Exchange 2010 Enterprise Multiple CAS Servers

We have 4 exchange 2010 servers in our environment:  2 - Mailbox Role (DAG); 2 -CAS/Hub Transport (No CAS Array).    Previously servers were separated by sites, but have since been moved to one site - within the same subnet.  Failover between the DAG/CAS has never been successful, but hopefully some of our issues will be resolved once we upgrade to SP3.

Currently Emails typically route out both exchange CAS servers:  is there a way where we can route emails out through just one CAS server in the event we have an issue with one of the CAS servers.  I've been told that when the servers were at separate sites, emails were only going out one CAS Server, but since moving them to the site together - emails route through either or both CAS servers.  Basically can we control what CAS server emails get sent out from?
Thank you,
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Initially I wrote that you could use scoped send connectors, but then I re-read what you wrote and noticed that you collapsed the AD environment that contains your Exchange Organization into a single site.. Scoped send connectors are scoped to a site, not to a specific HT role holder.

If you really need to have a single HT handle your email then you need to either consider adding a HT role holder to a different site (possible licensing issues) or removing the HT role holder from one of your CAS/HT servers (again, not commenting on specific implications of this, as I've never done this before).

What is it that you are trying to accomplish, besides losing redundancy sending/receiving emails between your organization and the Internet-at-large? Remember, even local mailbox servers uses HT to transfer emails between mailboxes within the organization.
trinity2007Author Commented:
If we have a problem with one of the CAS servers how can we re-route all outbound through one CAS server.  Eventually, we would like full redundancy for the environment, but previous attempts by previous staff have failed.  We have the DAG for the mailbox role, and I understand we can setup a CAS Array with NLB eventually.  I would like to get SP3 installed here in the next month and when doing one of the CAS servers I would like email to go out specifically through the other CAS server without interrupting mailflow while we do the SP3 update.
Ok - I seem to have been hungup on some of your terms. Emails don't route through CAS, they route through HT.. You mention HT, but the problem that you have isn't HT. HT is, by and large, probably the most trouble-free piece of Exchange.

First, you need to get the  CAS-Array record created. This is not in of it self the problem, it is simple enough to create and modify your databases with it. The real problem that you will have is updating all your Outlook clients to reflect the CAS-Array object verses a specific Exchange server.

A good read through this two-part blog will help immensely:

Note: CAS-Array object is not redundancy.

The most troubling part is getting the pre-existing Outlook clients to recognize the new CAS-Array object: has several suggestions on how to tackle this.

In a somewhat similar situation as yours, I was migrating Exchange 2010 through OS upgrades and there was no CAS-Array object in the organization. Fortunately it was a Citrix environment, so it was all online Outlook clients.

Good luck!
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trinity2007Author Commented:
I apologize I meant HT and shouldn't have used CAS in the explanation.  Thank you for the links, I will read through these.
Adam FarageSr. Enterprise ArchitectCommented:
SommerBlink basically covered what a CAS is, but you are referencing HUB transport.
Its pretty simple configuration.. you would make a single Send connector and add both CAS/HUB into it as a source server. This allows either server to use the Send connector in the event something goes down, as a send connector is an organization object and not a per server object.

As for Receive connectors you should have them identical to each other, including permissions and allowed networks. Then on the internet side (where you push emails through) list both internal addresses of both CAS / HUB. This will allow email to continue to flow if one server is down.

For example (inbound traffic, which is handled by the receive connector) you have a smarthost (filter software). You would add the IP addresses of both CAS / HUB. If you are doing a straight NAT (1:1 public to private) then you would need two public IP addresses and then two MX records at the same weight (unless your router can do some smarthost functionality and load balance the inbound load). You also have the option to load balance inbound traffic with a software / hardware load balancer over TCP 25, thus creating a single point of entry (to the virtual IP of the load balancer).

A few options for you the play out there, the receive connector being a bit more complex then the send connector.

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trinity2007Author Commented:
Gotcha...I see on the send connector both CAS/HT servers listed, so basically either server would route emails out - as you stated.  Inbound we have a couple different Receive Connectors set up and are set up the same on both CAS/HT servers.  
My next question would be if I 'removed' one of the CAS/HT server from the send connector - technically email should route out through the one CAS/HT server.  I'm asking for the purpose of bringing down one CAS server for updates/SP3 install, etc.
Future intentions would be to use a NLB, CAS Array, etc. for better redundancy.
Thank you,
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