Thursday, January 5, 2012



How does round robin work with Exchange 2010 CAS Array deployment was a question I come a cross and wanted to clarify few things.


Now Lets say you have 4 CAS servers names

Lets say you set your CAS Array and modified RPCClientAccessServer attribute to be and all your databases tagged with same attribute.

Now each time your MAPI client want to connect to their respected mailboxes, they will want to locate this name , which is, then your workstation will do DNS query to configured DNS servers on its TCP/IP properties and will ask , hey get me the IP address for, I want to connect to my mailbox, the DNS server will go to its database and see there are 4 host records for and will provide you the first one first now you get the IP address for which is CAS01 and CAS01 will let you connect to you your respected database and you start seeing your mails within outlook.

Now imagine 3 other users will do same and each time they go to DNS DNS will perform round robin and hand out IP address for in fair order. So at the end of the day if you have 100 MAPI users your CAS load will be something like this


Now all you have done is, to leverage the CAS load , if any of your servers goes down let’s see what happens ?

If CAS01 goes down , now 75 Users will get ***(Disconnected)*** , You opened DNS console in your DC and delete the A record Server IP, until the Client DNS catch expires, the client will keep trying to establish connection to its cached DNS information thus outlook continue to go bananas.

Even if you lower the DNS TTL record for


You can do the math if this is a good configuration or not, or you could purchase HLB and point to HLB ( Hardware load balancer) IP and let HLB to handle the MAPI load and deal with situations like above without making your clients get disconnected. You can look it up on the net to see what HLB solutions available, some of them are very reasonable to be honest.


Oz Casey, Dedeal ( MVP north America)
MCSE 2003, M+, S+, MCDST
Security+, Project +, Server + (Blog) (Blog)


iphone roulette said...

Very well done this is great... keep going...

Hoegi said...

Very well done, easy to understand.
Just some info, instead to buy a HLB we set up our CAS-Servers with Windows NLB and that works very good so far.


Mr.Macchiato said...


DNS Round Robin is used for the purpose of having a minimum number of Exchange servers (2) with all roles installed and running a DAG.

DNS Round Robin is the only way to go other than hardware load balancer if you want to have a DAG and CAS Array running on 2 servers.

anand said...

what ps command did u use to show the number of owa and rpc connections on each cas

Anonymous said...

I would love to know this PS command as well to show current connections

Anonymous said...