Hotspot Question
You have an Active Directory domain named Contoso com. The domain contains Hyper-V hosts
named Server1 and Server2 that run Windows Server 2016. The Hyper-V hosts are configured to
use NVGRE for network virtualization. You have four virtual machines that are connected to an
external switch. The virtual machines are configured as shown. To which virtual machine or virtual
machines can VM1 and VM3 connect? (To answer, select the appropriate options in the answer
area.)
Answer: 
Explanation:
The GRE keys must match. To separate the traffic between the two virtualized networks, the GRE
headers on the tunneled packets include a GRE Key that provides a unique Virtual Subnet ID for
each virtualized network.
https://networkheresy.com/2011/10/03/nvgre-vlxan-and-what-microsoft-is-doing-right/