Exchange 2010 DAG members with different levels of Service packs


I have 3 exchanger servers, with each server installed with CAS, HUB, Mailbox.

Ex2010A SP2 Ver. 14.2 (Build 247.5) Active DAG member (site A)
Ex2010B SP2 Ver. 14.2 (Build 247.5) Passive DAG member (site B) Active CAS server
Ex2010C SP3 Ver. 14.3 (Build 123.4) not part of DAG (Site A)

AD schema 47
Exchange forest schema 14734 (support 2010 SP3)
Total users 400

Obviously not properly design when all users are in site A but CAS is in site B. I am planning to upgrade my Exchange to all SP3 level and switch some roles.

Options 1:  Add Ex2010C to DAG member and upgrade Ex2010A to Sp3. Move CAS from Ex2010B to Ex2010C and upgrade Ex2010B as well. ( I don't see any down time in this and look very simple, only issue new ex2010C has different SP. though I know from TechNet "You can safely move a database between servers running Exchange 2010 SP1, SP2 or SP3. Even though you can move databases between mailbox servers running different service pack levels)

Option 2: Make Ex2010B active and upgrade Ex2010A to SP3 and vise versa. Then add Ex2010C in a DAG ( only issue is since all users in site A and when I make Ex2010B (Site B) as primary there will latency issues which is I am afraid. Though upgrade does not take that long but what if its unsuccessful )

Option 3: Move mailboxes from Ex2010A to Ex2010C and move CAS from Exch2010B to Exch2010C. Once upgrade create a new DAG.

Any other ideas are much appreciated.
Thanks guys
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Scott CSenior EngineerCommented:
You are best off keeping all of your versions of Exchange as close as possible.

 You need to upgrade to SP3 first on all of your machines then install RU10.

 The installation order is important.

 Order to apply the Service Pack 3 and a few hints…

The first thing when you are planning is the order to apply Service Pack 3 on your servers. These are the key rules to apply a new Service Pack in your organization.

 0.Edge Servers (if you have any)
 1.Client Access Server (if you have multiple sites internet facing sites first)
 2.Hub Transport
 4.Unified Messaging

 Install SP3 in the above order and once that's done install RU10 in the same order.

 Once you have everything at the same SP and RU level then you can do your moves.  I would not chance it in an unmatched environment.

 Make sure you have a good and tested back up before you start.
numanmalikAuthor Commented:
Agreed. I'm new to this environment and I have been told don't install SP3 on Ex2010A it will break. So the idea was to switch roles and decommission it without a try.
Scott CSenior EngineerCommented:
Ok.  That makes it clearer now.  Since you can't install SP3 on EX2010A, That does limit the options.  Option 3 would work, but you'd have to move users twice.

Your task would be to move the users off, flatten the machine, build it from scratch, get it up to the proper patch level and then move the users back.

Option 4
Stand up another Exchange server, get it patched and move everybody off of EX2010A to the new server.  You then decommission the old server and you only have to move the users once.

Are these server virtual or physical?  Virtual would give you more flexibility in terms of hardware.

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numanmalikAuthor Commented:
I think option 4 is the best it will safe me the effort. Yes they are all virtual and I have only 3 roles client, hub, and mailbox with smarthost.
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