NLB does not check IIS status

Rember: Network Load Balancing (NLB) does not check services status. It works directly on TCP/IP and it is not aware of upper services status (IIS, etc.).

If you have a NLB system running and you shutdown a machine, the "cluster" converges in some seconds. The http connections are than served by the remaining machines. But if you stops the IIS instance of one server, NLB continues to send http traffic to that machine.

Reference: http://support.microsoft.com/?id=234151



PS: I do not agree with calling NLB a "cluster" solution. Windows 2003 offers a real cluster solution. NLB is only a "light cluster".