C) Configure Forest Trust
Next step is to configure trust between contoso and fabrikam forest. Following steps may help you with trust configuration.
Login to contoso domain controller and open Active directory domains and Trusts;
Right on contoso domain and select properties;
Select trust tab and click on New Trust as highlighted below;
Click next on new trust snap in ;
Enter fabrikam.com for creating the trust and click next;
Select forest wide trust and click next;
One way trust is sufficient to build the resource forest topology. I have selected two way trust so that all feature will work as expected. Select two way trust and click next;
Select this domain only option and click next;
Select forest-wide authentication and click next;
Enter the trust configuration password and click next;
Click next to complete the configuration;
Confirm outgoing trust check box and click next;
Also confirm incoming trust and click next;
Click on finish and trust configuration has been completed successfully.
You should be able to see fabrikam.com in the forest trust list.
Lync 2013 resource forest deployment part -3 is available in following link ; http://blogs.technet.com/b/saleesh_nv/archive/2014/06/04/lync-2013-resource-forest-deployment-with-manual-sync-part-3.aspx