Although you can use Microsoft’s InterOrg Synchronization connector to create a combined Global Address List (GAL) from multiple Exchange Server organizations, the tool is slow and doesn’t really scale to meet the needs of large organizations. Another disadvantage of the InterOrg connector is that it doesn’t accommodate directory feeds from messaging systems other than Exchange Server, a requirement that often surfaces in heterogeneous corporate messaging environments.
When Compaq acquired Digital Equipment, two very large Exchange organizations had to seamlessly mesh from the first day of the merger. The Compaq organization had 35,000 mailboxes, and Digital had 65,000. The Exchange Server Move Server Wizard wasn’t available in early 1998, so even if merging the servers had been technically possible, the logistics of such an operation prevented Compaq from moving Digital's servers into the Compaq organization overnight. Therefore, the companies changed their goal to enabling interconnectivity between the two organizations and merging their GALs.
To accomplish these tasks, the companies used the Lightweight Directory Access Protocol (LDAP) Directory Synchronization Utility (LDSU). Exchange uses the IMS for all interorganization message traffic and LDAP for automatically updating the Exchange directory in each organization with information from the other. Exchange Server 5.5 is the first version to support LDAP 3.0, which supports LDAP read-write access to the directory. Because LDSU uses LDAP, it's fast.
The initial replication process—when bridgehead servers in each organization updated each other—took several hours. Digital mailboxes became custom recipients in the Compaq organization and vice versa. The organizations also exchanged custom recipients and distribution lists (DLs). At the same time, LDSU smoothed over differences in attributes to achieve a consistent look and feel in the unified GAL. For example, LDSU amended descriptive text in mailbox names (mostly to distinguish between mailboxes with similar names) with the organization name. I became Tony Redmond (Digital) in the Compaq GAL. Within 24 hours, LDSU had replicated the unified GAL to all servers worldwide, and all the employees of the new Compaq company could communicate with each other.
You can purchase LDSU from Compaq Services. For more information, go to http://www6.service.digital.com/ems/ldsu/. To read more about Compaq and Digital used LDSU was used in the companies' merger, see the article in EMA News at http://www.ema.org/html/pubs/mmv4n6/emerging.htm.