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CAS Array

Posted on 2014-12-28
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CAS Array how to configure it...
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Question by:Exchange_Don
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Expert Comment

by:Seth Simmons
ID: 40520604
what version of exchange?
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by:Exchange_Don
ID: 40520616
2013
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Gareth Gudger earned 1000 total points
ID: 40521041
There is no CAS array any more in 2013.

If you need to load balance multiple CAS servers then you can configure load balancing. This could be a hardware or virtual load balancer such as a KEMP. Or it could be Microsoft IIS running ARR. You could technically even use DNS Round Robin for load balancing.

If you are looking on how to configure CAS from a namespace, DNS and certificate perspective then take a look at my article.
https://supertekboy.com/2014/07/08/designing-simple-namespace-exchange-2013/
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by:Adam Farage
Adam Farage earned 1000 total points
ID: 40521275
To add to what Gareth said..

Exchange 2010 mainly required Layer 7 load balancing when doing a CAS with Load Balancing, and the CAS Array object was a strict "AD object" that was utilized for a database attribute (RPCClientAccessServer). The CAS Array object never load balanced anything, just provided the RPC endpoint to the databases, which would be the CAS LB VIP.

In Exchange 2013 the CAS array object is now gone since the CAS is a stateless session server.. meaning all it does is authenticate the clients coming inbound, proxy the connection to the appropriate cas / mailbox or redirect the connection to the appropriate CAS. Since this is now stateless, persistence is no longer required so the Layer 7 load balancing requirement is out the window. Exchange 2013 can do Layer 4 and Layer 7.

I would not recommend Windows NLB due to the vast functionality limitations (multicast has to be enabled, no healthcheck, either the server is completely down or not before it is taking out of the pool so an individual virtual server could be down but Windows NLB will not remove it from the pool so it will still send active requests to it) along with scalability issues (in Exchange 2010 it was not recommend to have more than 7 CAS within a single Windows NLB cluster).

You have  several options when it comes down to load balancing multiple CAS that are within the SAME active directory site (you can check this by the subnet, or active directory sites and services);

- hardware load balancer, such as F5 or KEMP
- software load balancer such as F5, Kemp or IIS AAR (which I linked in the other question, this also acts as a reverse proxy)
- DNS round robin

A few things to consider:

- Although Layer 4 is supported, I would recommend against it. Its complex to setup and can be complex to troubleshoot (the part I hate the most is how you have to setup the default gateway on the servers as your load balancer port). Layer 7 is the way to go in most situations.
-  Steer clear from DNS round robin. Such as the solution with Windows NLB, most DNS solutions are not service aware.. meaning if a service is down and does not respond the DNS will still send weighted requests to that client (which is having issues) without notice.
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