Virtual Network Identifier

A virtual network identifier (VNI) is a value that identifies a specific virtual network in the data plane. It is typically a 24-bit value part of the VXLAN header, which can support up to 16 million individual network segments. (Valid VNI values are from 4096 to 16,777,215.) There are two main VNI scopes:

Network-wide scoped VNIs: The same value is used to identify the specific Layer 3 virtual network across all network edge devices. This network scope is useful in environments such as within the data center where networks can be automatically provisioned by central orchestration systems.

Having a uniform VNI per VPN is a simple approach, while also easing network operations (such as troubleshooting). It also means simplified requirements on network edge devices, both physical and virtual devices. A critical requirement for this type of approach is to have a very large number of network identifier values given the network-wide scope.

Locally assigned VNIs: In an alternative approach supported as per RFC 4364, the identifier has local significance to the network edge device that advertises the route. In this case, the virtual network scale impact is determined on a per-node basis versus a network basis.

When it is locally scoped and uses the same existing semantics as an MPLS VPN label, the same forwarding behaviors as specified in RFC 4364 can be employed. This scope thus allows a seamless stitching together of a VPN that spans both an IP-based network overlay and an MPLS VPN.

This situation can occur, for instance, at the data center edge where the overlay network feeds into an MPLS VPN. In this case, the identifier may be dynamically allocated by the advertising device.

It is important to support both cases and, in doing so, ensure that the scope of the identifier be clear and the values not conflict with each other.