How can I throttle network utilization between two Exchange Server systems?

The answer depends on what kind of traffic you want to control. Exchange has a set of mechanisms for limiting certain kinds of network traffic. Windows Server 2003 and Windows 2000 Server add controls for Active Directory (AD) replication. Win2K Server and later can also use TCP/IP Quality of Service (QoS) extensions to let you control, on a per-protocol basis, the amount of bandwidth used for network communications. Each of these methods has pros and cons.

You can use Exchange mechanisms to schedule SMTP connectors (which you can use as a primitive throttling tool) and to have Exchange route all messages bigger than a certain size to a separate queue that has a separate schedule. You can set a schedule for public folder replication, and you can specify the size of the replication messages that Exchange generates. However, you can't throttle traffic between servers in the same routing group, nor can you control the rate of network utilization that occurs between Outlook clients and your Exchange servers.

You can control AD replication by setting up site links and site-link bridges, changing the frequency of replication, and changing the replication schedule. None of these actions directly involve Exchange, but in some organizations, the volume of directory changes (as employees change their names, move between offices and departments, or get new cell phone numbers) can mount quickly and justifies AD replication control.

QoS is a network-layer protocol and thus is moderately difficult to implement. After it's in place, however, QoS lets your network routers prioritize traffic so that important or time-sensitive data gets first claim on bandwidth. For example, I use QoS on my Vonage Voice over IP (VoIP) phone to make sure that voice packets aren't dropped during heavy network activity. You can use QoS to specify bandwidth allocations according to protocol (e.g., you can specify that SMTP traffic always gets at least 15 percent of your bandwidth), but it can't help you control bandwidth on a more granular level.