In this article let's have a look at steps to configure cross-forest availability between two Exchange forests.

By using the Add-AvailabilityAddressSpace commandlet which has been introduced from Exchange 2013 we would be able to share the Exchange free busy data between two forests.

We need to have a trust relationship between the source forest and target forest to execute this command. Only then the below command will be successful.

If a trust relationship exists between the two forests, run the following commands.

In our example let's think of sharing the freebusy information between domain Exchangequery.com and toybox.com.

In order to share the free busy info between these two forests we need to perform the below steps.

In the source forest perform the following tasks (ExchangeQuery.com):

Add-AvailabilityAddressSpace -ForestName toybox.com -AccessMethod PerUserFB -UseServiceAccount $true

The above command adds the target domain's address space in source domain to share the free busy information in a secured way.

Below types of access methods can be used.

PerUserFB - used to access the FB data in All Exchange Servers group.
OrgWideFB - used to access the FB data in specific group in the target forest.
InternalProxy - used to proxy the request in the latest version of exchange in the site.

The type of access method can be selected according to our requirement.

Now we need to run the below command in the target domain (Toybox.com):

Get-Exchangeserver | Add-AdPermission -AccessRights ExtendedRight -ExtendedRights "ms-exch-epi-token-serialization" -User "ExchangeQuery.com\Exchange Servers"

The above command will add required permission for source domain on the target domain Exchange Servers to access the free busy information.

In a trust relationship scenario, run this command in the target forest toybox to export the SCP from the target forest to the source forest:

Export-AutodiscoverConfig –DomainController “LocalForestDomainController” -TargetForestDomainController "(toybox.com)" -TargetForestCredential (Get-Credential) -MultipleExchangeDeployments $true

Type (Toybox\Administrator) password when prompted.

Now we need to perform the same tasks in the target forest toybox.com to share the exchangequery.com address space for sharing the freebusy data.

In the target forest perform the following tasks (Toybox.com):

Add-AvailabilityAddressSpace -ForestName exchangequery.com - AccessMethod PerUserFB -UseServiceAccount $true

Run the below command in the Source domain (ExchangeQuery.com):

Get-ClientAccessServer | Add-AdPermission -AccessRights ExtendedRight -ExtendedRights "ms-exch-epi-token-serialization" -User "Toybox.com\Exchange Servers"

Now  run this command in the forest Exchangequery.com to export the SCP from the target forest to the source forest:

Export-AutodiscoverConfig –DomainController “LocalForestDomainController” -TargetForestDomainController "exchangequery.com" -TargetForestCredential (Get-Credential) -MultipleExchangeDeployments $true

Type Exchangequery.com\Administrator password when prompted.

Important Note: This command Add-AvailabilityAddressSpace is available and applicable only for Exchange 2013 servers and Office 365.

The required trust relationship, contacts and address space between the two different organizations must be already be created and replicated between them. Only then free busy information will be working.

Reference - http://technet.microsoft.com/en-us/library/bb124122(v=exchg.150).aspx