XCON: Maximum Number of X.400 Connectors That Can Be Installed on One Exchange Server Computer

ID: Q243632

The information in this article applies to:
  • Microsoft Exchange Server, version 5.5
IMPORTANT: This article contains information about editing the registry. Before you edit the registry, make sure you understand how to restore it if a problem occurs. For information about how to do this, view the "Restoring the Registry" Help topic in Regedit.exe or the "Restoring a Registry Key" Help topic in Regedt32.exe.

SUMMARY

The maximum number of X.400 Connectors that can be installed on a single Exchange Server computer varies depending on practical limits for each server, such as hardware, network bandwidth, and so on. This article offers some suggestions for installing a large number of X.400 Connectors on a bridgehead server.


MORE INFORMATION

WARNING: Using Registry Editor incorrectly can cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk.

For information about how to edit the registry, view the "Changing Keys and Values" Help topic in Registry Editor (Regedit.exe) or the "Add and Delete Information in the Registry" and "Edit Registry Data" Help topics in Regedt32.exe. Note that you should back up the registry before you edit it. If you are running Windows NT, you should also update your Emergency Repair Disk (ERD).

  • Be sure to use Exchange Server version 5.5 and the latest service pack. There are some significant message transfer agent (MTA) performance and diagnostics enhancements in Exchange Server 5.5, and each service pack improves upon these.


  • Set the TCP/IP control blocks value according to the number of X.400 Connectors installed on the server. You may need to manually adjust this value if more than two X.400 Connectors are installed on an Exchange Server computer. It is found at the following registry location:
    HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\MSExchangeMTA\Parameters\TCP/IP control blocks
    By default, this value is set to 20 (decimal). However, if there are more than two X.400 Connectors on the server, allocate enough control blocks (10 X the number of X.400 connectors) to allow for the maximum number of associations over each connector (this maximum number is 10, 9 associations for low and normal priority mail and 1 association is reserved for urgent priority mail).

    The actual maximum entry in the registry for is 1,250 decimal, but this is taken from a pool of control blocks for the TCP/IP, TP4, and X.25 transport stacks. These registry values correspond to TCP/IP control blocks, TP4 control blocks, and Eicon X.25 connections, respectively. By default, all of these values are set to 20 decimal, so the TCP/IP control blocks value can be increased up to 1,210 decimal without a problem. This permits a maximum of 121 X.400 Connectors on a single server, each using the maximum number of allowable associations. This number is only the theoretical value. The capacities of the bridgehead server may limit the actual number of X.400 Connectors to a value less than this.

    It is unlikely that each X.400 Connector would process enough mail to require the maximum number of associations for each connector. Furthermore, if the X.25 transport stack is not in use, this value can be reduced to a value of zero, thus increasing the available control blocks for the TP4 or TCP/IP stack by 20. TP4 control blocks can be set below the default value of 20, however, doing so does not allocate any more control blocks for TCP/IP.

    If you set the maximum number of (pooled) control blocks allowed too high, you will get the following error message:
    Could not start the Microsoft Message Transfer Agent service on <servername>. Error 2140: an internal Windows NT error occurred.
    You will also see event ID 4300 in the application log:
    Event ID: 4300
    Source: MSExchangeMTA
    Type: ERROR
    Category: Configuration

    Unable to initialize due to a bad configuration. Contact Microsoft Technical Support. Error code=<variable> [1 POP4 MAIN BASE 1] (16)
    For example, using the above guidelines, if you have 30 X.400 Connectors installed and functioning on an Exchange Server computer, set TCP/IP control blocks to 300 decimal for maximum performance. If this setting is too low for the number of X.400 Connectors installed on the server (for example, 70 decimal), and all the associations are in use, event ID 9156 may appear in the application event log:


    • Event ID: 9156
    Source: MSExchangeMTA
    Type: Warning
    Category: Resource
    Description: A resource limit has been reached while attempting to open an association. There are no free control blocks available for network type 1. The configured count is 70. [BASE IL MAIN BASE 1 282] (10)
  • Consider splitting a large number of X.400 Connectors across several servers (30-50 for each server) to reduce the impact of a single-point-of-failure caused by either network or hardware issues.


  • Performance Monitor logs help gauge how the server is performing as more connectors are added. It is a good idea to have a baseline of normal usage for the processor, disk, page file, and so on to judge how many connectors a server can handle.


For additional information regarding how the Exchange Server 5.5 MTA handles control blocks, click the article number below to view the article in the Microsoft Knowledge Base:
Q194589 XCON: Event ID 57 from the Exchange Message Transfer Agent
For proactive as well as reactive MTA troubleshooting and monitoring, see the "MTA Troubleshooting Guide" on TechNet or at the following Web site:
http://technet.microsoft.com

Additional query words: 9156, 57

Keywords : exc55
Version : winnt:5.5
Platform : winnt
Issue type : kbhowto


Last Reviewed: November 19, 1999
© 2000 Microsoft Corporation. All rights reserved. Terms of Use.