What is NVGRE (Network Virtualization using Generic Routing Encapsulation)?
NVGRE stands for "Network Virtualization using Generic Routing Encapsulation." It is a tunneling protocol that uses GRE (Generic Routing Encapsulation) to encapsulate Ethernet frames into IP packets, enabling the creation of virtual Layer 2 (L2) segments on top of Layer 3 (L3) networks.
Since the mid-1990s, VLAN (Virtual LAN) has been a commonly used technology for creating logical (virtual) L2 segments on physical switches. However, the VLAN standard only allows for a 12-bit VLAN ID to identify logical L2 segments, which limits the number of segments to approximately 4,000, making it unsuitable for large-scale networks. NVGRE was developed to resolve this limitation.
With NVGRE, the VSID (Virtual Subnet ID) for identifying logical L2 segments is extended to 24 bits, enabling the creation of up to approximately 16 million L2 segments. Additionally, because NVGRE constructs logical L2 segments on top of L3 networks, it allows for the creation of single logical L2 segments that span multiple VLANs connected by routers.
This capability is particularly advantageous in situations like virtual machine live migrations between different physical servers. Normally, live migrations across L2 domains can result in issues such as changes to the IP subnet, requiring new IP addresses for the migrated virtual machines, and forcing client-side adjustments to access the updated addresses. However, by maintaining a unified L2 segment even across routers, NVGRE mitigates these issues effectively.
The practice of using tunneling protocols to create logical networks over existing physical ones is referred to as network overlay. Other technologies using network overlay, similar to NVGRE, include VXLAN. Network virtualization through overlay techniques is a key component in implementing SDN (Software Defined Networking), a software-based approach for managing network configurations.