Exchange has an internal process called MSExchangeDSAccess Topology Discovery, which runs every 15 minutes and rediscovers the contents of Active Directory (AD). For each DC it discovers, the process runs a series of tests against the DC and selects one or more DCs that contain the information that Exchange requires (i.e, a configuration DC, a domain DC, and a Global Catalog–GC).
You can check the status of the Topology Discovery process by viewing the Application event log. The chosen servers are logged in event ID 2081, as the following example shows:
Event Type: Information
Event Source: MSExchangeDSAccess
Event Category: Topology
Event ID: 2081
Date: 8/3/2004
Time: 7:17:25 PM
User: N/A
Computer: Raptor
Description:
Process MAD.EXE (PID=1708). DSAccess will use the servers from the
following list:
Domain Controllers:
Raptor.alibutt.com
Global Catalogs:
Raptor.alibutt.com
The Configuration Domain Controller is set to Raptor.alibutt.com
For more information, click
http://www.microsoft.com/contentredirect.asp.
If selected GCs are unavailable, the process will attempt to contact them every 5 minutes until the next topology discovery. If the GC is unavailable when the Topology Discovery process is run, the process removes it from the list of DCs it can use. When the GC comes back online, the Topology Discovery process detects it during the next check
