Could someone please comment on this proposed setup. We have an environment of 150 users and plan on implementing Exch2010 (no current exchange server). EXCH1 will host MB/CAS/HT and be the primary server. We would like to create a DAG using a backup server and another 2008 box acting as the witness. The backup server will have minimal load on in it during daytime hours. We only want to use the backup server in case of failure to the primary server. We do not plan on using a load balancer. If the server does fail, we'd have the users logoff/logon and upon logon a script with a .prf file will run to re-direct the outlook clients to the backup server. A short period of downtime is acceptable (1 hour). External mail access (OWA) is not available to users. Will this work? Am I missing anything? Thanks

One other followup, is 2008 enterprise edition required for this setup? We will just be using one mailbox database.

So to reiterate...you want to do 2 servers. (1) HUB/CAS/MB and (1) MB, the two MB roles will be configured in a DAG. So you will have a few issues with this if I understand correctly. First off, the HUB, CAS, and MB roles are all required. So if you had a failure to your "primary" server and the DAG flipped over to the secondary MB server then nothing would work. Email flow relies on the HUB server and ALL client access relies on the CAS server. So you would be better off with 2 servers, both with HUB/CAS/MB roles installed with a DAG. If you don't want to create a CAS Array (with a 2 node setup a hardware loadbalancer would be required). Then when you failed over, you would just have to update the RPCClientAccess parameter on the databases to the secondary CAS. To answer your other question....Windows Enterprise edition is required for DAGs as it relies on the Windows Failover Clustering service (only available in Enterprise Edition).Tim Harrington - Catapult Systems - http://HowDoUC.blogspot.com

Just to add - with 150 users and the capacity for 4 databases with Exchange 2010, I would be recommending to split users up over databases. This not only aids recovery, but also if you have to reseed the DAG at any time, you will have less data to reseed - as it is unusual to have to reseed all data. Enterprise edition of Windows, but standard edition of Exchange 2010 that is required. Simon.Simon Butler, Exchange MVP. http://blog.sembee.co.uk , http://exbpa.com/