Exchange CAS 2010 and NLB: changing from Unicast to IGMP Multicast

We have a cluster of 2 Exchange 2010 CAS servers running WNLB in Unicast Mode. It all runs fine, except the broadcast flooding on the Cisco switches. Thus, we want to change the NLB configs to IGMP Multicast to stop that.

After changing NLB to multicast, NLB complained that the interfaces were misconfigured and the “configuration conflicts with Cluster: cluster traffic mode differs” and the cluster could not converge.

From the System Logs in event viewer, I saw this: “A duplicate name has been detected on the TCP network.  The IP address of the computer that sent the message is in the data. Use nbtstat -n in a command window to see which name is in the Conflict state.”

Another error in the NLB manager itself: “Cluster IP address (172.16.xx.xx) not added to TCPIP properties.”

Each CAS server has 2 dedicated NIC's, one for LAN and the other for NLB.

Any suggestions as to what I am missing when making this change from Unicast to Multicast? Do I need to add the NLB Cluster VIP  to each NIC?

Thanks.
alibrisAsked:
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msmamjiConnect With a Mentor Commented:
It seems to be default behavior.
http://support.microsoft.com/kb/264645
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alibrisAuthor Commented:
Thanks for the possible answer. We'll have to put the KB to the test. I will update shortly.
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Senior IT System EngineerIT ProfessionalCommented:
Hi, for the exchange server HT role, what is the correct NLB mode ?

Is that Unicast or IGMP Multicast ?
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lawlibraryCommented:
I have the same question as above....which is the correct NBL mode?
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