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An Exchange Routing Group is a collection of Exchange servers joined by a persistent network connection that has relatively low latency and is within a single network hop of the routing group master. Within a Routing Group, SMTP messages are sent directly from one server to another. Messages sent between Routing Groups are sent to a bridgehead server in the recipient's Routing Group.
All information about routing groups and connectors is stored in the Active Directory configuration-naming context. The configuration-naming context is replicated to all domain controllers in the Active Directory forest, which means that all Exchange routing information is known to all Exchange servers in the organization. However, you must still configure connections between routing groups. If you change the underlying network infrastructure by adding new network links, the Exchange routing topology also may change to take advantage of the modified network if desired. There are cases in which your routing topology may not need to follow your network design.
Exchange routing group boundaries should be based on the availability and reliability of the underlying network bandwidth. Unlike Exchange 5.5 sites, communication between servers within the routing group is not based on synchronous RPCs, which requird reliable, high-bandwidth, low-latency connections. Exchange 2003 servers in the same routing group communicate with each other using SMTP, which has less stringent requirements for network bandwidths and latencies. Perhaps the most important factor for determining routing group boundaries is the stability of the network connection, rather than high bandwidth. You should place servers in separate routing groups if the network connection between the servers is not stable.
After you define routing group boundaries, you must connect the routing groups using a connector. The primary options for connecting routing groups are the Routing Group Connector, the SMTP Connector, and the X.400 Connector. These options are explained in the following sections.
The Routing Group Connector is efficient, easy to configure, and the preferred method for connecting routing groups. It uses SMTP as its transport mechanism between servers and uses the Routing Engine's link state database for making routing decisions.
Routing Group Connectors can be configured to use multiple bridgehead servers. With a bridgehead server, all e-mail passes through the bridgehead server, which handles transmission of the message to other routing groups. Using a bridgehead server facilitates message tracking. Multiple bridgehead servers provide a degree of load balancing and redundancy in case one of the bridgehead servers should fail. If you do not specify a bridgehead server (the default setting), then all Exchange servers in the routing group will share responsibility for transmitting messages to other routing groups.
With Routing Group Connectors, you can configure message priority restrictions, message size limits, message types (i.e., system messages, nonsystem messages, or both), a message delivery schedule, and a different schedule for messages greater than a specified size.
The Routing Group Connector is unidirectional, meaning that you must configure a Routing Group Connector for both the local and remote routing groups if you want to create a bidirectional link. However, if the Exchange System Manager (ESM) console discovers that you have configured only one end of the connection, it will ask whether you want it to automatically create the missing Routing Group Connector. You can have ESM automatically create the missing Routing Group Connector if you have the appropriate permissions in the Administrative Group where the remote Routing Group is homed.
If you are operating in a mixed environment that contains both Exchange 2003 and Exchange 5.5 servers, you can use a Routing Group Connector to connect to an Exchange 5.5 site. When used to connect to an Exchange 5.5 site, the Routing Group Connector will automatically use the MTA and remote procedure calls when communicating with the Exchange 5.5 site.
An SMTP Connector can be deployed between two routing groups, between two independent Exchange organizations, or between Exchange and any SMTP-compatible messaging system, such as the Internet's SMTP servers. When connecting two Exchange routing groups, the SMTP Connector will exchange link state status information with other routing groups in the same Exchange organization, although it uses Domain Name System mail exchanger records for routing decisions.
Although both the SMTP Connector and Routing Group Connector use SMTP as their transport mechanism, the SMTP Connector provides additional configuration parameters for fine-tuning the connection, such as multiple authentication methods and the ability to hold mail for clients that connect periodically. Most of the additional configuration parameters have limited value for connecting Exchange routing groups, but they are very useful for connecting to other environments.
Exchange 2003 continues to offer an X.400 MTA, although it is no longer used as the primary transport for Exchange. The X.400 MTA is similar to its Exchange 5.5 counterpart but with a few changes. On the positive side, Exchange has implemented RFC 2156 (Multipurpose Internet Mail Extension [MIME] Internet X.400 Enhanced Relay) to allow full X.400 and SMTP interoperability. Exchange also supports the use of Lightweight Directory Access Protocol directory lookups instead of XDS. On the negative side, Exchange no longer supports X.400 connections over TP4 because Windows 2000 and Windows 2003 do not support TP4.
There are two primary uses for the X.400 Connector:
Connectivity to external X.400 systems, including other X.400 MTAs and X.400 service providers
Connectivity between two Exchange routing groups
The Routing Group Connector is the preferred method for connecting routing groups, and the SMTP Connector is usually a good second choice. Both of these are efficient and easy to configure. Both of these connectors also allow you to configure multiple bridgehead servers to supply a degree of load balancing and redundancy.
Unfortunately, with an X.400 Connector, you can only achieve the same type of load balancing and redundancy by implementing multiple X.400 Connectors. The X.400 Connector is also less efficient than the other two connectors because it enforces strict handshaking and acknowledgment rules. However, its strict enforcement of rules, combined with its check-pointing recovery capabilities, makes the X.400 Connector a good choice for use over a network connection that has minimally acceptable bandwidth, reliability, or latency. Its check-pointing capabilities also make it a good choice if you regularly send large files.
You sometimes do not have a permanent network connection to all locations where you may need to deploy Exchange. Exchange 5.5 included a Dynamic Remote Access Service Connector that could be configured to periodically connect two Exchange sites using a modem over an asynchronous dial-up line. However, Exchange 2003 no longer includes the Dynamic Remote Access Service Connector. To connect routing groups when you do not have a permanent network connection, you should consider using a Routing Group Connector, an SMTP Connector, or an X.400 Connector over an on-demand connection supplied by the Windows Routing and Remote Access components.
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