Within Exchange 2000 and Exchange 2003, the Directory Service Access (DSAccess) component serves to control how other Exchange components talk to your Active Directory. The answer to your question isn't based on whether you're running against a Windows 2000 or Windows 2003 global catalog server; the process is essentially version agnostic. DSAccess discovers your Global Catalog Servers by ensuring some prerequisites are met, and then through a round-robin process and suitability weightings that I'll explain.
First, DSAccess makes a random connection to the first domain controller it finds in the domain it's running in. DSAccess requests a list of all global catalog servers in the domain, and checks the following:
- Is the server in a domain in which DomainPrep has been run? Note that the server does not have to be in the same domain as the Exchange server.
- Is the server in the same active directory site as the Exchange server?
Once this has been accomplished, DSAccess confirms the following:
- Is the server listening on port 3268?
- Can the server's security descriptor be read in the Configuration Naming Context of Active Directory?
- Is the server in synch with the rest of the domain?
- What is the weighting of the server's associated Service (SRV) Resource Record in DNS? High weightings will be given higher priority for client requests than lower weightings.
DSAccess then maintains a list of up to 10 global catalog servers that meet all these criteria. Finally, DSAccess ranks these servers by assessing two further factors--response time and number of outstanding requests. This permits efficient load distribution across these global catalog servers.
So in summary, Exchange won't prefer either Windows 2000 or Windows 2003 global catalog servers -- rather than version of Active Directory, Exchange uses other criteria to make this decision.
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