XCON: Route Selection Criteria (193322)

SUMMARY

After the message transfer agent (MTA) has used the Gateway Address Routing Table (GWART) to determine an appropriate list of possible connectors, the MTA must then decide which connector is the best one to use. The following criteria are processed in sequential elimination order to determine the appropriate connector or connectors. Each connector or group of connectors that meets the criteria for each step is passed to the next step for testing.

Eliminate any inbound connector that may have delivered the message from another site.
Select the connectors that are not at maximum retry. The selection process checks two counters: Retry Count and Max Open Retries. The Retry Count counter of the message is compared to the Max Open Retries counter of the connector. Only connectors that are not at maximum pass this step.
Select the connectors that will connect the soonest, also called the activation schedule, which has four states:
- Active Now
- Will Become Active in the Future
- Never
- Remote Initiated
Select the connectors that have tried to connect the fewest times. The message Retry Count counter and the Max Open Retries counter are compared again. This time, only the connectors that have been attempted the fewest times are selected.
Select the connectors that are not currently retrying a connection. Each MTA maintains an Open Retry timer. This timer indicates how long it has been since the MTA attempted a failed association. The MTA can only know the Open Retry timer for any connectors on itself. Because of that, if the connectors being considered are on different computers, this step is bypassed.
Select the connectors that have the lowest cost value assignment.
Select local connectors over remote connectors. Local connectors exist when the MTA that is processing the selection criteria has the authorization to send the message directly to the remote site (for example, an X.400 messaging bridgehead server). Remote connectors require the MTA that is processing the selection criteria to pass the message to a messaging bridgehead server before it can be passed to the remote site.

MORE INFORMATION

Exchange Server version 4.0 Service Pack 4 and Exchange Server version 5.0 Service Pack 1 offer a registry entry to alter the selection criteria.

For additional information about instructions on how to enable this feature, click the article number below to view the article in the Microsoft Knowledge Base:

166567 XCON: How to Enable Lowest Cost Routes Only

This feature adds a new Use Lowest Cost Routes Only registry flag. If this flag is set, the MTA always routes to the lowest cost link. The one exception is that, if the activation schedule of the lower cost link is currently inactive and the higher cost link is active, the rerouting is to the higher cost link.

NOTE: A route with an address space of * is chosen over an address space of blank and overrides the cost value for connectors. The most common occurrence of this is with the Internet Mail Service (Internet Mail Connector in Exchange Server version 4.0).