Federation trust during cross forest migration

Dear experts,

I´m involved on Exchange cross forest migration between Exchange 2010 and other forest with Exchange 2013. I have designed migration phases, currently have installed and configured Exchange 2013 new environment and finishing some details with Forest trust, but I have one question about calendar.
Source Exchange have about 20 authoritative domains that must to be on destination forest, and of course calendar must be accesible on both Exchange forest. I try to create a federation trust to share calendar, but How can I do when sorce and destination are same domains? Since we are talking about 2000 mailboxes, forest coexistence will be long enough users ask about calendars of migrated users. Must to configure all domains on destination forest as federated domains or as authoritative?

Any input and detailed steps are welcome.

Thanks.
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Miguel Angel Perez MuñozAsked:
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Jamie McKillopIT ManagerCommented:
Hello,

The first thing you will need is a placeholder domain for your 2013 environment. This will be used for routing and calendar sharing.

On your 2013 environment, setup your placeholder domain as authoritative and set it up as a federated domain. Setup all your other domains as internal relay and create send connectors for those domains, which point to your 2010 Hub Transport servers for delivery.

On your 2010 environment, setup your placeholder domain as internal relay and setup a send connector that points to the 2013 Hub transport servers. Setup all your other domains as federated.

Change your e-mail address policy so that an address with the placeholder domain is added to objects.

You can now setup the Federated Trust between the organizations.

You will need to use Microsoft FIM to sync your mailboxes as contacts from your 2010 environment to the 2013 environment.

At this point you will need to change your MX records (or firewall rules) so that incoming mail hits the 2013 servers first. Mail will then be routed to the 2010 server for mailboxes that haven't been migrated.

Now, as you migrate mailboxes, you will specify your placeholder domain as the target delivery domain. When users on your 2010 server go to look up free/busy for mailboxes that have been migrated, they will be looking at the contact and will be redirected to the 2013 server. When users on the 2013 server look up free/busy for users that haven't been migrated yet, they will be looking at the contact created by FIM and will be redirected to the 2010 server.

-Jamie

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Miguel Angel Perez MuñozAuthor Commented:
Dear Jamie,

This placeholder domain must be public, can not be internal, isn´t it?

Thanks.
Jamie McKillopIT ManagerCommented:
Correct. It would need to be public so that Microsoft can Federate you.

-Jamie
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Miguel Angel Perez MuñozAuthor Commented:
There is any way to do without using placeholder domain?
Jamie McKillopIT ManagerCommented:
You could use one of your 20 domains but it would be more complicated to setup and there would be greater risk.

-Jamie
Miguel Angel Perez MuñozAuthor Commented:
I will try using forest trust to share calendar: https://technet.microsoft.com/en-us/library/bb125182.aspx since looks easy than made Federation trust.

Thanks for your assistance.
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