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My naive understanding of how the internet works is as follow :

Physical layer (1)
Link layer (2) - Inside frames
IP layer (3) - Inside IPv4 or IPv6 packets
Transport layer (4) - Inside TCP or UDP
Application layer

Where does L2TP protocol stands? The name suggests it is a layer 2 protocol, but the wiki article says L2TP packets are inside UDP datagrams. This, to me, suggests it is an application protocol.

Is L2TP a layer 2 protocol? Is it using another "kind of frame" different thant the common one I am used to? Or is it actually an application protocol contained within "normal" frames, IP packets and UDP datagrams?

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  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 14 '17 at 22:44
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Fixed layer count models and tunneling don't match up very well.

Our stack might look something like.

  • Appliction data.
  • TCP (inner network)
  • IP (inner network)
  • Ethernet framing (inner network)
  • L2TPv3
  • UDP (outer network)
  • IP (outer network)
  • Ethernet framing (outer network)
  • Ethernet medium access control
  • Ethernet physical

What layer we regard L2TP as depends on what perspective we are looking from. From the perspective of the "outer" network it looks like an application protocol. From the perfective of the "inner" network it looks like an Ethernet port.

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Tunneling protocols like L2TP, GRE, IPSEC and MPLS don't fit well into the OSI model. L2TP tunnels (encapsulates) L2 frames in UDP so it can be transported over layer 3.

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  • Not sure I understand your answer. So it is actually not the kind of frame that is normally used in a standard communication? – Gradient Jul 28 '16 at 16:47
  • I'm saying that trying to understand tunneling protocols in the context of OSI will only confuse you. L2TP is as standard as any other protocol. – Ron Trunk Jul 28 '16 at 16:51
  • So where does it stands? When L2TP is involved in a communication, are notions like MAC addresses, IP addresses, ports, etc. involved somewhere? – Gradient Jul 28 '16 at 16:53
  • Of course. The L2 frame (including MAC addresses are encapsulated in a L3 packet. The L3 packets are themselves encapsulated in separate L2 packets. – Ron Trunk Jul 28 '16 at 16:57
  • Where is L2TP in that? – Gradient Jul 28 '16 at 17:00

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