SR-IOV virtual machine using synthetic network adapter instead of SR-IOV

Question

A virtual machine configured to use SR-IOV networking is falling back to synthetic NIC performance, even though the host hardware and physical network adapter both support SR-IOV.

Looking for the recommended verification checklist to identify the cause, including BIOS settings, NIC driver and firmware, virtual switch configuration, and any other required SR-IOV prerequisites.

Answer 1

Hi Mendoza Joshua,

Thanks for reaching out.

If a VM configured for SR-IOV is falling back to the synthetic network adapter, verify the following prerequisites:

• SR-IOV is enabled in BIOS/UEFI, including VT-d/IOMMU support. Some systems require a full power cycle after changing these settings.
• The physical NIC supports SR-IOV and is using the latest vendor-recommended driver and firmware. Check with:

    Get-NetAdapterSriov
  

• The external Hyper-V vSwitch was created with SR-IOV enabled. SR-IOV cannot be enabled on an existing non-SR-IOV switch after creation. Verify:

    Get-VMSwitch | Select Name,IovEnabled
  

• SR-IOV is enabled on the VM network adapter and has a non-zero IovWeight value.

No VM network policies are preventing VF assignment, as some virtual switch features require traffic to stay on the synthetic path and will disable SR-IOV automatically.

For a quick health check, please run:

Get-VMHost | Select IovSupport,IovSupportReasons
Get-NetAdapterSriov
Get-VMSwitch | Select Name,IovEnabled
Get-VMNetworkAdapter -VMName <VMName> | Select Name,IovWeight,IovUsage

These commands will usually identify why the VM is not receiving a Virtual Function (VF) and is instead using the synthetic adapter.

Hope this answer has brought you some useful information. If it did, please hit “accept answer”. Should you have any questions, feel free to leave a comment.  

Best regards,

Titus Bui