Exchange 2010 Design Q's (CAS related)

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HI All,

I'm currently doing my first Ex2010 design and have some confusion on the cas role I want to clarify

It's a 3 geographically dispersed site design with decent links but nothing flash, all 3 sites are also defined in AD and all prerequsits are / will be met to go from 2003 to 2010. Client access is currently via outlook 2003 with Outlook 2010 on the horizon but won't happen before Ex2010

1> 2 servers is each site with CAS/HT/MB (in a DAG) roles on both, can I put the load balancer in front of these 2 servers rather than dedicated CAS boxes?

If I don't load balance and a client on server A goes down, DB fails over to Server B does the client fail to reconnect to server B and requires intervention such as running a PS script and a restart of outlook or isn't it that simple?

If I do load balance and a client on server A goes down, DB fails over to Server B does the client just redirect automatically, I've seen comments about the DAG failover happens it asks outlook to restart

I'm at the design phase once I get go ahead I can then start spending time testing senerios but nice to get a design fairly nutted out beforehand



Dave Baird


I'll answer the bits I can:

When you're running multiple roles on one box (CAS/HTS/MBX) and using DAG, you can't use NLB. Therefore, you are right that you will need to put a hardware load balancer in front of the CAS boxes. If you do this, you can create a CAS Array and use the load balancer address, and the load balancer will then direct the traffic to either of the CAS Servers. This also means you will associate your mailbox databases with the CAS Array rather than a single CAS Server, so you will not need to update the database>CAS association when it fails over between DAG nodes.

I'm not sure about Outlook having to restart. It's MAPI termination point is the CAS Server now, so it shouldn't even be aware that the database has failed over in the background.

Just bear in the mind the Outlook 2003 issues regarding Encryption and polling methods if you will be connecting any Outlook 2003 clients to your Exchange 2010 servers.

Hope this helps.



Please see the answer in this thread

If you do have load balance, you have to set another CAS server to the database (Example 2 in here)

James Luo



1) HLB is manadatory you cannot use NLB with DAG

2) Each Exchange 2010 mailbox database has an attribute called RpcClientAccessServer. When creating a new mailbox database in an Active Directory site where a CAS array has not been created, this attribute will be set to the first CAS server installed in the AD site, so if the CAS failed you will need to change the RPCClientAccessServer to the new one.

3). for HLB you need to make sure that stickiness is configured correctly my team posted part one of that here

Regards, Mahmoud Magdy Watch Arabic Level 300 Videos about Exchange 2010 here: Read pretty advanced Exchange stuff I and other MVPs post here: Or follow my blog: or our corp blog: and if you Liked my post please mark it as helpful and accept it as an asnwer
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