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E2K10 SP2 RU3 cross site failover - Is DNS record update requirement? or recommendation?

My 4 node DAG spread across two AD sites.
2 in Primary and other 2 in Secondary site.
We also have 2 CAS in Primary and other 2 in Secondary (Netscaler VIP in each site with different DNS name).

This blog says DNS IP change from Primary CAS to Secondary is "recommended" or "guide line". Not definitely a "requirement" if I've read correctly. It's reasonable because Primary CAS is totally down, no one can tell Outlook the new CAS name in Secondary site.

http://blogs.technet.com/b/exchange/archive/2012/05/30/rpc-client-access-cross-site-connectivity-changes.aspx

"changing the primary datacenter's DNS records to point to the IP addresses in the secondary datacenter, that is still our recommendation"

However this blog guy succeeded to perform the event WITHOUT changing DNS record.

http://exchange2010admin.blogspot.com/2013/06/testing-for-outlook-connectivity-during.html

It seems Outlook retrieves the new CAS name from AD without relying on primary CAS name.

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I'd like to understand :
- It looks DNS change is not "requirement". Why MS guy still keep this procedure as "recommendation"?
- What happen if we don't follow MS guidance?
- What is the technical query processes that Outlook to get CAS name. Where's the attribute that Outlook determine the new CAS name in new site?

Cross site failover is an event in very extreme situation and we most probably can't expect DNS team guy to be available timely. It's much better if we can just rely on AD replication.
Not sure why the guidance keep this process manual.

Thanks.
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kimakabane
Asked:
kimakabane
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Simon Butler (Sembee)ConsultantCommented:
All the designs and procedures I have written have involved updating the DNS record.
I wouldn't depend on Autodiscover picking up the change.

I do this for speed - if I have done cross site it is quickest to update the DNS entry (which will have the lowest TTL time).

The client will find the new server from the information within the domain related to the CAS Array. Remember the CAS Array is two parts - the DNS entry and the configuration in Exchange (new-clientaccessarray). CAS Arrays are also AD site specific. Therefore if you change the database activation value, then the clients should find the new CAS Array for that site.

I will admit it isn't very clear exactly how, particularly as you change the RPC entry on the database. That is why most guides will say switch the DNS record across, because that is very easy to predict and do, rather than depending on Exchange "detecting" things have gone away or making modifications to the DAG configuration.

Simon.
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kimakabaneAuthor Commented:
Simon,

Thanks for your comment as always.
I had a chance to perform cross site failover (Primary site down scenario) and confirmed DNS  change was definitely required in my environment.
I've waited about an hour to complete the AD replication but Outlook didn't find CAS array automatically until DNS record was updated.

Thanks.
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kimakabaneAuthor Commented:
Good information was provided to my question.
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