He covers "how rich coexistence is configured between 3 separate Exchange forests with different versions of Exchange deployed".
As reason for a coexistence he describes: "... One reason could be that company policies states that specific entities within a company must run isolated from the other entities. This is typically because of legal requirements. It could also be because of a merger or acquisition, where one company purchased another company or several other companies. The companies could continue to run isolated or be consolidated into one large Active Directory forest...."
As basic conditions Walter assumes ". . . an infrastructure using DNS name resolution between each forest as well as an established trust relationships between them..."
We experience that a lot of companies do not want or are not allowed to establish a trust relationsship. To publish the own internal DNS structure is mostly also forbidden.
So we developed a smart software to overcome these limitations: with NETsec GALsync you can share directory information and free/busy between different forests using different Exchange versions. You do not need a trust or a "shared" DNS.
Link: http://www.netsec.de/en/products/galsync/
