A new capability was added to Windows Server 2012 R2 whereby the MTU size that is being reported to a guest virtual machine could be reduced by using Hyper-V Network Virtualization (HNV) or any other switch extension. This capability was implemented to reserve additional space in Ethernet frames for overhead from encapsulation. However, it was discovered that this capability does not function correctly and can affect NAT and Site-to-Site gateway connectivity for Linux virtual machines that are hosted on HNV on Windows Server 2012 R2.
A fix was released to reduce the reported MTU size from the Hyper-V host and pass it to the network stack of the guest virtual machine (for example, Linux). New code has also been submitted to the Linux community for driver support of this fix. The community reviewed the code, and the code is checked in to the mainline branch. Advanced users can reference the source-code-level change in the Linux kernel here, incorporate the source-code-level fix into their Linux kernel, and rebuild the Linux kernel. Other users will have to wait for specific Linux distributions to incorporate this change.