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Q232182: XCON: MTA Problems w/High Network Use on Low Bandwidth WAN Link

Article: Q232182
Product(s): Microsoft Exchange
Version(s): winnt:4.0,5.0,5.5
Operating System(s): 
Keyword(s): exc4 exc5 exc55
Last Modified: 19-DEC-1999

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The information in this article applies to:

- Microsoft Exchange Server, versions 4.0, 5.0, 5.5 
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SUMMARY
=======

An Exchange Server site can span geographical locations provided there is
adequate bandwidth available for remote procedure call (RPC) based
communication. In a single Exchange Server site with multiple physical locations
connected by a low bandwidth WAN link, that is 128 Kbps or lower, there may be
high network usage, and the Exchange Server Message Transfer Agent (MTA) may
occasionally stop responding (hang) or not function properly.

MORE INFORMATION
================

For example, an Exchange Server site that has 27 physical sites and seven of
them are connected by 128-Kbps links. These sites with low bandwidth WAN
connectivity could experience the following problems.

- High network usage at low bandwidth WAN link because of network applications,
  business software, and databases trying to share the same bandwidth at remote
  physical locations.

- MTAs may have messages queued up occasionally, and the MTA may hang when
  processing large messages such as public folder replication messages, which
  can be several megabytes in size.

Exchange Server contributions to this high network usage are:

- Within a site, the Directory service on an Exchange Server computer
  replicates with every other Exchange Server computer's directory every five
  minutes. This can cause excessive traffic itself, depending on how often
  directories are changed.

  Keep in mind that within a site, the Directory service communicates directly
  with the Directory service on other servers without going through the message
  transfer agent (MTA), so this traffic is not visible through the MTA.

- If public folder replication is scheduled, it puts a severe load on the
  network, and depending on the size of the folder replication, it can saturate
  the network and eventually hang the MTAs.

- Exchange Server itself causes messaging traffic.

Recommendations
---------------

Keep the following recommendations in mind when you design a site with the
above-mentioned criteria and existing low speed WAN links, or, if an existing
Exchange Server site starts to see this problem:

- Upgrade or increase the available network bandwidth. This is the best
  solution because it provides enough bandwidth for all applications. How much
  additional bandwidth is needed depends on network use.

- Make physical sites with low bandwidth separate Exchange Server sites. They
  can then be connected by X.400 connectors. X.400 connectors have several
  tuning and performance optimization settings that can be configured for
  connectivity across slow WAN links.

- Set message size restrictions on the MTA to reduce network traffic. This can
  be done in the Microsoft Exchange Server Administrator program, on the
  General tab of the MTA Properties page. This way users are restricted from
  sending large messages that can cause severe problems on the network.

- Schedule public folder replication for after hours, or for times when there
  is extremely low traffic or no traffic on the network.

Additional query words:

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Keywords          : exc4 exc5 exc55 
Technology        : kbExchangeSearch kbExchange500 kbExchange550 kbExchange400 kbZNotKeyword2
Version           : winnt:4.0,5.0,5.5
Issue type        : kbinfo

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