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NLB - unicast Vs multicast

Posted on 2013-01-30
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Last Modified: 2013-01-31
Hi - can someone please describe in lay-man's terms the difference between these two modes in network load balancing, it is so confusing what I've found online...

Thanks!
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Question by:fredostare
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by:giltjr
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Do you know and understand the difference between unicast and multicast as it relates to IP in general?
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by:fredostare
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HI giltjr, well, my limited understanding would be that unicast would be broadcasting to one specific host and multicast would be broadcasting to many... is that anywhere near?
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That is correct.  It's not really broadcasting, but it is sending traffic to more than one destination using a single MAC/IP address and a single frame/packet.

In NLB unicast you must have 2 NIC's.  One that will be used for the NLB traffic and one for the two servers to talk to each other.  This is because in unicast mode, both hosts will use the same IP address for NLB traffic.  

In NLB multicast mode you can get by with a single NIC, but it is not recommended generally.  This is because MS uses a multicast MAC address which can exist on multiple devices.  MS does break standards here because they use a multicast MAC address with a unicast IP address, which is not really valid to do.

Both modes have their advantages and disadvantages because of the way switches work.  In fact because different switches work different ways unicast may be better on some switches while multicast on others.

Read this and see if it helps:

http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1006580
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