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Dell Configuration Guide for the S4048–ON System 9.14.2.6

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VXLAN Frame Format

VXLAN provides a mechanism to extend an L2 network over an L3 network. In short, VXLAN is an L2 overlay scheme over an L3 network and this overlay is termed as a VXLAN segment. VMs within the same VXLAN segment communicate with each other through a 24–bit ID called VXLAN Network Identifier (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 traffic cross-over

Components of VXLAN Frame Format

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 figure:

Figure 1. VXLAN Frame Format
VXLAN Frame Format

Some of the important fields 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 first hop router's MAC address when the VTEP is on a different 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 field is transmitted as zero. When a packet is received with a UDP checksum of zero, it is accepted for decapsulation.

VXLAN Header :

  • VXLAN Flags : Reserved bits set to zero except bit 3, the first bit, which is set to 1 for a valid VNI

  • VNI: The 24-bit field that is the VXLAN Network Identifier

  • Reserved: A set of fields, 24 bits and 8 bits, that are reserved and set to zero .

Frame Check Sequence (FCS):
Note that the original Ethernet frame's FCS is not included, but new FCS is generated on the outer Ethernet frame.

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