Deploying Lync has a cost both on time and money. In addition, the more features, availability, and quality we deliver, the more resources will be required. It makes sense for a company to dampen costs, deploying Lync Server 2013 in a single domain/forest and making Lync services available to users whose accounts reside on separate forests outside the single corporate boundary. There are two different topologies we are able to deploy to achieve the previously mentioned result: Lync in a Central Forest or Lync in a Resource Forest. In both scenarios, we have a two-way forest trust between the forest-hosting Lync and the external forests where the user accounts reside. In the first topology, the forest that contains our Lync deployment is called the central forest.

There are active user accounts in all the forests, including the central forest, and the ones that have no Lync Server available are called User Forests. A resource forest topology is similar, and we have the Lync Server installed in a single resource forest. However, our active user accounts are located only in the user forests. We can easily understand this by looking at the following image: