In unicast mode,Microsoft NLB shares a common MAC address.Vmware ESX server sends a RARP packet when a node is powered on.This exposes the Cluster Mac address asap NLB node is on.This causes single node will response to network traffic.you have two options in this case.
1.Use multicast for NLB.
2.in ESX server-->Configuration-->Advanced Settings-->Net
set the value Net.NotifySwitch=0
but this is a global option,all esx virtual machine will be affected.Vmotion may not work properly.
http://kb.vmware.com/selfservice/microsites/search.do?cmd=displayKC&docType=kc&exter
nalId=1556&sliceId=1&docTypeID=DT_KB_1_1&dialogID=26310418&stateId=0 0 26304946
Microsoft Network Load balancing in Vmware
