Network Load Balancing (NLB) clients cannot connect to the Windows Server 2008 NLB cluster by using the virtual IP address when NLB is running in multicast mode
Many organizations use Network Load Balancing (NLB). Some times clients may experience intermittent connectivity failures when they try to communicate with a server that is part of this NLB cluster. A supported hotfix is available from Microsoft. However, this hotfix is intended to correct only the problem that is described in this article. Apply this hotfix only to systems that are experiencing the problem described in this article. This is a link to KB article I’m referring to: KB960916.