What happened to CAS array in Exchange Server 2013?
How CAS load balancing in Exchange Server 2013?
How CAS high availability works in Exchange Server 2013?
How outlook clients connects to Exchange Server 2013?
I personally had all these questions in my mind and got it cleared after gone through some good articles. So thought of share that will you all.
What is CAS array?
Introduced with Exchange 2010, CAS arrays provided a method to group a set of CAS servers together in such a way that they could be addressed as a single entity (and had a single IP address and FQDN). Individual servers could join and leave the array over time and the array would keep functioning as long as a single server was active. Its not a load balance mechanism.
CAS role in Exchange Server 2013
The Exchange 2013 version of the CAS is a much simpler beast as it is purely an authentication (are you authorized to connect to Exchange?) and proxy/redirect (where do you need to go to find your mailbox?) server. No processing is performed of mailbox data by the CAS; all it does is to send on client requests to connect to the mailbox server that hosts the currently active copy of their mailboxes via HTTPS (no MAPI RPCs).
How outlook connects to 2013 CAS
The RPC Client Access namespace, which was introduced in Exchange 2010 to handle the concept of RpcClientAccessServer described above is no more and Exchange no longer uses FQDNs of CAS servers or arrays to locate user mailboxes. Instead, CAS uses the unique GUID assigned to the mailbox. When an incoming client connection must be processed, CAS looks up Active Directory to find details of the mailbox via its GUID (including the database that hosts the mailbox) and Active Manager will tell CAS what mailbox server currently hosts the active copy of the database.
There are many reasons why Microsoft has taken this route for Exchange 2013, but perhaps the most basic is to uncouple the functionality of CAS from mailbox servers so that both can function independently of each other in terms of geographic location (a CAS in one datacenter can service requests going to a mailbox server in another) and software versions (you won’t have to update CAS and mailbox servers with the same software in future).
CAS high availability in 2013
As mentioned earlier outlook clients will access their mailboxes using MAPI over HTTP/HTTPS method.
When we look the outlook anywhere configuration of each CAS server, the InterHostname property was configured with its host name by default. So when the outlook connects with that CAS and if that is offline, outlook will take time to switch the connection to other available CAS server through AutoDiscover.
To overcome this we can configure single namespace instead of unique server FQDN. we also need to make sure that the DNS record created for that name space and points to the CAS servers. In this way we can use DNS round robin for load balancing till we get HLB.
without HLB the outlook takes around 20 secs to time out and then re-establish connectivity to the other IP address that the name space resolves to.