It is RECOMMENDED that the MTU of
the physical network accommodates the larger frame size due to
encapsulation. Path MTU or configuration via control plane can be
used to meet this requirement.
In a network with Hyper-V Network Virtualization (using NVGRE encapsulation) the MTU (Maximum Transmission Unit) size is 42 Bytes smaller than in a traditional Ethernet network (where it is 1500 Bytes). The reason for this is the NVGRE encapsulation which needs the 42 Bytes to store his additional GRE Header in the packet. So the maximum MTU size with Hyper-V Network Virtualization is 1458 Bytes.