Keith and Mike from this week’s 10233 deploy and design of Exchange 2010 class asked about a hurdle they will have to get over. Two forests, both 2003. If they can’t collapse the AD structure before migrating to 2010, how can they keep things like the GAL, and availability up and running during the upgrade??

First off if you haven’t seen or used the Exchange Deployment Assistant, then your missing out!

Cross-forest A cross-forest topology is one with multiple Exchange forests. Here is an overview of what you need to do to deploy Exchange 2010 in a topology with a multiple forest:

  1. You must first install Exchange 2010 in each forest. For more information, see Understanding a New Installation of Exchange 2010.
  2. Next, you must synchronize the recipients in each of the forests, so that the Global Address List (GAL) in each forest contains users from all the synchronized forests. See the “GAL Synchronization” section below for more details.
  3. Finally, you must configure the Availability service so that users in one forest can view availability data for users in another forest. For more information, see Configure the Availability Service for Cross-Forest Topologies.

Here are some deeper dive links!

Deploy Exchange 2010 in a Cross-Forest Topology

Exchange 2003 – planning roadmap for upgrade & coexistence

more specifically here are some answers to the GAL and sync questions

Configuring cross-forest connectors

Configure the Availability Service for Cross-Forest Topologies on using IORepl in Exchange 2010

MSFT really wants you to use ILM for sync’ing the two GAL’s.