I've been pondering (translated: trying to figure out) Exchange 2000 Server routing and wondering what consequences we'll see as we move from Exchange Server 5.5's remote procedure call (RPC) and X.400-based routing to Exchange 2000's SMTP-based routing. Will we really miss the Gateway Address Routing Table (GWART)? I don’t think so.
In Exchange Server 5.5 and earlier versions, both administration and routing occur from the standpoint of the site. If you route messages between servers within a site, Exchange 5.5 uses RPC connections. If you send messages to servers at other sites, you need some sort of routing mechanism in place—usually the Site Connector or the X.400 Connector. Both connectors rely on GWART entries to determine how a message gets to its destination.
Exchange 2000 has an entirely new routing architecture based on SMTP. SMTP is a more ubiquitous and open protocol standard (compared to X.400), so the drastic change is justified. Active Directory (AD) and DNS replace the GWART. The Exchange 5.5 Internet Mail Service (IMS) demonstrated that SMTP is a fast and effective message transfer agent (MTA). Leveraging work from the Microsoft Commercial Internet System (MCIS) product, Microsoft developed Exchange 2000 to use SMTP. And because SMTP is now part of Windows 2000 (Win2K), Exchange 2000 can leverage this technology as the new default protocol. In addition, Internet protocol (IMAP, POP3, and HTTP) clients use SMTP to submit messages for delivery. SMTP is part of Internet Information Server (IIS) and is configured as a transport stack. When you install Exchange 2000, you add the routing service and some enhancements to the base Win2K-supplied SMTP component. Instead of the Site Connector, Exchange 2000's default connector is now the SMTP connector (stronger, better, faster IMS), but the X.400-based MTA is still available and will be installed in a mixed-mode environment.
Another noticeable difference for Exchange 2000 routing will be the change in the concept of a site. Because Win2K now has sites, Exchange 2000 needed another administrative concept that wouldn't cause further confusion (I'm not sure it succeeded). Therefore, Exchange 2000 has routing groups. Win2K sites still mean the same thing as Exchange 5.5 sites. However, your message routing topology might not always perfectly overlay your Win2K sites. Routing groups consist of Exchange 2000 servers connected by continuous, reliable connections and are a subset of administrative groups (another new concept for Exchange 2000). No special connectors are required within a routing group; servers use SMTP to communicate with each other. If your entire organization has good connectivity, you might not ever need to worry about routing groups. (By default, routing groups are not visible unless you enable them in the Exchange System Manager.) Most large organizations that want some degree of management and control over message routing, however, will need to establish and maintain routing groups.
Message routing is another completely overhauled concept that you need to familiarize yourself with when planning your move to Exchange 2000. Luckily, Microsoft and other organizations are devoting significant time to this topic in the form of white papers and conference sessions. If you haven’t explored this topic yet, you might find it worthwhile to investigate. Just add it to your to-do list for Exchange 2000 deployment and migration.