XCON: Least-cost Routing Doesn’t Take Open-retry State into Account in Routing Algorithm

• Microsoft Exchange Server, version 5.5

SYMPTOMS

When message transfer agent (MTA) least-cost routing is enabled, as described in Knowledge Base article Q166567, the routing algorithm does not take into account the open-retry state of the connector. Therefore, when a message is sent to be re-routed, the normal load-balancing logic is used, and the MTA tends to send the message back to the connector that is in the retry state. Additionally, the way in which the MTA performs load balancing now tends to favor the first selected connector in the early stages of message delivery, and then this tapers off to a more equal ratio as message flow and time increase.

For example, in the following scenario, there are three servers in two sites:

Site #1, Server #1
Site #2, Server #2
Site #2, Server #3

Connector #1 between Server #1 and Server #2, cost of 1 and with least- cost routing enabled.
Connector #2 between Server #1 and Server #3, cost of 1 and with least- cost routing enabled.

1. When messages are sent from Site #1 to Site #2, the messages should load-balance over Connector #1 and Connector #2.



2. If Server #2 is shut down and messages sent from Site #1 to Site #2, messages should be sent over Connector #2 (after Connector #1 starts open-retry).

CAUSE

The only scenario affected by the problem should be least-cost routing where multiple links of equal lowest cost exist. The reason is that for least-cost routing, the MTA does not filter connectors based on whether they are in open-retry mode.

STATUS

Microsoft has confirmed this to be a problem in Microsoft Exchange Server version 5.5. This problem has been corrected in the latest U.S. service pack for Microsoft Exchange Server version 5.5. For information on obtaining the service pack, query on the following word in the Microsoft Knowledge Base (without the spaces):

S E R V P A C K

Keywords : XCON
Version : 5.5
Platform : winnt
Issue type : kbbug