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Cross-forest Exchange Migration, notes from the field Part 3, Coexistence

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In the first part of this series I had an overview of
Exchange migration which can be found here.

In the second part of this series I provided details on how
to check for inconsistencies on user attributes and set for UPN which can be
found here.

In this part of the series I will give you details on how to
setup the coexistence. So here are the steps to configure coexistence:

- Conditional
Forwarding: As you have 2 different forests you will need to have DNS name resolution between the domains. You can use DNS conditional forwarding feature to do this.

**Trust<br>relationship:** some of the tools that would be needed for migration (Hint  
ADMT) will need Windows trusts to be configured between the 2 forests. You will  
also need to configure Windows trusts for cross-forest availability.
  • Directory
    synchronization:     After you start migrating the users, you need to make sure
    users are available on both sides. The recommended approach is to use FIM to
    synchronize users, distribution groups and contacts. While you are configuring
    you need to plan for migrating the users through your migration planning and
    will need to configure the new object provisioning through FIM. (Ex: What will
    happen when a new user is created in old forest during coexistence)
  • Control
    panel:     remember our scenario. We are moving only Exchange functionality to
    the new forest. In this case you might consider using control panel to manage
    Exchange properties of the users. If this is the case you might have to do
    configuration on your Control Panel.
  • Coexistence
    Server:     In order to migrate users and provide mail flow you can use an
    Exchange Server 2010 in the old forest. This will provide you with the new
    mailbox replication proxy functionality. You would also use this server
    together with the Exchange in new forest for providing availability services in
    cross-forest migration scenario. You would need to have certificate installed
    on this server that would be trusted by the new forest Exchange servers.
  • E-mail
    address policies:     In order to flow mail between two organizations you will
    need to configure secondary e-mail addresses for each side.
  • Send and
    receive connectors:     These will be needed on both sides to enable mail flow
    between 2 Exchange organizations acting as a single organization.
  • Cross-forest
    Availability:     During mailbox migration you may want to have each side of
    your Exchange servers to be able to query availability information for
    respective recipients. For more information have a look here.
  • Auto
    discovery:     You will need to configure auto discovery services so that one
    you start migrating the users, they will be able to reconfigure themselves for
    the new forest. Please keep in mind that this will work for seamlessly for
    Outlook anywhere and ActiveSync but if you configure the coexistence server as
    your Internet facing CAS servers you will only get a redirect, which means
    migrated users will be prompted for authentication on new servers. You can use
    an Access gateway solution to provide seamless redirection when the mailbox has
    been moved and they are accessing through OWA.

These are the basic steps you will need to do configure
Coexistence between 2 forests.