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
outlook.Telnet25.org 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 outlook.Telnet25.org, 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 outlook.Telnet25.org, I want to connect to my mailbox, the DNS server will go to its database and see there are 4 host records for outlook.Telnet25.org and will provide you the first one first now you get the IP address for outlook.Telnet25.org=10.10.10.101 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 outlook.Telnet25.org 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 outlook.Telnet25.org=10.10.10.101=CAS 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 outlook.telnet25.org.
You can do the math if this is a good configuration or not, or you could purchase HLB and point outlook.Telnet25.org 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.