We've implemented Microsoft NLB before, but it's not a common thing for our environment, consequently I've not looked at the config in a while. The topology is:

EDGE == 3750X's (stack of 2) == N5K's (vpc'd together) == Blade switches == servers

Looking through our existing config.. I see static ARP entries in the 3750 (which make sense to me), and in the N5K I see static MAC address-table entries. the thing is that there are 3 static ARP entries (all 3 are NLB clients of ours), but only one static MAC address table entry, but all 3 are working without issue. So it got me thinking..

Do I need to include the static MAC address entries in the N5K? I am pretty sure the only contention with NLB was telling the L3 device, that 'yes you can send this unicast response back to this multicast MAC' by making a static ARP entry. Thanks for any clarification.

1 Answer 1


You should not need to manually configure any static MAC address entries in any of your switches for MS NLB to work. As a point in fact, doing so can break a MS NLB setup as the MS server/cluster can no longer shift instances between links or servers.

Normally you would configure IGMP snooping on the Cisco devices and it will learn the hosts who want to be part of the multicast group and the ports they are on. IOS will then automatically add or remove static multicast entries on the switch as needed.

Rather than your assertion about unicast/multicast response, this limits where the multicast traffic should go. Normal switch operations would be to flood multicast traffic out of all ports, which isn't generally necessary and only adds unnecessary traffic to a number of devices. IGMP snooping gives a mapping to the switch of which ports should be sent traffic for the multicast destination.

I would recommend reading (and possibly rereading) documents on MS NLB, Cisco IGMP snooping and general multicast. Searching in your engine of choice should find you any number of documents.

  • Cool, I will definitely read up on this, thanks for your help!
    – A L
    Oct 1, 2013 at 19:15

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