If you were to search online for high availability for Exchange Server 2007, you most likely would receive several results related to clustering database servers. The emphasis
When an Edge Transport server is in use, every message that comes into your organization passes through it. If that server were to fail or become overwhelmed, then inbound email would cease to flow. Using multiple, load-balanced Edge Transport servers can alleviate this; however, implementing them can be somewhat difficult.
Hub to Edge Transport server synchronization
The Hub Transport server synchronizes Active Directory data to the Edge Transport server's Active Directory Application Mode (ADAM) partition. For this synchronization to succeed, the Hub Transport server must have exclusive access to the ADAM partition to establish a lock with the Edge Transport server's ADAM partition. If the lock fails, then synchronization will also fail.
Synchronization failure tends to complicate things if your goal is to create Edge Transport server redundancy, because the Hub Transport server can only synchronize with one Edge Transport server at a time. Additionally, each Edge Transport server needs to receive identical Active Directory data.
It is possible to subscribe a single Hub Transport server to multiple Edge Transport servers. If the Hub Transport server can't establish a lock with an Edge Transport server, it then assumes that the server is synchronizing with another Edge Transport server. The two servers will retry at the next scheduled synchronization.
While this architecture works, it's inefficient and puts the Hub Transport server at risk of becoming a single point of failure. An alternative is to put a Hub Transport server into each site, and then establish a 1:1 ratio of Hub Transport to Edge Transport servers. In such a configuration, each Hub Transport server is synchronized to only one Edge Transport server.
Load balancing with multiple Edge Transport servers
The question is: If the Edge Transport server represents the entry point of email flowing into your organization, how do you share the workload equally over multiple Edge Transport servers?
I recommend connecting each Edge Transport server to a separate Internet connection through different ISPs. Servicing multiple Edge Transport servers with a single ISP creates a potential single point of failure. Using multiple ISPs ensures that if you will still have Internet access, even if there is an ISP problem.
Whether you use one or several Internet connections, you still must route your email through Edge Transport servers. An MX record specifies the IP address of the organization's mail server. If you have an Edge Transport server in place, then the MX record should point to that server's IP address.
If you have multiple Edge Transport servers, you must do things differently. A DNS server can contain multiple MX records for a domain. You can create a separate MX record for each Edge Transport server. By instructing the DNS server to use round-robin processing, you can load balance by spreading inbound email somewhat evenly across your Edge Transport servers.
About the author: Brien M. Posey, MCSE, is a Microsoft Most Valuable Professional for his work with Exchange Server, and has previously received Microsoft's MVP award for Windows Server and Internet Information Server (IIS). Brien has served as CIO for a nationwide chain of hospitals and was once responsible for the Department of Information Management at Fort Knox. As a freelance technical writer, Brien has written for Microsoft, TechTarget, CNET, ZDNet, MSD2D, Relevant Technologies and other technology companies. You can visit Brien's personal Web site at www.brienposey.com.
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This was first published in December 2007