Mac Address Spoofing broken in Hyper-V 2019 RRS feed

  • General discussion

  • I believe we have come accross a bug in Windows Server 2019 Hyper-V.
    Does anyone in here know how to report this to Microsoft?

    To make some virtual network appliances work, we need to activate the function "Mac Address Spoofing" on the vNIC of the VM's internal interface.

    This is needed for VPLS setup and for a VPN concentrator setup.

    The function is working, but with highly reduced performance, especially upload goes from 800 Mbit/s to 30 Mbit/s, and ping latency from 1-2 ms to 5-8 ms.

    We have tried Mikrotik routers, pfSense, SoftEther on Windows, and we even set up a Windows Server as a router, and the issue is present i all cases.

    It only appears then the VM acts as a router, that is traffic coming in through one interface and out through another, and only when Mac Address Spoofing is activated.

    We have tried similar setup on VMware where the similar setup requires something called "promiscuous mode", and it works fne with the same VM's.

    To make the test complete, we also tested with a Server 2016 Hyper-V, and that works as well.

    So Mac Address Spoofing Works in Windows server Hyper-V 2016, but not in Windows server Hyper-V 2019.

    Best regards, Ulrich - novaram

    Wednesday, August 14, 2019 12:54 PM

All replies


    Simply call Microsoft and open a support case.  If Microsoft determines that it is a bug and not a configuration issue, they will not charge you for it.


    Wednesday, August 14, 2019 1:48 PM
  • Hi Tim,

    Thank you for that. I cannot find a phone number on, only a wizzrd to create a ticket, and it wants me to pay for creating a ticket.

    can you help me get a nnumber for them?

    Thank you


    Best regards, Ulrich - novaram

    Wednesday, August 14, 2019 1:59 PM
  • Yes, you create a ticket.  If it is a bug, they don't charge you.


    Thursday, August 15, 2019 1:55 PM
  • Hi Ulirch.

    Did you figure this out?
    I', having the exact same problem.

    Kind Regards
    David Eriksson

    David Eriksson

    Wednesday, October 30, 2019 10:10 AM
  • Hi David,

    Unfortunatelly not, I have not had the time to persue the issue.

    One of my techs have installed Hyper-V on a Windows Server 2016, and we have moved the 5 vm's to this node for now, but it is by no means optimal as we do not have HA, as the 2016 Node cannot be part of the cluster.

    Wednesday, October 30, 2019 10:16 AM
  • Hi again Ulrich, thank you for your reply.

    Can you remember if it was related to 10Gbit NICs?

    It seems to be working fine for us if we use 1Gbit adapters at physical host, although it's not a permanent solution for us.
    We use LBFO Team with 1 x 10Gbit NIC as active and 1 x 1Gbit NIC as standby, when we switch and use 1Gbit as active everything works fine.

    Running MikroTik CHR

    David Eriksson

    Wednesday, October 30, 2019 10:50 AM
  • We only have 10Gbit/s in our infrastructur, so I have not tried less.

    For us it is shown on multiple systems, both pfsense, Mikrotik CHR, and others.

    We have tried VM's in VMware with their similar setting, and it works fine.

    On Hyper-V on server 2012 R2 and Hyper-V on Server 2016 it also works...the problem is only present on Hyper-V running server 2019.

    Wednesday, October 30, 2019 10:58 AM
  • What make/model is the NICs you are using?
    I've tried forcing the 10Gbit NIC to 1Gbit but it doesn't help so i'm thinking this may be a driver error.

    David Eriksson

    Wednesday, October 30, 2019 11:17 AM
  • It seems the problem for us is related to Receive Segment Coalescing (RSC).
    I've disabled it both on physical NIC and on vSwitch and now everything seems fine.

    It first looked fine when just disabling on physical NIC but there where still problems with IPSEC-tunnels.

    After running "Set-VMSwitch -Name vSwitchName -EnableSoftwareRsc $false" it all looks good.
    Apparently it's a new feature to have it implemented at software level on vSwitches in Windows Server 2019.


    David Eriksson

    Wednesday, October 30, 2019 1:12 PM