Microsoft has introduced a new dedicated role in Lync 2010 and its known as Directory role. In OCS 2007 and R2 this role existed but was not a explicit role. it was just a frontend server with out any users homed on it.

It’s server which is generally placed before the front end pool. Its purely a optional server and it can be a single Directory role server or pool of servers behind a hardware load balancer or DNS load balancing. It can disadvantage if you have a single directory role server when it goes down. So its recommended to have multiple servers into the directory pool to avoid single point of failure. One more way of avoiding single point of failure is add multiple SRV records. One SRV record for Directory pool and other one for the Front end pool with different preference.

This role can only be deployed on the sever running Lync server2010 Enterprise edition and it cannot clubbed with any other role.

figure 1. Directory server/pool placement.

Director role acts has a mediator between Lync 2010 client and front end pool. Lync 2010 client can be coming form the Internal or Internet and service offered by the director server varies depending on the client source(Internal or Internet)

Director role service for Internal client

During deployment SRV record should be pointing to the director pool. So when the client issues a request on the SRV _sipinternaltls._tcp.<domain>.com record , then the service is handled by the director pool and it determine the front end pool where the users are located from its local database and and redirect to the correct pool. Its one more useful when you have a multiple front end pool.

Once the client determines its front end pool then director role server will not be communicated any more.

Director role service for Internet client

The main purpose of director role is for the users/client coming from the internet. Though its optional, its recommended to implement for security reasons and it allows and authenticates  clients are connecting from Internet. When users from the internet tries to connect the Lync server, it talks to the edge server and it will be forwards to director for the authentication. Once client is authenticated then it proxies the client request to the appropriate front end pool. It also maintains the communication path between the client and the user’s home pool as well as the Edge Server.

Refence link from DR Rez