It appears to be a common misconception regarding DNS Load Balancing (DNS LB) in that it means we don’t need Hardware Load Balancers (HLBs) anymore.
First off, let’s break open our DNS books. There is no such thing as DNS LB in any of those books. Why? Well, because it really isn’t a DNS function, it is a Lync 2010 feature. As far as DNS goes, all we are doing is creating multiple records with the same host name by using different IPs for each entry. That is, for all intents and purposes, the same thing as DNS round robin (DNS RR). So, nothing new from the DNS side. DNS RR, by itself, is not a high availability solution.
However, in the Lync world, life is so much better when using DNS LB. Lync is able to use DNS LB (yes, DNS RR entries) to load balance traffic such as SIP and media traffic between Lync clients and Lync front-end servers. You still need HLBs to handle the HTTP and HTTPS traffic. So, the HLB is needed for port 443, and 80 traffic [corrected from the original post]. We need to use the HLB for these ports because the HTTP and HTTPS protocols are session-state–oriented. We need to use HLBs to ensure thats if connections are initiated to a particular server, the Lync client would maintain its association with the server and continue to connect to the server to complete the entire request. Using DNS LB prevents the client from having to rely on the initial server connection.
So, what is the purpose of DNS LB if it doesn’t replace HLBs? Great question, and I am glad you asked. The purpose is to reduce the complexity of the rules required for implementing the HLBs. The purpose is not to replace HLBs by using DNS LB.
DNS Load Balancing can be used for pools of Directors and for pools of Front-end servers. However in almost all cases with Directors and Front-end pools, HLBs are still required.
DNS LB can also be used for Edge servers (for the internal and the external interfaces). You can pretty much get away with using just DNS LB (meaning no HLB) when connecting to Edge pools. There are a couple of pretty big issues when using DNS LB and Edge servers, though, when it comes to federation with companies that use previous versions of OCS and when federating with PIC and XMPP environments. Basically, it will not work.