Upgrade Exchange 2013 CU1 to CU5 question - DAG & HA

Hi there guys.

I have 2x Exchange 2013 CU1 in DAG that needs upgrading to CU5. As I do not want to cause any downtime, I wanted to first upgrade one of the servers & test for a few days before upgrading the other server. Question is, will the DAG & all other services still work OK if one server is on CU1 and the other on CU5?
Should I first upgrade the passive member in failover or the active? Can you help me with a roadmap?
Lastly, I also need to upgrade the operating system from Server 2012 to Server 2012 R2 - should I do this before or after upgrading Exchange and why?

Here's my configuration scope:

CONTOSO1 - Server 2012 - Exchange 2013 CU1 - Primary Exchange Server - MB & CAS Roles - Active member in Failover Cluster
CONTOSO2 - Server 2012 -  Exchange 2013 CU1 - Secondary Exchange Server - MB & CAS Roles - Passive member in Failover Cluster
CONTOSO3 - Server 2012 - DAG Witness Server

Thanks,

Rod
LVL 3
Rodrigo Costa dos PrazeresNetwork AdministratorAsked:
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ChrisCommented:
best practise is to have them up to the same Patching level pretty much straight away. there are enough changes in between CU1 and 5 that it will cause some issues with long term running.

CU5 is much better and more stable that CU1 so i would say Patch Passive - failover to it and make sure it comes up and then patch the active

this lists the steps for sorting out the failover during patching - although its for CU1 the steps are still relavent
http://exchangeserverpro.com/exchange-2013-installing-cumulative-updates/
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Adam FarageEnterprise ArchCommented:
I would first patch the passive member, then fail back to the passive and patch the active member. Exchange 2013 CU1 > CU5 will not stop you from failing over between both members, but you want to make sure they are both patched.
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ChrisCommented:
just to make sure its not confusing

Active Node = Node1
Passive Node = Node 2

patch node 2 first
Fail over from node 1 to node 2
Patch node 1
fail back from node 2 to node 1

there is at least one bug in CU1 in relation to DAG's so you are safest running both on CU5 as soon as possible
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Rodrigo Costa dos PrazeresNetwork AdministratorAuthor Commented:
Thank you guys. That's extremely helpful.

Lastly, I also need to upgrade the operating system from 2012 to 2012 R2. Would you say it's best practice to upgrade Exchange to CU5 first and then upgrade the OS or the other way around?

Rod
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suriyaehnopCommented:
Exchange 2013 not support in place upgrade. If your existing exchange 2013 running on Windows 2012 and you want to upgrade the OS to Windows 2012 R2. The best practice prepare another server with Windows 2012 R2, install Exchange 2013 with latest CU then move the mailbox to it.
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Rodrigo Costa dos PrazeresNetwork AdministratorAuthor Commented:
Thank you very much.
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