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Exchange servers periodically perform what's known as topology discovery. Every 15 minutes, Exchange attempts to "relearn" the contents of Active Directory; it looks for new or different AD domain controllers, tests them, and then talks to the DC that holds the roles it needs to obtain specific information (such as the global catalog server, for instance).
Topology discovery events appear in the application event log, under Event ID 2081, for a process named MAD.EXE. If one of these topology discoveries fails, MAD.EXE will try to poll every five minutes until they work again.
There are a number of problems that can cause topology discoveries to fail. Most common is if the needed DC roles are not assigned correctly (for example, this can happen in a broken migration scenario). Another common reason is if the network itself is malfunctioning -- a bad routing table or incorrectly configured internal DNS.
Yet another circumstance, often overlooked but much rarer, is if the clocks on the Exchange machine and the other DCs are heavily out of sync. Generally, server clocks should be within one minute of agreement with each other; if they're not, something is wrong.
Most people may have encountered this problem on a desktop machine at one time or another -- no matter how many times the clock is reset, it either speeds up or falls behind, sometimes by minutes in the course of a day. If the server's clock is constantly falling out of sync, the on-board CMOS battery may be bad or (even worse) the motherboard itself may be defective or in need of a BIOS upgrade.
A possible workaround for the problem involves using a third-party program to synchronize the server with an external clock once a day, but failing hardware needs to be replaced sooner rather than later.
It's also been reported that a badly-grounded outlet or line noise can cause clock drift, since the waveform that comes out of the wall socket is used to derive the clock frequency in the first place. (If the server isn't using a UPS or some kind of power conditioning system, install one immediately.)
About the author: Serdar Yegulalp is editor of the Windows Power Users Newsletter and a regular contributor to SearchExchange.com.
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Related information from SearchExchange.com:
Topics Library: Exchange replication and synchronization
Topics Library: Exchange hardware management
