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Cross-forest Exchange Migration, notes from the field Part 1, Overview

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Exchange migration has always been a topic of interest for
organizations. As more and more organizations depend on Exchange as their core
infrastructure downtime during upgrades have been noticeable by the clients and
need detailed planning. When there is a need to change the forest as a part of
this upgrade, the problem becomes a complex migration exercise.

Let’s take a hypothetical organization running Exchange 2007
which wants to move to Exchange 2010 in a new forest. For the sake of argument
lets say customer only wants to migrate exchange functionality to the new
forest and the old forest will remain where Exchange will be uninstalled. When
the number of clients involved is large, the mailbox move process can take
longer than the organization can tolerate downtime and coexistence is needed.
Coexistence can be defined as where you have 2 exchange organizations in 2
different forests and it acts like a single organization. Here are important
points to consider for kind of migration:

- Mail
flow: generally mail flow to and from Internet is done from the old organization during coexistence phase and mail between the organization is done with connectors in between.

**Mail<br>Access:** For OWA users this will depend on where the mailbox is hosted at a  
given particular time. Exchange can provide redirection to the new environment,  
more on this later. For Outlook anywhere and ActiveSync users Autodiscover will  
need to be used in cross-forest configuration. In order for autodiscover to  
work you will need to have Outlook 2007 as a minimum on clients.
  • Availability:
    During coexistence you need to be able to query free/busy information. Exchange
    2010 supports several methods to get this information.

In order for these functions to work, you need do analysis
on source forest:

- Single
sign on: During coexistence you will need an entry point that can connect to both forests and would receive credentials from clients only once.

**Account<br>names:** If customers are using Domainname\\username format to logon, this  
will need to change when Exchange moves to the new forest. One way to solve  
this problem will be to use UPN. Users accessing old forest can start using UPN  
and the new forest will also have the same UPN but different forest name. Generally  
you would want to have UPN the same as the e-mail of the user.

This part clearly shows you need to have a lot of
preparation before you do the migration. We will focus on more details in later
series.