I have two instances of Windows Server 2008 running in Hyper-V VM's. The two instances are configured in a multicast NLB. I can ping either node from another subnet, but I get a timeout when pinging the NLB name.

If I stop NLB on one node and take a network trace on the remaining node, I see that an ARP request for the NLB's IP address gets to the VM... but the VM never sends an ARP reply.

Can anyone suggests causes and solutions? There are many articles out there about the router not forwarding ARP requests to NLB nodes, but that doesn't appear to be the problem here, as the NLB nodes do get the ARP request; they just don't respond.

EDITS: MAC address spoofing is enabled on both nodes in Hyper-V manager.

2 Answers 2


If you're using NLB on Hyper-V guests you need to enable MAC address spoofing in the Advanced Features of the vNIC on each guest.

  • Thank you, Joe. I did do that; I'll see if I can edit my question to include that.
    – Nevo
    Aug 3, 2015 at 18:06


"In Multicast cluster operation mode, when the source MAC address is masked, the ARP response from an NLB host includes a substitute source MAC address in the Ethernet frame, but contains the correct NLB cluster MAC address in the ARP header. Some Layer 3 switches and routers are confused by this response and cannot perform the ARP mapping automatically. In this case, create a static ARP entry on the affected switch/router which maps the NLB virtual IP address to the NLB cluster MAC address."

  • Thanks, Greg. However, a Wireshark capture from the NLB node shows that the ARP request is received, but that a response is never made. I suppose it's possible that Wireshark is lying to me, but I've never seen that happen before.
    – Nevo
    Aug 4, 2015 at 11:54

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