Now that Exchange 2000 Server Release Candidate 1 (RC1) has hit the streets (I know, not everyone’s street), are we clear about how an Exchange 2000 server will work in an Exchange 5.5 environment? Because the Windows 2000 (Win2K) Active Directory (AD) replaces the Exchange 5.5 directory in Exchange 2000, Microsoft had to devise a method for these two environments to coexist. Two key services allow this coexistence: the Active Directory Connector (ADC) and Site Replication Services (SRS).
The ADC service runs on a Win2K server and allows synchronization of the Win2K AD and the Exchange 5.5 directory. This synchronization occurs through connection agreements and uses Lightweight Directory Access Protocol (LDAP). The ADC provides for mixed-mode (both Exchange 5.5 and Exchange 2000) coexistence. However, here are a couple of caveats. First, you can move mailboxes only within Exchange 2000 administration groups, rather than anywhere within the AD environment, which is the case in a native-mode (all Win2K or Exchange 2000) scenario. Another minor irritation is that Exchange 2000 servers must have a site service account configured so that the Message Transfer Agent (MTA) can route mail to the legacy Exchange 5.5 environment.
The SRS spoofs its way into the Exchange 5.5 directory family by pretending it's another Exchange 5.5 server running the directory service. The first Exchange 2000 server that you install in an Exchange 5.5 site automatically has the SRS installed. Also, any bridgehead servers that you upgrade also get the SRS so that the replication for bridgeheads in other sites functions properly. SRS is really just a lean version of the Exchange 5.5 directory service complete with the dir.edb file.
The ADC and SRS work together to manage the coexistence of Exchange 5.5 servers and Exchange 2000 servers within and across sites. For example, the SRS uses the ADC to create and manage connection agreements within the site. Keep in mind that the SRS uses mail-based replication (i.e., Exchange 5.5), and the ADC uses LDAP-based replication. The SRS is also savvy enough to know whether replication activity needs to occur using Exchange 5.5 replication methods (object-based) or AD replication methods (attribute-based) and will select the best method.
As you plan your testing, pilot, and rollout of Exchange 2000, you'll need to understand how the ADC and SRS work together to provide interoperability for Exchange 5.5 and Exchange 2000 environments. I suggest you install a test environment and experiment with both ADC and SRS. Microsoft has done the work; we just have to figure it out.