What is VXLAN (Virtual Extensible LAN) in 2021?

Basics of VXLAN (Virtual Extensible LAN)

VXLAN or Virtual Extensible LAN is an overlay tunnelling scheme where we can extend the layer 2 domain over a layer 3 underlay network.

vxlan basics also cover its major use in extending VLAN across geographically spread Data-Centers. vMotion across data-centres requires us to have the same L2 domain across data-centres and hence this can be easily achieved with the help of VXLAN overlay.


Related – VXLAN Interview Questions in 2020

VXLAN encapsulated Packet Format:

vxlan header

VXLAN Header


VXLAN Terminologies:

  • VTI (VxLAN Tunnel Interface): It is a switch port linked to a UDP socket to be shared between many VLANs. VXLAN header encapsulation and de-multiplexing occur at this interface. Encapsulation at the headend of VXLAN tunnel and de-multiplexing at the tail-end.
  • VNI ( VXLAN Network Identifier or VXLAN Segment ID): It is a 24-bit number that distinguishes between VLANs being carried over the VTI.
  • VTEP (VXLAN Tunnel Endpoint): It is an entity where either a VXLAN tunnel originates or terminates.
  • VXLAN Segment: A network-wide layer-2 domain implemented as an overlay network of VTEPs interconnected using VXLAN Tunnel Interfaces.


Ways to Implement VXLAN –

There are two ways to implement this based on the use cases in Data Centers:

  1. Bridging: When the two hosts communicating are on the same subnet and no gateways are required on the VTEPs. In this case, packets can be simply bridged over the VTIs from source VTEP to destination VTEP.
  2. Routing: When the two hosts communicating are on different subnet and gateway is required on the VTEP. A packet will be routed from the source VLAN to the destination VLAN on the first hop VTEP. And then will be bridged to remote VTEP.


VXLAN Control Plane Options –

  • Multicast
  • HER (Head End Replication) aka Ingress Replication in Cisco.

Sample Configuration of VXLAN Tunnel:

interface vxlan1

vxlan source-interface loopback <lo-int>     # Specifies the Source Interface for VXLAN traffic

vxlan udp-port 4789                                      # Destination UDP port for VXLAN Encap Traffic

vxlan vlan <vlan-id> vni <vni-id>                  # One to one mapping between VLAN and VNI

vxlan flood vtep <remote-vtep-ip> <remote-vtep-ip> # Floodlist to encapsulate BUM traffic and replicate to remote VTEPs.


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