Hi, I have 2 Active Directory sites (Headquaters and branch office). Site HQ has subnet 172.16.100.0/24, Site BO has subnet 172.16.200.0/24. Both sites have Exchange 2010 SP1 RU5 installed with all roles-HUB, CAS, MBX. I need to deploy high availability (HA) of Exchange. I know, how to make HA of HUB or MBX roles, but how can I make HA of CAS role? I read that it is not possible do CAS array between two AD sites-is it true? If yes, so how can I design it? Just now I have primary CAS with redirecting technology. Thank you for advices!Petr Weiner

True, you can only have one CAS Array per AD site. You will need some sort of load balancer, I believe using WNLB is now not recommended anymore and a hardware solution (mayve virtual too) should be used. Create a CAS Array, add the CAS servers to the Array and load balance the connection using the DNS names you created. Configure OLK to use the CAS Array name. Set the RPCClientAccess value on the DB. If you only have one CAS then you will obviously need another.Sukh

Hi, thanks for confirming my idea, but tell me how can I make my Exchange HA? If I am not able to do a CAS array between two AD sites and my one site will crashed, how can I provide CAS "services" to clients? HUB and MBX (DAG) roles are clear for me, but I still not understand how to do HA of CAS role. Petr Weiner

Hi Yo know what to do with the mbx & hub as you've mentioned this already. The CAS array you will have to do as mentioned above. Is there something you don't understand? Maybe read through this http://www.msexchange.org/articles_tutorials/exchange-server-2010/high-availability-recovery/load-balancing-exchange-2010-client-access-servers-using-hardware-load-balancer-solution-part1.html http://blogs.technet.com/b/ucedsg/archive/2009/12/06/how-to-setup-an-exchange-2010-cas-array-to-load-balance-mapi.aspxSukh

I do not understand HOW can client(s) connect Exchange 2010 when site where is CAS array located will failed/crashed. Should I need CAS array also in the second AD site or how can I archieve this? As I mentioned above-I have two AD sites, both have one Exchange server with HUB+CAS+MBX roles installed, so if I need HA of CAS role in my organization should I have to make CAS array in both AD sites and also create WNLB or used HW load balancer? Petr Weiner

You need a min of 2 CAS servers in a CAS Array in each AD site. This needs to be load balanced. If one CAS in the array fails then the other will provide services. If both CAS servers fail in the same AD site then OLK won't work. Use a HW Load balancer, 1st, it's not recommended to use WNLB anymore, secondly, you have All in one roles deployed, which mean you can't use WNLB on a server which is also part of a DAG as the DAG use OS clustering feature which conflicts with WNLB (not supported).Sukh