DNS load balancing is introduced in Microsoft Lync Server 2010 communications software. The objective of DNS load balancing is to provide a native load balancing mechanism option in Lync Server 2010. A Hardware Load Balancer is still required for Load Balancing the Web traffic.

Domain Name System (DNS) load balancing uses DNS as a way to load-balance across multiple servers. DNS load balancing is implemented at the application level in both servers and clients. They both participate in the load-balancing logic.

Hardware Load Balancer is still required for web traffic. Both HTTP and HTTPS are session-state–oriented protocols. With DNS load balancing, there is no sticky-session state that can be set up. As a result, there is no way to ensure that a session is going to be continued on the correct server. Hardware Load Balancing specifically addresses this session problem by caching the client-server state information. For web-based traffic DNS load balancing is not a solution.

DNS load balancing is not supported in all scenarios.

DNS load balancing supports automatic failover only for servers running Lync Server 2010 and Lync Server 2010 clients. Earlier versions of clients and Office Communications Server can still connect to pools running DNS load balancing, but if they cannot make a connection to the first server that DNS load balancing refers them to, they are unable to fail over to another server in the pool.

Additionally, if Exchange Unified Messaging (UM) is used, only Exchange 2010 SP1 has built-in support for Lync Server 2010 DNS load balancing. If an earlier version of Exchange is used, failover capabilities for the following Exchange UM scenarios will not be available:

  • Playing their Enterprise Voice mail on their phone
  • Transferring calls from an Exchange UM Auto-Attendant

All other Exchange UM scenarios will work properly.