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I'm trying to understand how a receiver knows that the received packet is a VXLAN-encapsulated.

Looking at the VXLAN RFC, I can't see anything specific that defines that the packet contains VXLAN header. I see (parsing the packet from the beginning):

  • outer MAC (L2) header
  • outer IP (L3) header
  • outer UDP (L4) header
  • VXLAN header
  • inner MAC header
  • inner ... etc

But I don't see anything specific in the outer UDP header that says that the next header is VXLAN.

What am I missing?

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  • Linux system uses its default UDP 8972 port number for VXLAN because VXLAN was implemented on it before IANA has assigned a udp port neumber for VXLAN, whereas other systems use IANA assigned VXLAN UDP 4789 port number.
    – bear
    Commented Feb 21, 2021 at 14:18

1 Answer 1

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The destination UDP port in the outer UDP header is specified in the VXLAN specification (Port 4789). This means it is a well-known service. So an UDP packet that arrives on Port 4789 is expected to be a VXLAN packet¹ in the same way that a TCP packet that arrives on Port 80 is expected to be a HTTP packet¹.

The draft you linked to is outdated and is missing this port number (although it mentions that the port number is to be obtained from IANA).

¹) When I talk about VXLAN/HTTP packets I mean of course the respective UDP/TCP packets with VXLAN/HTTP header/protocol inside.

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  • Thanks! My problem was an outdated RFC, indeed. Note however, that this whole mess with RFC versions and port numbers caused a mess in VXLAN support. Turns out that before port 4789, there was port 8472, and different implementations might still have this old port (such as VMvare ESX).
    – kliteyn
    Commented Apr 25, 2014 at 13:53
  • Another implementation that uses 8472 is flannel github.com/flannel-io/flannel
    – morhook
    Commented Jun 22, 2021 at 16:11

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