Hi,

I'm in a process of upgrading a single Exchange 2010 server to multiple Exchange 2013 servers.

Here's what I have:

Site A :
- Existing Exchange 2010 server
- New Exchange 2013 CAS server
- New Exchange 2013 Mailbox server

Site B:
- New Exchange 2013 CAS server
- New Exchange 2013 Mailbox server

the WAN link between the 2 sites is >100Mbps

I did create a DAG for the 2 Mailbox servers and everything is working fine.

However, it is the CAS part that I'm not sure of. How can I configure some high-availability for the CAS part?  What I would like is that clients in site A always connect to CAS in site A and clients in site B always connect in site B UNLESS one of the CAS fails. Note that I have NO load balancers.

From my understanding, I would have 2 options:

1) Do DNS round robin with the 2 CAS but I dont like that option because half of the clients in site A will connect to the CAS in site B and half of the clients in site B will connect to CAS in site A. This is not optimal.

2) Install the CAS role on the Mailbox servers. Configure Windows Load Balancing (one in site A containing both the CAS and Mailbox/CAS servers and another one in site B for CAS and Mailbox/CAS in site B). Configure the WLB in both sites so that the CAS server receives 99% of the client requests. This way, the CAS role on the Mailbox server would only be used when the CAS server fails. This way, I believe that if a site fails (let's say Site A fails), I would need to change the DNS host record  for the CAS VIP in site A so that it now point to the CAS VIP in site B.

Are these my only 2 options? Do you have any other recommendation?

Thanks

You really have no high availability in site so a load balancer doesn't really make sense to me (nor does separating the CAS role).  I would recommend going with multirole servers spread across both datacenters.  Then you can have  a different namespace per datacenter and if anything happens to either server you can move activate the database copy in the other site.  

Thanks for your input. However, different namespace per Datacenter would not give me site resiliency right?

Hi chucky,

Thank you for your question.

Option1:

We could use DNS RR to configure Exchange CAS high availability. But Internal outlook client connect to Exchange CAS server by SCP which is in AD, we could not control that client in site A will connect to Exchange server in site A and client in site B will connect to Exchange server in site B.

Option2:

If you install CAS on mailbox server and configure NLB, the DAG will not work. Because WNLB is incompatible with Windows failover clustering. If you're using an Exchange 2013 DAG and you want to use WNLB, you need to have the Client Access server role and the Mailbox server role running on separate servers.

We suggest you use HLB for CAS high availability. The reason could be referred by the following link:

https://technet.microsoft.com/en-us/library/jj898588(v=exchg.150).aspx

If there are any questions regarding this issue, please be free to let me know.

Best Regard,

Jim