange

Question

Copied from the Active Directory forums as the suggestion of replies.
I didn't find exactly what I was looking for so decided to create my own question to get some direct feedback.
Currently we have a single domain environment with two domain controllers located at two separate sites. When the domain was first set up, no configuration was done in the Sites and Services module for Active Directory. The two domain controllers we have are
currently located in the Default-First-Site-Name container. We do not have any subnets configured with the Sites and Services module.
These two domain controllers are located at two different sites with different IP schemes and the sites are connected with a high speed site-to-site VPN. We also have 2 satellite offices with their own IP schemes as well with more offices to come. In the future
domain controllers will be placed at these satellite offices which are connected with a slower site-to-site VPN to the main offices.
All replication and network functions are working well now, but I would like to know what the effects would be and what to watch out for if I create sites for our environment. I am particularly concerned about our Exchange 2010 server and need to make sure
that the change will not disrupt communications between it and the domain controllers.
I would like to create a site for each of our locations and link the subnet to that site now so that when we install the domain controllers the configuration is ready.
Any suggestions or input is highly appreciated thank you in advance.

Answer 1

Exchange will be an issue only if your Exchange servers span sites when your new Windows sites are created.  If you have Exchange servers all in a single location, adding sites to your Windows forest will cause no issues.  However, if you have
Exchange servers in both locations, as soon as a new site is defined for an Exchange server in a separate location from your other Exchange servers, you will start having issues.  Let me give some examples so you can see what problems might occur:
Two datacenters, one Windows site, Exchange mailbox servers in both locations (primary and DR), but hub and CAS roles only in the primary datacenter:
In this situation, as soon as your second site is defined, the server in the DR datacenter will no longer be receiving mail - there is no hub to deliver it - and users will no longer be able to access their mailboxes - there is no CAS to support them. 
Solution:  Add hub and CAS to second datacenter and all is well with the world.
 Two datacenters, one Windows site, Exchange multirole servers in both locations (primary and DR), but CAS Array defined:
Now we have a little bit better setup, since we have all roles in both locations.  However, the CAS array in the primary site isn't going to be able to support your client connections in the DR site - so users will be connecting directly to the CAS
servers in the DR site (not optimum).  Solution:  Define a second CAS array for the DR site, with its own load balancer and configure the databases in your DR location to use that CAS array as the RPC Client Access Server.
There are other oddities, but as you can see, there will definitely be issues if your Exchange servers aren't all in the same location and you start defining Windows sites ...