NVGRE
Network Virtualization using GRE — Microsoft's overlay tunneling protocol (RFC 7637), a competitor to VXLAN. It encapsulates L2 frames in GRE and uses the GRE key field as a tenant ID (VSID, 24 bits). Mainly in Hyper-V/Windows environments.
Background: around 2012 there was a standards war, VXLAN (Cisco/VMware) vs NVGRE (Microsoft). VXLAN won broadly thanks to UDP encapsulation (better ECMP hashing in existing hardware) while NVGRE's GRE-based design was harder to load-balance. NVGRE survived mostly in Microsoft's ecosystem and then faded. Both solve the same thing: 16 million virtual networks over shared physical infrastructure. Historically instructive: a technically equivalent protocol lost on practical details (hashing/offload support). Geneve came later as the flexible successor option.