Advantage of Microsoft Cluster over Microsoft Network Load Balancer

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http://serverfault.com – Till recently, I assumed that Microsoft NLB worked at an OS/Machine level rather than an Application Level. i.e. the NLB just monitors heartbeats on the machine to check if machine is alive and then switches off a particular node if it's gone down. However, I found this comment on a server fault question which claims differently. As per the comment NLB just routes connections to the TCP port that is open. If your application closes the port then NLB won't route connections to it any more until the port is open again. Is the above true? Does NLB monitor applications at a port level? If (HowTos)