DNS Round Robin on CAS Array

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I have 1 2003 exchange server and will transition to 2 exchange 2010 servers environment.

I installed 2 Exchange 2010 servers and have implemented DAG and CAS Array. The cas array fqdn is casarray.org.com and created 2 A records pointing to the 2 exchange servers (svexc01 and svexc02) Both servers running CAS, HUB and MBX

casarray.org.com to match ip addr of svexc01

casarray.org.com to match ip addr of svexc02

I've test HA on DAG and CAS array and works fine.

I would like to know if DNS round robin is supported with 2 Exchange server 2010 environment running DAG and CAS Array. WLB is NOT installed on these server



yes DNS round robin is supported well it is simply not related to exchange, you are practically relying on DNS to send users randomly to server 1 or 2

what you should know however is that if one of the servers goes down you will need to delete in the DNS the entry corresponding to its IP or 1/2 users will stay disconnected

a small tip lower the TTL for these record

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Neil Hobson

Supported? Yes. Recommended? Not really. :)

You have to consider the fact that it's more manual, client caching causing slow failovers and the fact that affinity is broken for things like OWA and ECP.

Principal Consultant, Silversands | www.silversands.co.uk | www.msexchange.org/neil_hobson

Sembee [MVP]

Round Robin doesn't really help with a CAS Array. If one of the servers goes down, then the DNS will continue to send clients to that host because has no awareness that the server is down. I don't see how you say that it works fine. You really should be using at least WNLB so that there is some server availability awareness, or you are going to need to have a process to remove the down host from DNS - which could take some time depending on when the failure occurs.

Is there any reason why you aren't using at least WNLB?


Simon Butler, Exchange MVP
Blog | Exchange Resources


Hi, Simon

So I could use a 3rd server with WNLB and load balance the CAS array?

I was reading somewhere that you cant install WNLB on any DAG members seeing I only have 2 servers running DAG and CAS array.



You are correct that you cannot install WNLB on a DAG server since it cannot coexist with Windows Failover Clustering. As stated above, DNS roundrobin is not recommended for a CAS Array. To do this right and provide true HA for all roles, you need to split the CAS role off of your DAG/MB servers. So you need two new servers, move the CAS role onto those new servers with WNLB installed and configured. If you want to stick with the 2 Exch server solution, the proper way is to load balance with a hardware load balancer. Here is a link that explains the scenaios:


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