Microsoft KB Archive/232182

= XCON: High Network Usage over Low Bandwidth WAN Link May Cause MTA Problems =

Article ID: 232182

Article Last Modified on 10/28/2006

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APPLIES TO


 * Microsoft Exchange Server 4.0 Standard Edition
 * Microsoft Exchange Server 5.0 Standard Edition
 * Microsoft Exchange Server 5.5 Standard Edition

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This article was previously published under Q232182



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.

Keywords: kbinfo KB232182

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