The Lync Director role is one of the most confusing roles in the Lync environment. This follows on from the confusion that existed in OCS where I for one was never sure of when a Director should be deployed. One thing that has changed is that Microsoft have made an explicit Director role in Lync, whereas in OCS, the Director was simply a pool without users homed on it. Therefore Microsoft obviously believe that the Director has a role to play in Lync designs.
The function of the Director is different dependent on whether the clients are internal or external.
For internal clients, the purpose of the Director is to re-direct users to the correct pool in a multi-pool deployment. Without a Director, clients through autodiscovery, will receive from DNS a pointer to only one pool. If this is not their home pool then they will be redirected to the correct pool. This leads to an overhead on the pool pointed to by the DNS records. The Director will take on this role, redirecting clients to their home pool. Once redirected, the Director is no longer in the communications path for internal clients.
The fact that the Director role in Lync is a dedicated role means that users cannot accidentally be homed on it, unlike the situation in OCS where the Director was a front end pool but was designed not to home users.
For external clients, the role of the Director is different. In this situation, it is acting to offload the authentication process from the Front End server. Also, once authenticated the Director will stay in the communications path, which also traverses the Edge server. For this reason, the Director can scale to 15,000 remote users. The Director will act as a further layer of security for external clients. If a Denial of Service attack is initiated against the Lync system, this will affect the Director but will not affect your Front End servers and will therefore minimize the impact for internal users who can continue to use the services of the Front End servers.
The platform requirements for the Director role are the same for other Lync roles with the exception of requiring only 4 GB of memory.
Dr Rez on the Director
Another Reason for the Director from Doug Deitterick's Blog