Question: How do I cluster Exchange Server 2010?

At the design stage of Exchange Server 2010 projects I’m often asked about clustering. For Exchange Server 2010 there are basically two reasons to “cluster” different server roles.

  1. For high availability
  2. For load balancing

Clustering Exchange 2010 for High Availability

High availability for Exchange Server 2010 can be achieved by “clustering” two different server roles.  This can be for high availability within a single site or across multiple sites (site resilience).

Client Access servers can be made highly available by deploying a Client Access server array.  A CAS array can consist of two or more Client Access servers configured as a Windows Network Load Balancing cluster.  In the event that one server in the cluster fails the others can continue to accept client connections.  This is important for Exchange 2010 environments because the Client Access server accepts all client communications, including Outlook MAPI connections.

Mailbox servers can be made highly available by deploying a Database Availability Group.  A DAG is up to 16 Mailbox servers configured to replicate mailbox databases between them.  If one database or server fails another replica is brought online to continue serving clients.

Database Availability Groups can’t be used to provide high availability of public folders.  Public folder replicas are used for that purpose.

Clustering Exchange 2010 for Load Balancing

Load balancing for Exchange Server 2010 can be achieved by deploying a CAS array.  Just as the CAS array provides high availability the Windows NLB cluster also load balances the traffic across the NLB members.

However a Database Availability Group, while it can be used to spread the mailbox database load somewhat, doesn’t provide any dynamic load balancing.

Other Server Roles

Other Exchange 2010 server roles can be made highly available or handle load balanced traffic but they do this by deploying them in configurations that don’t involve clustering, for example:

  • Multiple Hub Transport servers will automatically load balance internal email traffic
  • Multiple Edge Transport servers can be load balanced using a hardware load balancer or multiple MX entries in DNS
  • Multiple Unified Messaging servers can load balance traffic if the PBX system is configured to use multiple UM servers

About the Author

Paul Cunningham

Paul is a former Microsoft MVP for Office Apps and Services. He works as a consultant, writer, and trainer specializing in Office 365 and Exchange Server. Paul no longer writes for

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