High network usage occurs while Outlook clients download the offline Address Book from Exchange 2003 at the same time

When you deploy Microsoft Office Outlook 2003 in cached mode, you find that a period of very high network usage occurs where all the Outlook clients in your company try to download the whole offline Address Book at the same time.

In Outlook 2003, when you click the arrow next to Send/Receive Status in the Outlook status bar and then click Details, the status bar displays a message that is similar to the following: In the message, file name might be the browse file, the index file, the search file, or another file.

When you deploy Outlook 2003 in cached mode, a company-wide download of a whole copy of the offline Address Book to each Outlook 2003 client may occur for a variety of reasons.

A download of the whole offline Address Book may occur for various reasons. Typically, each Outlook client downloads an offline Address Book .diff file every day. The offline Address Book .diff file contains the updates in the directory for the past 24 hours. The offline Address Book .diff file download is generally very small. Occasionally, an Outlook 2003 client has to download the whole offline Address Book. Outlook 2003 also has to download the whole offline Address Book when Outlook 2003 is first installed. The size of the offline Address Book can be large and depends on the size of the organization. The download can take a long time. If one client downloads the whole offline Address Book, the download typically does not cause much network traffic. However, if an event occurs that causes all Outlook 2003 clients to download the whole offline Address Book, excessive network traffic may occur.

Reasons for a download of the whole offline Address Book

The following changes might trigger a company-wide download of the whole offline Address Book.

Changes to many users in the directory

When Outlook 2003 has not downloaded a .diff file for several days, and the sum of the .diff files is greater than 1/8th the size of the whole offline Address Book, Outlook 2003 downloads the whole offline Address Book. If there are many Active Directory objects that have a change to an attribute that is in the offline Address Book, that change may cause the .diff file to be greater than 1/8th the size of the whole offline Address Book. Therefore, all Outlook 2003 clients try to download the offline Address Book.

For example, the following changes might cause the .diff file to be greater than 1/8th the size of the whole offline Address Book:

• You update the phone numbers of a large set of users with a new area code.
• You add each user's department information to the directory.
• You add a new address type or a new address book.

Changes that cause the offline Address Book indexes to change

If the parent distinguished name (PDN) table changes, either because there is a new PDN or because a PDN is removed, all Outlook 2003 clients try to download the whole offline Address Book. A parent distinguished name is the part of a domain name in an earlier Exchange format that does not include the final relative distinguished name (also known as RDN). For example, the relative distinguished name /o=org/ou=site/cn=Recipients/cn=bob has a PDN of /o=org/ou=site/cn=Recipients.

The PDN table is the set of all PDNs that are in the directory. They come from the legacyExchangeDN attribute and the proxyAddresses attribute, and they start with the following prefix: X.500 addresses are only included if they start with /o=Orgname, where Orgname is the name of the local Exchange organization. This means that X.500 addresses for users who migrated from another organization are not included.

You might cause an update to the PDN table and therefore trigger the download of the whole offline Address Book in the following ways:

• The download may occur if you manually modify a legacyExchangeDN attribute to create a PDN that did not exist before. A new PDN can be created by accident. For example, in native mode, a new PDN might be created by accident if you change a legacyExchangeDN by using the ADSI Edit snap-in after you move a mailbox across administrative groups.
• The download may occur if you modify a legacyExchangeDN correctly but neglect to enter the old distinguished name as an X.500 address. If this is the only object that has that PDN, the PDN disappears from the directory.
• In Microsoft Exchange Server 5.5, the download may occur when an Active Directory Connector (ADC) is configured, and you do the following:
  1. You create a new container in Exchange 5.5.
  2. You insert an object into that container, or you delete the last object in an Exchange 5.5 container.
• In Microsoft Exchange Server 5.5, the download may occur when the following conditions are true:
  • An ADC is configured.
  • ADC is set to replicate the container hierarchy to Exchange 5.5.
  • You create an object and mail-enable the object in a new Active Directory container.
  The ADC then creates the new container in Exchange 5.5 and back-replicates the new Exchange 5.5 distinguished name as the legacyExchangeDN attribute of the Active Directory object. In other words, changes in the Active Directory hierarchy can add new PDNs, and the new PDNs may cause the download to occur.
• The download may occur after you add an administrative group. The first mailbox that is created on a server in this administrative group causes a new PDN to appear in the directory.
• The download may occur after you delete the last object that has a particular PDN in its legacyExchangeDN attribute or its proxyAddresses attribute. For example, you might delete the last object that has a particular PDN when you delete the last mailbox from a site that you consolidated and deleted three years ago. The X.500 placeholder is removed, and the size of the PDN table is reduced.
• The download may occur after you add, remove, or modify an X.500 proxy address by using the Active Directory Users and Computers snap-in. If the X.500 address is in the local organization, but the organizational unit and the containers are new or have mistyped names, a PDN is added or deleted from the table.

Note In Microsoft Exchange 2003 Service Pack 1 (SP1), the administrator can override the default behavior and limit the bandwidth that is used by full offline Address Book downloads from a particular public store. For additional information, click the following article number to view the article in the Microsoft Knowledge Base: