Concept Guide
called VXLAN Network Identier (VNI). This 24–bit ID allows up to 16 million VXLAN segments to coexist within the same administrative
domain. The VNI scopes the inner MAC frame originated by the VM. This model has over-lapping MAC addresses without trac cross-
over
VXLAN achieves L2 over L3 overlay using the tunneling mechanism. VTEPs are the end points of a tunnel located within the hypervisor on
the server that hosts VMs. The VXLAN frame format is shown in the following gure:
Figure 156. VXLAN Frame Format
Components of VXLAN Frame Format
Some of the important elds of the VXLAN frame format are described below:
Outer Ethernet
Header:
The Outer Ethernet Header consists of the following components:
• Destination Address: Generally, it is a rst hop router's MAC address when the VTEP is on a dierent address.
• Source Address : It is the source MAC address of the router that routes the packet.
• VLAN: It is optional in a VXLAN implementation and will be designated by an ethertype of 0×8100 and has an
associated VLAN ID tag.
• Ethertype: It is set to 0×0800 because the payload packet is an IPv4 packet. The initial VXLAN draft does not
include an IPv6 implementation, but it is planned for the next draft.
Outer IP Header: The Outer IP Header consists of the following components:
• Protocol: It is set to 0×11 to indicate that the frame contains a UDP packet .
• Source IP: It is the IP address of originating VTEP.
• Destination IP: : It is the IP address of target VTEP.
Outer UDP Header: The Outer UDP Header consists of the following components:
• Source Port: Entropy of the inner frame. The entropy could be based on the Inner L2 header or Inner L3
header.
• VXLAN Port : IANA-assigned VXLAN Port (4789).
• UDP Checksum: The UDP checksum eld is transmitted as zero. When a packet is received with a UDP
checksum of zero, it is accepted for decapsulation.
1146 Virtual Extensible LAN (VXLAN)