Network Load Balancing and Virtual Machines: Tuneup

SharePoint 2010 Web-Front-End NLB Tuning (points to be noted before configuring Network Load Balancing and Virtual Machines on Windows Server 2008 R2):

When running NLB as a Virtual Machine (VM) guest using Windows Server 2008 R2 Hyper-V you need to be aware of some specific configurations settings in Hyper-V host prior to configuring NLB. If you do not enable spoofing of MAC address prior to configuring NLB on the VM you could potentially have problems with the NLB cluster. 

When configuring NLB in unicast mode on Hyper-V with enable spoofing of MAC Address disabled you may see some of the following symptoms:

• When initially configuring NLB you will lose network connectivity on the network adaptor NLB was configured on.
• Either of the nodes NLB status might show converging instead of converged and healthy status.
• There will be an NLB error event in the Windows Event Log stating that the network adaptor does not support dynamic MAC address updates.
• After rebooting the server, NLB will appear to be bound to the network adapter, but the cluster VIP will not have been added to the network adaptor.
• The cluster MAC address will still be the original MAC address associated with the network adaptor prior to configuring NLB.   Use CMD> ipconfig /all to view the MAC address.  It should start with "02-BF-***"
• If you ignore all previous symptoms and manually add the VIP you could get an IP conflict if there are other nodes in the cluster that have the same VIP.

With that said, to allow VM guests to run NLB you need to set the VM property for "Enable spoofing of MAC Address". 

To enable spoofing of MAC Addresses open the Hyper-V management console.  Make sure the VM is stopped open the properties of the VM.  Select the Network Adaptor for the NLB VM and check the "Enable spoofing of MAC Address" and click OK.  Then start the VM.

also refer articles from technet here http://technet.microsoft.com/en-us/library/cc917873.aspx