Exchange Server 2007 (RTM and SP1) Hub Transport servers are resilient by default. This means that if you install more than one Hub Transport server in an Active Directory (AD) site and one fails, the other(s) will continue to accept connections. In addition, when you have more than one Hub Transport server deployed in an AD site, connections will be load balanced automatically between the Hub Transport servers. There is only one exception to this rule, and that is when a Hub Transport server role is installed on a server also holding the Mailbox server role. In this specific scenario the Hub Transport server local to the Mailbox server will always be preferred over other Hub Transport servers in the AD site.
The resilient behavior that by default is built into the Hub Transport server works just fine for many organizations, but there are situations where you as an Exchange messaging administrator or consultant, for example, have a line of business (LOB) application, Microsoft Office SharePoint Server 2007 (MOSS 2007) portal, or perhaps a System Center Operations Manager 2007 (SCOM 2007) service management solution which, in order to submit messages to an Exchange organization must use a SMTP relay as these applications cannot log on to a mailbox using MAPI and then send the messages as that mailbox.
So what are your options in these types of scenarios? Well, with the Exchange Server 2007 RTM version, it was not supported to load-balance Hub Transport servers using Windows Network Load Balancing (WNLB) technology. This meant that if you had an application which needed to use your Exchange 2007 messaging environment to relay messages, you either had to specify two SMTP servers in the application (often not possible), use DNS round robin (not as intelligent as NLB) or MX records (not viable if the application only allows you to specify a smart host).
As mentioned load balancing Hub Transport servers in Exchange 2007 RTM was not a supported scenario, but now that Exchange Server 2007 Service Pack 1 (SP1) has been released guess what? Yes you’re right, it’s supported to load balance Hub Transport servers using a hardware load balancer or standard WNLB technology.
Although it’s now supported to configure Hub Transport servers in an NLB, please note that it isn’t supported to load balance connections between Hub Transport servers on your internal corporate production network using this method. You should only use NLB to load balance inbound SMTP connections from applications (such as LOB application, MOSS, and SCOM 2007 etc.) and other non-Exchange sources as well as client connections (in order to send messages, POP & IMAP clients uses the default client receive connector on a Hub Transport server).
In this article series, I’ll show you step by step how you configure Hub Transport servers in a NLB using WNLB. We’ll also verify things works as expected as well as take a look at how fault tolerance and load balancing works for outbound message flow (messages leaving the Exchange organization).
Environment used in this article
If you want to deploy and test the solution explained in this article series in your own environment (you should of course always start out in your lab environment), you will need the following:
- 1 x Windows 2003 Server SP2 Domain Controller and Global Catalog (DC01)
- 1 x Windows 2003 Server SP2 with Exchange 2007 SP1 Mailbox and Client Access Server role installed (Mailbox01)
- 2 x Windows 2003 Server SP2 with Exchange 2007 SP1 Hub Transport Server role installed (HT01 & HT02)
Note:
Because the NLB cluster configured in this article series is configured in unicast mode, you need to install two network interface cards (NICs) in each Hub Transport server.