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.