Exchange Server 2003 Message Routing Topology


Topic Last Modified: 2005-10-23

The following figure illustrates an Exchange Server 2003 organization routing topology with multiple internal routing groups connected through routing group connectors and a connector that connects the Exchange organization to an external messaging system. In this topology, routing group A represents a central routing hub. All messages to remote routing groups (routing groups B and C) and the non-Exchange messaging system are routed through routing group A. Multiple bridgehead servers are deployed in routing group A to provide redundant message transfer paths.

An Exchange Server 2003 message routing topology


The message routing topology shown in the figure above includes the following key components:

  • Routing groups   These are logical collections of servers, used to control mail flow and public folder referrals. Routing groups share one or more physical connections. In a routing group, all Exchange servers communicate and transfer messages directly to one another, using Simple Mail Transfer Protocol (SMTP) virtual servers. In a native mode organization, routing groups can include servers from different administrative groups. However, in a mixed mode organization, routing groups cannot span multiple administrative groups, due to backward compatibility with Exchange Server 5.5. This is because the routing topology in Exchange 5.5 is defined by sites, and sites provide the functionality of both the administrative group and the routing group.

    SMTP works well over any type of TCP/IP connection. Therefore, a routing group does not necessarily define regions on a computer network with high network bandwidth. Routing groups can span slow network connections, if the connection is permanent and reliable. For example, if all servers in Figure 5.1 can communicate directly through TCP/IP, you might consolidate all Exchange servers into one routing group, thus eliminating four of the five bridgehead servers and all routing group connectors. This significantly streamlines the routing group topology. In Figure 5.1, the bridgehead server running a connector to the non-Exchange messaging system must remain connected to the external messaging system. Note, however, that all servers in a routing group periodically poll the routing group master. Gaining control over server-to-server communication might require you to implement multiple routing groups, which might be especially important if communication over wide area network (WAN) connections generates costs. For more information about the design and configuration of routing group topologies, see Exchange Server 2003 Transport and Routing Guide (
  • Routing group connectors   A routing group connector enables message transfer between two routing groups. The following Exchange connectors can be used to establish message transfer paths between routing groups:

    • Routing group connectors   A routing group connector provides a one-way connection path in which messages are routed from servers in one routing group to servers in another routing group. Routing group connectors use Simple Mail Transfer Protocol (SMTP) to communicate with servers in connected routing groups. Routing group connectors provide the best connection between routing groups.

      The Routing Group Connector (note the capitalization) is a specific type of connector that can only be used to connect routing groups with each other. Other connectors that can connect routing groups are the SMTP connector and X.400 connector. However, these connectors can also be used to connect an Exchange organization to an external messaging system through SMTP or X.400. To avoid confusion, this guide uses "Routing Group Connector" to refer to the specific connector that can only be used between routing groups and "routing group connector" to refer to all types of connectors that can be used to connect routing groups.
    • SMTP connector   An SMTP connector can be used to connect routing groups, but this is not recommended. SMTP connectors are designed for external message delivery. SMTP connectors define specific paths for e-mail messages that are destined for the Internet or an external destination, such as a non-Exchange messaging system.

    • X.400 connectors   Although you can use X.400 connectors to connect routing groups, X.400 connectors are designed to connect servers running Exchange with other X.400 systems or to servers running Exchange Server 5.5 outside an Exchange organization. A server running Exchange Server 2003 can then send messages over this connector using the X.400 protocol.

      X.400 connectors are available only in Exchange Server 2003 Enterprise Edition.
  • Connectors to non-Exchange messaging systems   These connectors support message transfer and directory synchronization between Exchange and non-Exchange messaging systems. When appropriate connectors are implemented, the user experience is similar on both messaging systems and the transfer of messages and other information between the Exchange and non-Exchange messaging system is transparent to the user. However, some message properties might be lost during message conversion from an Exchange format to a non-Exchange format, or vice versa.

  • Mailbox servers   A mailbox server is an Exchange server configured to host mailboxes. Users can access their mailboxes through a variety of clients, such as Microsoft Office Outlook, Microsoft Office Outlook Web Access, Microsoft Office Outlook Mobile Access, Post Office Protocol version 3 (POP3)-based clients, and Internet Message Access Protocol version 4rev1 (IMAP4)-based clients. In the Exchange Server 2003 routing topology, mailbox servers are typical sources and destinations of e-mail messages.

  • Bridgehead servers   A bridgehead server is a connection point that performs message transfer from one routing group to another routing group, or to an external messaging system. In large organizations, bridgehead servers are often separated from mailbox servers to avoid performance bottlenecks. Mailbox servers create bottlenecks due to increased processing requirements during peak messaging hours. Bridgehead servers are identified as local or remote bridgehead servers, as follows:

    • Local bridgehead servers   This server hosts a connector and uses it to transfer messages. When you create a connector, you designate at least one Exchange server as a bridgehead server. You can also designate multiple bridgehead servers for load balancing, performance, and redundancy. For example, the default option for routing group connectors is Any local server can send mail over this connector. In this case, all Exchange servers in the local routing group can use the connector to transfer messages.

  • Remote bridgehead servers   The remote bridgehead server, specified in a connector configuration, is the server (in the connected routing group or non-Exchange messaging system) that receives all messages transferred over a connector. Routing Group Connector can have multiple remote bridgehead servers (that is, remote virtual SMTP servers). SMTP connector and X.400 connector, however, can have only one remote bridgehead server per connector instance. Remote bridgeheads are also named target bridgeheads.


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