CAS Arrays inc DR Failover (Exchange 2010)

Please can someone confirm this basic design for CAS array setup + DAG & fail over in a DR scenario. I have read some conflicting articles and wanted to confirm my understanding

Site 1
Exchsvr1 - 3 roles
Exchsvr2 - 3 roles
80% of users here, the rest on remote sites

Site 2 (DR)
Exchsvr2 - 3 roles
DR facility

CAS Array
CAS Array 1 - casarray1.domain.local (5min TTL) - DNS RR to exchsvr1 & 2
CAS Array 2 - casarray2.domain.local - DNS A to exchsvr3

DR situation
Manually edit internal casarray1 DNS record to point to exchsvr3

One DAG (DAC enabled) File Witness on site1 member server, secondary File Witness on site 2 member server
A copy of all databases on all servers (50% of active on exchsvr1 & 50% of active on exchsvr2) preference to be set so standby db replica (site 1) is used before DR replica on site 2)

DR situation
Users will connect to and use mbx db on exchsvr3 (due to manual DNS change of CAS array)
StuartTechnical Architect - CloudAsked:
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Will SzymkowskiSenior Solution ArchitectCommented:
The outline that you have provided above looks correct. As you have stated already a DR failover will always need manual intervention. The only thing you should not have to do is the manual DNS change for the CAS array in the DR site when you fail over. When you enable DAC (and it is configured properly) it sets your databases primary in the DR site. So when users Open Outlook autodisvoer will look for where the active mailbox is located and use the DR CAS Array to connect to it.

Not to say that changing the CAS array DNS name will not work, but it should not be required. As long as DAC is enabled your mailboxes are active on the DR site your clients will use Autodiscover to find the mailbox and connect to it.

I answered a vary similar question like this a couple weeks back. See the PAQ below.


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StuartTechnical Architect - CloudAuthor Commented:
Thanks for the link somehow i missed that question when searching on here :(

I didn't realise Outlook has the intelligence to redirect to a different CAS array, i thought the config was saved in the users .ost file.. and if you wanted to change the CAS array it would be a manual change or script.
Will SzymkowskiSenior Solution ArchitectCommented:
Its really not the intellegency of Outlook more the Autodiscover Service that it uses. As long as your clinets can reach the DR site, when Outlook is launched it will use Autodiscover which will find what server the Active Mailbox resides on. Then it uses the CASArray in that AD Site to connect to the mailbox.

StuartTechnical Architect - CloudAuthor Commented:
Understood thanks!
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