CAS Array/RPCClientAccessServer and Outlook profile experiences

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Trying to achieve the following for a Exchange 2010 DR plan. Site A - primary Datacenter with 3 CAS Servers in CAS Array, 3 mailbox servers. Site B - same setup as Site A - no active clients - only for Disaster Purposes. Will have a DAG with all Mailbox servers from Site A and Site B. Site A - all clients point to this - Site A contains some local clients as well as hosting mailboxes for entire company from other sites. With entire failure of Site A, manual switchover to Site B will occur - will change the DNS record of the CAS Array to point to the CAS Array in Site B - and switchover mailbox databases.

The one scenario I'm trying to plan for is this - Site A is not reachable - decision is made to switchover to Site B. However, I want my local Site A clients to still use the servers in Site A since otherwise they would be entirely down. Yes, there is Exchange Cached Mode - but our internet link would still likely be up in this scenario and it would be nice for not only my users at Site A to be able to e-mail each other, but to be able to send out e-mail (although they wouldn't be able to send to other employees outside of Site A and have it be delivered - just to internet users). In any case, I could have my local Site A users always point to a dedicated CAS Server - but I'd be losing the benefits of a CAS Array. If I switch my CAS Array DNS entry to Site B they will no longer function. If I change the RPCClientAccessServer value to one of the Site A CAS servers it works - but it only works IF I either recreate the Outlook Profile or do a Profile Repair. Since the original CAS Array would no longer be reachable, a redirect cannot happen. I've tested this and the only way I can get it to work is to have a new Outlook profile created or do a repair. I would have expected AutoDiscover to be able to pick up where the active CAS server is for my database and reconfigure my Outlook profile? I've read some ramblings that this might be a bug with Outlook 2007/2010? Any suggestions greatly appreciated!!


After much testing I've determined that AutoDiscover WILL finally occur when there is a failure of a CAS Server - but it's not consistent and I'm not sure why. So, if I point a database towards a specific CAS via the RPCClientAccessServer value and the original CAS server is unavailable, Outlook does seem to start the AutoDiscover process - but it seems like it takes almost 10 minutes for this to happen. My impression is that after failure of a CAS, AutoDiscover would automatically start. Or, upon reboot of Outlook it would do an AutoDiscover. I'm not seeing this happen and it's confusing. Perhaps I have something set wrong, but if I go and do a AutoDiscover test via Outlook it immediately shows where the mailbox CAS is. If I do a new Outlook profile or a profile repair - it immediately works. My goal is to have my clients pick this up automatically. Any ideas on how this really should work or if I have something set wrong?

Brian Day MCITP

The key thing to remember is Autodiscover isn't happening all the time. It only happens I think at startup and then every so many hours if the client is left open. The newer the client the better the experience, but you are correct in that there are a few " surprises" getting it all to work and yes some clients do seem to just plain ignore some settings AutoD passes to them which is entirely frustrating. :)

The easiest way to keep things working is to do as you did and flip the DNS entry to point to the CAS Array in Site-B if the Site-A CAS servers are gone. If the CAS servers in Site-A are still available then they will simply do a cross-site RPC connection to the mailbox servers in Site-B. Typically the recommendation is that you want the CAS server a client uses to be as close to the mailbox server as possible where the DB is mounted; so having the clients in Site-A continue to use the CAS servers in Site-A would not be the best performance-wise for them if the DBs have been activated in Site-B.

I'm not entirely following your comment on Site-A users being able to send mail to Internet recipients if you flip the DNS record to Site-B. Are you saying perhaps the WAN connection between A & B is dead, but there is still a working Internet connection in Site-A?

Microsoft Premier Field Engineer, Exchange
MCSA 2000/2003, CCNA
MCITP: Enterprise Messaging Administrator 2010
Former Microsoft MVP, Exchange Server
My posts are provided "AS IS" with no guarantees, no warranties, and they confer no rights.


Thank you for responding. That's exactly the scenario I'm trying to accommodate - where the WAN connection is dead (site A is not reachable), but I still have internet access through another pipe - and local Site A users who we want to keep 'alive'. Now, I realize this is not a perfect scenario - because if I've switched everything over to Site B (except my one database for local Site A users), all my external e-mail is going to flow into Site B - and obviously e-mail from Site A to Site B isn't going to be possible. So all I'm really going to gain is that local Site A users can communicate with each other and send OUT to the internet. That's a scenario my company desires to have accommodated.

So, my plan was to utilize a CAS Array in Site A and B and when the rare time comes where Site A is not reachable but still up, that's when I'd do the following:

1. Site A - Datacenter (Active), Site B - DR Site (Passive)

2. Switchover all databases to Site B (with the exception of the database hosting local Site A users)

3. Switch the CAS Array DNS record IP address to the CAS Array IP in Site B

4. Switchover incoming e-mail record to go to Site B (I have to do this because Site A is still accessible from the internet)

5. Change the RPCClientAccessServer value of the local Site A database to a single Site A Client Access Server

6. Hope that AutoDiscover works for those local Site A users and switches connectivity from the non-reachable CAS Array to the new RPCClientAccessServer value

I wish I could figure another way to do this via DNS for the local Site A users. The only other way I can think is to always point that local Site A database to a dedicated CAS. I just hate to do that because I'm losing the advantage of the CAS Array and local balancing. That's why I've been frustrated about the AutoDiscover not working as I expected. I'm testing with Outlook 2010 and even after an OS reboot it doesn't seem to pick it up. Sometimes if I leave it open and I have the Connection Status screen up I can see it switchover in about 10 minutes - but sometimes it doesn't happen.

Brian Day MCITP

Agreed that is a tough situation for the users located right in Site-A, but not all that uncommon. Would you happen to know the population split by any chance. Like is it 80% of your users are in Site-A and 20% coming in frome remote sites, or more remote users than users in Site-A?

I assume you want to use a DAG here, but without splitting the Site-A users up into their own seperate DAGs this may be somewhat complicated or not entirely possible to get going.

I have to head out, but I'll think about this for when I get back.

Microsoft Premier Field Engineer, Exchange
MCSA 2000/2003, CCNA
MCITP: Enterprise Messaging Administrator 2010
Former Microsoft MVP, Exchange Server
My posts are provided "AS IS" with no guarantees, no warranties, and they confer no rights.


How it will be is 100% of users will go to Site A, and local Site A users are roughly 25% of the total. I do plan on using a DAG between the two Sites - a single DAG (at least that's my initial plan). Site B will strictly be passive - no active users at all. The biggest scenario that is planned for is a complete Site A failure - then Site B would be able to be activated and used without any problem. That part I've tested and seems to work just fine. It's just this other tough situation. If I knew AutoDiscover would detect a failure and switchover within 5 minutes every time I wouldn't be too worried - especially with Cached Exchange Mode enabled on all clients. In my testing I just saw Outlook trying to Connect and was pretty much not able to do anything. A lot of times the WAN very well may be re-connected in a short period of time but if it is a few hours it would be useful to have my local Site A users somewhat able to do some work. They won't be able to send e-mail to Site B users (at least not that I'm aware of any way) but they can send out to the internet and to other Site A users. The only other idea I thought of which I've heard of before is maybe utilizing Outlook Anywhere for Site A clients - and switch everything over to Site B. I just think it would add complexity and would be a shame that I couldn't take advantage of my local servers. The other challenge I'd have with dedicating local Site A users to a single CAS - any external OWA/ActiveSync requests would always be proxied through the CAS Array - which is undesirable.


So just a little more info from recent testing. I think my stumbling block is now narrowed down to Outlook Cached Exchange Mode. When I have Outlook set to Online mode (Cached Exchange Mode is off), AutoDiscover works every single time upon an Outlook restart. So, if I change the RPCClientAccessServer value and restart Outlook - it goes to the proper CAS server. This is exactly what I want and that's great. When I turn on Cached Exchange Mode, that's where it doesn't seem to work. It keeps trying to connect to the old, failed CAS - and a restart of Outlook doesn't make any difference. Now, after about 10 minutes (roughly - it's been different each time, sometimes a little more/less) Outlook will connect to the right CAS and reconfigure the profile. It's almost like AutoDiscover is disabled when in Exchange Cached Mode - but I am not sure of this. Why it reconfigures it eventually I'm unsure of. Repeated starts of Outlook do nothing to help the situation. We definitely need to use Exchange Cached Mode as this is obviously advantageous for many reasons. Is anyone aware of why this would be occurring (or not occurring) with Exchange Cached Mode? I'm running Outlook 2010 by the way.


Well I think I may have this figured out after much testing. So with Outlook Cached Exchange Mode, if Outlook is RESTARTED after the RPCClientAccessServer value is set, then within 5 minutes Outlook will do an AutoDiscover and connect to the appropriate CAS Server/Array and reconfigure the Outlook profile. In my testing previously I wasn't restarting Outlook. With that occasionally Outlook would reconfigure the profile via AutoDiscover - but not consistently and usually only after 10-15 minutes. If Outlook is restarted AutoDiscover triggers within 5 minutes. I tested this scenario 10 times and 100% of the time it was within 5 minutes. So with that I think my plan will now work. Now, why Outlook takes 5 minutes in Cached Exchange Mode to trigger I've no idea. With Cached Exchange Mode off and Outlook restarted, it occurs immediately. If there is something I'm missing I'm all ears but that's my conclusion thus far.


Hi Birdman,

Do you have any updates on this Outlook behaviour?

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