A virtual network identifier and room for options
Geneve wraps a payload in a Geneve header, UDP and an outer IP header. UDP destination port 6081 is the default. The 24-bit VNI identifies the virtual network. A separate Protocol Type field identifies the payload, such as an Ethernet frame.
The base header can be followed by variable-length options. These let tunnel endpoints carry additional metadata without changing the base header. A receiver drops a packet if it cannot interpret a critical option, so the sending and receiving endpoints need compatible options.
Endpoint discovery is a separate job
Geneve defines the packet format and leaves the choice of control plane to the deployment. A controller can tell endpoints which VNI and tunnel destination to use. EVPN is a control plane for exchanging reachability and is distinct from the Geneve headers seen in the data packet. Check which combination your endpoints support.
Try it in a lab
- Inspect a packet between two Geneve-capable endpoints. Find the outer UDP destination, the VNI and the Protocol Type field that identifies the inner payload.
- Compare packets with and without a supported tunnel option. Verify that the option length and location of the payload change while the VNI can stay the same.