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Posted by Mike Howells on January 8, 2012

In all previous instances when working with Windows NLB (Network Load Balancing) I have always used Unicast. Recently, I ran into a scenario where Unicast was not allowed in a VMWare environment, which forced the use of Multicast instead. Apparently, using VMotion works with Multicast as opposed to using Unicast. Note: For a detailed description between these NLB options see the articles posted at the end of this article.

The three options available to Windows NLB are: Unicast, Multicast and IGMP Multicast. In short, Unicast is a configuration setting which instructs Network Load Balancing to change the MAC address of the cluster adapters to the same value for all hosts in a cluster. This is the default mode of operation. Multicast is a configuration setting which instructs Network Load Balancing to add a multicast MAC address to the cluster adapters on all hosts in a cluster. The adapters’ existing MAC addresses are not changed. IGMP Multicast is essentially the same as plain Multicast except that the IGMP (Internet Group Membership Protocol) helps eliminate switch flooding apparent with both Unicast and Multicast.

The one additional “issue” with Multicast is that you will most likely need to add a static ARP entry to your distribution switches or routers to map the NLB cluster MAC to each of the NLB cluster IP addresses.

So, how do you find out the MAC address of your network load balanced IP address?

If you are using Unicast, you can do this by issuing the IPCONFIG /ALL command from a command prompt:

Unicast MAC

Unicast MAC

But, what if you are using Multicast or IGMP Multicast? How do you find out what the cluster MAC address is in either of these cases? Hint: IPCONFIG will not help you.

The answer is your friend IP2MAC…

IP2MAC is a option available from the NLB.exe command found in the %SystemRoot%\System32 folder:



How do you execute this command?

Open a command prompt and change your folder path to %SystemRoot%\System32 and then type: NLB.exe ip2mac <cluster IP>

So, for example, if your NLB cluster IP address is you would type: NLB.exe ip2mac

The results will appear as follows:

IP2MAC results

IP2MAC results

You are now presented with the Unicast, Multicast and IGMP Multicast MAC addresses. You can now give this information to your network administrator so that the static ARP entry can be made to the distribution switches or routers (assuming Multicast or IGMP Multicast).

Interestingly, the Unicast and Multicast MAC addresses are very similar except the high-order octet for Unicast begins with an 02 whereas Multicast begins with an 03. IGMP Multicast is way out there in left field with hardly anything similar.

Using NLB with ISA Server, Part 1: How Network Load Balancing Works

Selecting the Unicast or Multicast Method of Distributing Incoming Requests


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