DNS Round Robin for Exchnage Server 2016 - CAS

Question

We have two node exchange server as follows

abc-exch01.test.com 10.10.100.2

abc-exch02.test.com 10.10.100.3

Created two 'A' records as follows

A - mail.test.com 10.10.100.3

A - mail.test.com 10.10.100.3

TTL 00:00:00 for both record

Expected behavior: when abc-exch01.test.com is down, the outlook should restore connectivity from abc-exch02.test.com as we have DNS round-robin in place. In my case, I have to close and reopen Outlook to restore the connection. Is there any way to get it fixed? Am I missing something here

Answer 1

Hi @Shakkeer Chalakkandi ,

As far as I know, normally we don't have to restart Outlook to restore the connection with DNS round-robin in place. For more information, you may refer to the Outlook client experience mentioned in the following two blogs:

• Exchange Server 2013 Client Access Server High Availability
  "Without the aid of a load balancer the Outlook clients takes about 20 seconds to time out and then re-establish connectivity to the other IP address..."
• Exchange 2016 DNS Round Robin Load Balancing
  "From the point of view of Outlook, nothing was noticed. Outlook was responsive throughout the failover and in fact did not disconnect."
  (Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.)

That being said, I'd like to suggest checking the configuration steps described in the links above to see if anything has been missed. Also please try to install all available updates for the Outlook clients and see if there would be any improvement.

Besides, as noted by Robert and the official article he shared, while we can use DNS round robin, a true load balancer is always recommended.

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Answer 2

Round Robin isn't redundancy but load balancing. So around 50% of traffic will go to server 1 and 50% of traffic will go to server 2. That means that when 1 server is down, 50% of the traffic will be lost and it is up to the client to try again. If the client is lucky, it will get the server that it up with its next request.

You'll have to look in your load balancer options to implement health checks. For instance to check whether OWA is up, you can use the /owa/healthcheck.htm request. If the load balancer doesn't get a 200 responds, then it should mark that server as offline and remove it from the Round Robin pool until it is up again.

Additionally see; Load balancing in Exchange Server