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Windows Server 2012 R2 Network Virtualization (NVGRE) MTU reduction

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With Network Virtualization the usable MTU (Maximum Transmission Unit) size for a virtual machine is 42 bytes smaller than the ethernet default value of 1500 Bytes because of the GRE (Generic Routing Encapsulation) header. This would lead to packet fragmentation in the communication between VMs on different Hyper-V Hosts or with resources behind a NVGRE Gateway. To avoid this packet fragmentation the MTU size of a vmNIC (virtual adapter of the virtual machine) gets lowered automatically during the initialization of the vmNIC . In most cases the MTU size of a virtual machine connected to a virtualized network will be 1458 Bytes.

The vmNIC MTU is not reduced in case Jumbo Frames are enabled on the provider network. In this case it is not necessary to lower the MTU.

If the following statements are true, the automatic MTU reduction will take place:

  • The Hyper-V host must be Windows Server 2012 R2 .

- A VM must be running Windows Server 2008 or newer.

- The vmNIC must be a synthetic NIC.

  • The VM must have been assigned to a VSID (virtual subnet ID) before boot.

  • Jumbo Frames must be disabled on the pNIC (physical NIC) that is used by the PA Network.

The amount of bytes the VM MTU gets reduced

  • is determined by the vmNIC and an the vSwitch only while booting the VM.

  • depends on as well the MTU setting of the VM and the bytes needed by the encapsulation header.

  • is persistent across save, restore and live migration.