NLB rejected by switch/routers

Hi I've been dealing with an issue recently whereby it appears NLB packets are being rejected by switches. When I read about this, it seems to be a fairly common issue with NLB and multicast however it turns out that the particular configuration
in use is Unicast.
I managed to find the following technet article which states:
http://technet.microsoft.com/en-gb/library/cc781160%28v=ws.10%29.aspx
Traffic alternates unexpectedly between the cluster hosts breaking TCP connections.
Cause: Unicast network addresses are causing problems with the switching hub. If you are using a switching hub to interconnect the cluster hosts, you must use Network Load Balancing multicast support; otherwise, the
switch is likely to behave erratically when the same unicast network is used on multiple switch ports.
Solution: Check that you have selected multicast support in the
Network Load Balancing Properties dialog box. If you do not want to use multicast support, you can interconnect the cluster hosts with a hub or coaxial cable instead of a switch.
Can anyone go into detail and explain why the switch would behave erratically when the same unicast network is used on multiple switch ports? Is there anything which can be done to circumvent the problem in unicast mode without switching to multicast?

Hi,
Based on your description, the cluster operation mode which you are using is Unicast.
When we select the Unicast Method, if the cluster is connected to a switch, incoming packets are sent to all the ports on the switch, which can cause switch flooding.
By using the multicast method with Internet Group Membership Protocol (IGMP), you can limit switch flooding, if the switch supports IGMP snooping. IGMP snooping allows the switch to examine the contents of multicast packets and associate a port with a multicast
address.
For more details and how to use the multicast method with IGMP, please refer to this article below,
Selecting the Unicast or Multicast Method of Distributing Incoming Requests
http://technet.microsoft.com/en-us/library/cc782694(v=WS.10).aspx
Limit switch port flooding
http://technet.microsoft.com/en-us/library/cc737869(v=WS.10).aspx
Hope this helps.
Best Regards,
Tina

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