Aren’t they the Same thing? Well not really, matter of fact they are not even closely related to each other. One DNS round robin is handled by the server. where DNS load Balancing is a function of the client. So what does that mean? And why should you care? With DNS round Robin you place multiple A records with the same name pointed to different IP’s I.E

1. sip.domain.com  192.168.1.5
2. sip.domain.com 192.168.1.6
3. sip.domain.com 192.168.1.7

So the server will hand out or “round robin” the entry’s i.e. 1 will be delivered to the client first, then the next client that makes a request will receive 2. And the 3rd client will receive entry number 3. The server continues to round robin this all the time. This is simply a way of using DNS to send traffic to different IP’s however there is not true way to load balance the traffic if one server is offline. Traffic will still be directed to that server. Some people have referred to this as the Poor Man’s Load Balancing. but as you can see it does not really guarantee true load balancing. Nor does it have any knowledge if the server does not respond for any period of time.

So with true DNS load Balancing the client is actually what determines how and when to use each entry. The client is actually smart in that it can determine a server is not responding and temporarily remove it from the list to try for a period of time. This time period is dependent on how the client is programmed. The client actually requests all entries for the DNS name it is querying if the DNS servers supports EDNS then it will respond with all entries for the requested Name. The client then uses an algorithm to determine which record to try to connect to first. So what clients actually use DNS load balancing.

Exchange Hub transports

Lync Clients

Even PC’s do when trying to contact a Domain Controller (to some extent)

True DNS Load Balancing comes from Enhanced DNS specifications but parts of it have been being used for some time.

It is important to understand the differences and to understand that this is not new technology. this has been working for some time and other systems use different pieces of it. It is a pretty solid solution.