Exchange Server Configuration for High Availability

Loffler_Paul used Ask the Experts™
I have a client who wants a redundant Exchange configuration but wants to limit the number of servers and Exchange licenses. I was thinking of two servers each running Exchange Enterprise with the mailbox, hub transport, and CAS roles installed on each server. I would then create a DAG for mailbox redundancy and a CAS array using two Kemp load balancers.

Is there any technical reason why I could not place all of the Exchange roles on each Exchange server and create the DAG and CAS array?
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Rajkumar DuraisamyIT Service Manager
Top Expert 2012

there is no issue in placing all the default server roles in two server and configuring them as a DAG and CAS array.

We are supporting a client in a same way. only 2 server both installed with M,C & H roles configured as DAG and CAS array and a Load balancer.

To place the FSW, they have a seperate file server.
Site Reliability Engineer
Most Valuable Expert 2011
As previously indicated, there are no issues with that configuration whatsoever. The crucial part to watch out for (and for which you have correctly accounted for) is the load balancing; with the failover cluster role installed for a DAG, Windows NLB is no longer workable for load balancing the CAS array, although that approach is no longer recommended by the Exchange product group, anyhow.

With an even-membered DAG, you do need a third voting member in the DAG to maintain quorum in the event of a failure. The Majority Node Set with File Share Witness (FSW) quorum method is the one used, so you do need another server to host that file share on. That server will have to have certain additional security settings set, and for this reason, it is preferable that you use a non-Domain Controller. If you must use a DC, there are implications for your attack surface, as you have to add certain permissions across the entire domain rather than just locally on one server.

This does, of course, assume the Active Directory infrastructure is highly available - you need a minimum of two DCs on two separate physical machines, each of which also holds the DNS Server role and is configured as a Global Catalog.

While you are at it, and depending on how "highly available" this solution needs to be, remember to consider the physical environment in which the equipment is placed. Redundant air cooling and redundant power feeds (even if separate phases split across two machine racks) can all make the difference - and no matter how much software you throw at the set up, any physical single-points-of-failure will call the shots when they break! (One of my large customers has been there, and found out the hard way - it will never happen again)

You will need Windows Server Enterprise for the Failover Clustering features, but there is no specific requirement for Exchange 2010 Enterprise, unless you intend to host more than 5 mailbox databases on your two servers. Exchange 2010 Standard also has DAG clustering support:


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