Exchange Server has multiple roles, namely the Edge Transport, Client Access, Hub Transport, Mailbox server and Unified Messaging roles. For a large enterprise, the presence of these server roles makes it possible to design deployment scenarios for greater redundancy and load-balancing that can span offices or even geographical regions.
Organizations that do not require the Edge Transport and the Unified Messaging role will find themselves questioning whether they should deploy the Client Access, Hub Transport Server and Mailbox server roles on the same machine. I address these below:
Multi-role server for simplicity
To be clear, a multi-role Exchange deployment involves collocating the Client Access, Hub Transport and Mailbox roles into the same physical server. The key advantage of taking this route has to do with the simplified initial configuration as well as the ease of subsequent administration. While it can be argued that deploying the Client Access and Hub Transport role into separate servers allows for greater scalability down the road, excessive processor overhead is a sheer waste of computing capability and RAM.
Another argument in favor of a multi-role server has to do with how simplicity helps keep things manageable, while the inherent complexity of deploying the various roles on separate physical servers increases risk. This may range from something as mundane as a LAN cable coming loose or improperly maintained documentation causing a new Exchange administrator to misconfigure a crucial IP address.
Instead of working with suppositions that a multiple-server Exchange deployment allows for greater future expansion, Microsoft has provided the free Exchange 2010 Mailbox Role Requirements Calculator to help administrators work out the RAM and processor utilization for a given set of hardware and various variables such as number of users and size of mailboxes.
Exchange administrators can use this tool to validate a new server, as well as to compare the merits of an Exchange deployment using multiple servers versus a single multi-role server. Remember to factor the cost savings of using fewer servers (over a multi-server deployment) into consideration.
Do you need high availability
As Exchange 2007 does not support Client Access or Hub Transport roles on clustered server, there is no way to have a high availability deployment unless the roles are segregated into separate physical servers. The use of a multi-role server may not be suitable in such a scenario. The introduction of Database Availability Groups (DAG) in Exchange 2010 removed that limitation though, and as such eliminates an argument against multi-role servers.
Microsoft does offer a recommended configuration for combining a Client Access and Hub Transport role into a single server. According to Microsoft, a minimum of two processor cores are required, with up to a recommended maximum of 12 processor cores. Minimum RAM should be 4GB, with a recommended maximum of 2GB per core.
Multi-role Exchange deployment should be adequate for the majority of businesses out there. As such, it makes sense to start with the assumption of a multi-role server for the simplest possible solution, and move on to more complex deployments only when there is a need to.
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