By default, the server running ADC attempts to communicate with
the Exchange directory bridgehead server on port 389 (379 for the
Site Replication Service), which is the most commonly used
Lightweight Directory Access Protocol (LDAP)
port. Under some circumstances, you must configure the Connection
Agreement for another port; for example, if Exchange 5.5 Server is
deployed on a Windows 2000 domain controller. Active Directory
components always start before the Exchange directory; therefore,
the operating system locks port 389. The Exchange directory still
starts, but LDAP communications are not possible. To work around
this, use the Exchange 5.5 Administrator program to reconfigure the
listening port for LDAP (usually, port 390 is a good choice), and
then specify the port number on the Connection Agreement to
match.
The Connection Agreement does not have any LDAP port
configuration for Active Directory because it always attempts to
communicate with Active Directory on port 3268. This is the
reserved port for global
catalog servers, which implicitly means that all
Connection Agreements must be made into global catalog servers and
not standard domain controllers.
Most communications that ADC establishes are based on LDAP;
however, ADC uses a few synchronous remote
procedure calls (RPCs) when you use Active Directory to
create a User object but the mailbox is specified to exist on an
Exchange 5.5 server. When the next replication cycle occurs, an
instance of the mailbox object is created and a call is made to
create new proxy addresses; for example, Simple Mail Transfer
Protocol (SMTP), X.400, or Microsoft Mail addresses. The proxy
address generator can be called only through RPC. This can be a
consideration if a firewall exists between the server running ADC
and the Exchange 5.5 bridgehead server.