Life Before Availability Service Sharing

In my previous blog post, I discussed how you can utilize Availability Service sharing to set up Free/Busy coexistence between your source and target forests. Now, I must reveal there’s a catch—that method only works if all of the elements in the lookup chain support the Microsoft Exchange Availability Service. What that means is your end users must be using Outlook 2007 (or greater), have their mailboxes on Exchange 2007 (or greater), and perform lookups against a mailbox in a different forest hosted by Exchange 2007 (or greater), as well.

What if your company is still on Exchange 2003 and using Outlook 2003 when you are migrating to Exchange 2013? Luckily, there is a Microsoft process available for this exact scenario. While it does involve making changes to your source environment, it will deliver the best experience for your users, and is very similar to the process I described in our earlier blog post.

Configuring Free/Busy Lookups Between Exchange 2003 and Exchange 2013 Forests Diagram

First, some background information
Before the Availability Service was introduced with Exchange 2007, Microsoft supported the lookup of Free/Busy information by way of the Schedule+ public folder. This method relied on the Outlook client to create specially-formatted EML messages containing a user’s availability, encoded as a long series of numbers. When read by an appropriate client, availability was then made visible. Each mailbox was represented by an EML file within the Schedule+ folder. If this information was ever deleted or became stale, Outlook would eventually refresh this data.

To support backwards compatibility, Exchange 2007 and Exchange 2010 maintained the ability to store Free/Busy information in a Schedule+ folder (just like before), in addition to supporting the new Availability Service. Unfortunately with the arrival of “Modern Public Folders” in Exchange 2013, you no longer have that option.

What now?
For enterprises that require Free/Busy coexistence between Exchange 2003 and Exchange 2013, there is a solution. First, an Exchange 2010 server with a current rollup must be installed within the Exchange 2003 Org. This server will ‘proxy’ any Free/Busy lookup requests between the two environments, as long as the environment is configured in accordance with in my earlier write-up. You’ll still need Mail Enabled User objects representing mailboxes in the opposing forests; and you’ll still need to run the commands to set up and configure Availability Service Sharing between the two forests (with a minor difference in the command execution). If you follow these steps, your users will experience Free/Busy lookup functionality—without additional lag.

Let’s review the steps in more detail:

1.    Install Exchange 2010 SP3 Rollup 7 (current as of this writing) within your source organization.

2.    Replicate the Schedule+ Free/Busy folder to the Exchange 2010 server. A replica MUST be present on the Exchange 2010 server.

3.    Cross populate Mail Enabled User objects in opposing forests, as described in my earlier blog post.

4.    Verify that Autodiscover can be resolved by both opposing forests, as described in my earlier blog post.

5.    If you have a trust in place, just as before, run these commands on the Exchange 2013 side. This time, there is one critical difference: where in the earlier article we set “AccessMethod” as “OrgWideFB,” now we must set “AccessMethod PublicFolder.” This instructs the Exchange 2013 server to request data from the Schedule+ Public Folder on the source Exchange 2010 server:

Get-MailboxServer | Add-ADPermission -Accessrights Extendedright -Extendedright "ms-Exch-EPI-Token-Serialization" -User “ForestB\Exchange servers"

Add-AvailabilityAddressSpace -Forestname -AccessMethod PublicFolder

6.    When running the opposing commands on the source Exchange 2010, you should continue to use “OrgWideFB,” as there is no Schedule+ folder present on Exchange 2013.

Voila! You should now be able to resolve Free/Busy between your Exchange 2003-hosted mailboxes, and those mailboxes on Exchange 2013.