Exchange 2013 Client Access HA via two-datacenters

Hi folks,

I'm wondering how to make the following a nice HA solution:

- 2 Datacenters on different locations
- On each DC there is a complete Exchange 2013 Infrastructure with dedicated ClientAccess Servers and dedicated Mailbox Servers with DAG

How to make CAS HA in case of DC blackout?

1st idea:

- DNS Round Robin to client access server (con in case everything's fine = 50% of the client requests may contact the wrong CAS first, con in case of failover = client requests won't be answered if contacting failed server, 20 seconds timeout then retry other IP)

How do you recommend make CAS an reliable deployment?

Regards and tia
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Will SzymkowskiSenior Solution ArchitectCommented:
When you are load balancing Exchange 2013 CAS role you have a few options, but really only 1 recommended option. Hardware load balancer (layer4 or 7) or WNLB.

You really only use WNLB in a lab environment due to its limitations and reliability. In a production environment it is recommended to use a Layer7 Hardware load balancer, or a Layer4 (lower cost).

You can have mailbox copies in other Active Directory Sites on another DAG member but CAS role can only be load balanced within the AD site. You cannot load balance CAS (1 from each site) because you require CAS in each active directory site where you are hosting a mailbox database. CAS and Mailbox role is required.

So what you can do it load balance 2 CAS servers in ADSite1 and load balance 2 more CAS servers in ADSite2.

Take a look at the Technet below for additional info.


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