Virtual eXtensible LAN (VXLAN)

VXLAN is a network encapsulation technique to extend VLAN over layer 3 network. It encapsulates an original layer 2 Ethernet frame within an UDP packet (MAC-in-UDP, over UDP port 4789) that runs as a virtual overlay on an existing Layer 3 network. VXLAN provides the same Ethernet Layer 2 network services as VLANs do.

           +----------+-------------+--------+----------+--------------------------+-----+
           | Ethernet |      IP     |  UDP   |  VXLAN   |         Original         | FCS |
           |  Header  |    Header   | Header |  Header  |      Ethernet Frame      |     |
           +----------+-------------+--------+----------+--------------------------+-----+
Bytes           14          20           8        24                                  4
About          VTEP        VTEP        4798       VNI          Original VLAN

           +-----------+-----------+-----------+-----------+
           |VXLAN Flag | Reserved  |    VNI    | Reserved  |
           +-----------+-----------+-----------+-----------+
Bits             8          24          24           8

VNI - VXLAN Network Identifier (or VXLAN Segment ID) functions like a VLAN ID.
VTEP - VXLAN Tunnel End Point. A device acts as an NVE (Network Virtualization Edge) to provide connectivity between overlay network and underlay networks for a VXLAN Segment.

BGP EVPN VXLAN

Q: Why is BGP EVPN needed for VXLAN?

VXLAN by itself does not provide optimal switching and routing capabilities in a network because it uses a “flood and learn” mechanism that limits scalability. "Flood and learn" mechanism is where the host’s information is flooded across the network for it to be reachable. To provide optimal switching and routing capabilities, a VXLAN overlay requires:

• An underlying transport network that performs data plane forwarding to allow unicast communication between end points connected to the fabric.

• A control plane that is capable of distributing Layer 2 and Layer 3 host reachability information across the network.

MP-BGP is a solutions to carry Layer 2 MAC and Layer 3 IP information. EVPN is the extension that allows BGP to transport Layer 2 MAC and Layer 3 IP information. This deployment is called a BGP EVPN VXLAN fabric.