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Understanding Routing Groups


Topic Last Modified: 2005-05-13

A routing group is a logical collection of servers used to control mail flow and public folder referrals. In a routing group, all servers communicate and transfer messages directly to one another.

In a routing group, all servers communicate and transfer messages directly to one another, as follows:

  1. A user in your Exchange organization uses a mail client to send mail to another user.

  2. Using SMTP, the sender's client submits this mail to the SMTP virtual server on the Exchange server on which the client's mailbox resides.

  3. The Exchange server looks up the recipient of the mail message to determine which server the recipient's mailbox resides on.

  4. One of two things occurs:

    • If the recipient's mailbox is on the same Exchange server, Exchange delivers the message to the recipient's mailbox.

    • If the recipient's mailbox is on another Exchange server, the first Exchange server sends the message to the recipient's home mailbox server, and it is the recipient's home mailbox server that delivers the message to the recipient's mailbox.

Although all servers communicate with each other directly in a routing group, this is not the case when a server in one routing group must communicate with a server in another routing group. To allow servers to communicate with servers in other routing groups, you must create a routing group connector. Although you can use an X.400 connector or an SMTP connector to connect routing groups, the routing group connector is specifically designed for this purpose and is the preferred method of connecting routing groups.

By default, all servers in a routing group can send mail over the routing group connector. Servers that can send mail over a routing group connector are bridgehead servers. These bridgehead servers are each a combination of an SMTP virtual server and an Exchange server responsible for delivering all messages through a connector.

When creating a routing group connector, you have the option of keeping all the servers as bridgehead servers for that connector or of specifying that only a selected set of servers act as bridgehead servers for that connector. The following table compares the advantages of each approach.

Number of bridgehead servers in a routing group

Number of bridgehead servers Advantages

All servers in a routing group

  • Provides more efficient message flow because all the servers in the routing group can directly deliver messages to other routing groups.

  • Takes advantage of those configurations where all the servers in a routing group have the same network connectivity to the servers in other routing groups.

Only a select few servers in a routing group

  • Makes troubleshooting message flow easier because there are limited points of contact between routing groups.

  • Distributes messaging if you anticipate heavy message flow between routing groups.

  • Makes mail flow more reliable and efficient in those configurations where some servers have better network connectivity than others.

The following figure illustrates the basic components of routing discussed thus far. This figure shows message flow between servers in a routing group and between routing groups. It also illustrates a topology that uses only a single bridgehead server in each routing group.

Communication in and between routing groups


When a topology is as simple as that shown in Figure 5.1, you do not have to consider how to best route messages between routing groups. As topologies become more complex, with large numbers of routing groups spread over varying geographical distances, message routing among groups becomes critical. You configure routing among routing groups by assigning costs to the routing group connectors that are used by these groups. When a user on a server in one routing group sends mail to a user on a server in another routing group, Exchange uses these costs (part of the link state information maintained by Exchange) to determine the most efficient route. Exchange always uses the route with the lowest cost unless a connector or server in that route is unavailable. So that every routing group knows what the various costs are for each connector and the status of those connectors, each routing group has a routing group master that updates and coordinates this information with all the other servers in a routing group.

For detailed instructions about working with routing groups, see the following procedures:

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