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PPTP, L2TP, PPPOE - these are tunneling protocols. And they are Data link Layer protocols. Why? As I understood, Layer2 protocols work only in same subnet. They don't use IP addresses and cannot be routed. They operate only with MAC addresses. These Layer2 members: LACP, WOL, CDP, Trunk/VLAN don't use IP addresses! Ofcource some of them could be routed using some broadcasting technologies. However, PPTP, PPPOE and L2TP protocols use IP addresses. Moreover, they use TCP ports, higher OSI layer. Why?

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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 post and accept your own answer.
    – Ron Maupin ♦
    Commented Mar 16, 2021 at 20:54

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PPTP, PPPoE, and L2TP all provide OSI Layer 2 services. That is, the user of these protocols (usually, a network layer protocol suite) thinks it's running over a "normal" link layer. However, each of these protocols provides the link layer service by transporting packets over another service, rather than over the physical layer.

  • PPTP provides PPP (link layer) service by running over TCP (transport layer).
  • PPPoE provides PPP (link layer) service by running over Ethernet (a different link layer)
  • L2TP provides PPP (link layer) service by running over UDP (transport layer).

So, each of these protocols plays two different roles. Each provides a service that's a link-layer service, so looking at it from above, it's a "link layer service provider". But each runs atop a different service, so, looking from below, it's a transport service user, network service user, or link service user.

Phrases like "X service user" and "X service provider" are generally more useful than the less distinct phrase "in the X layer". This user/provider distinction is an important part of the OSI layer model, though it may not be obvious from some tutorials.

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Note that layer three and above protocols can be encapsulated inside the protocols you mention but not layer two (or below). One way to look at it is that you build a protocol stack from the bottom up. So first we put down layers 1, 2, and 3. Then for layer 4 we put PPTP (for example) and that starts a NEW layer 2 on top of (inside of) the "real" layers 1, 2, and 3. Then you can put yet another layer 3 on top of the new PPTP layer two and go on up from there.

That's not exactly what's going on but remember that the OSI model is only a MODEL. And technically the OSI model was meant to be used with OSI protocols, which most commonly used protocols are not. As a way to understand networking in general, the OSI model helps a lot, even with non-OSI protocols, which is why it is used by so many networking professionals in many areas. But again, it's only a model, so it helps us get a grip on what's happening but it does not rigidly define what is happening, nor does it 100% reflect what is happening.

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  • From your words I understood,
    – Алдар
    Commented Mar 22, 2015 at 7:32
  • From your words I understood, for example, L2TP is a Session Layer Protocol it uses UDP 1701 port, and it creates 2nd Layer inside of the tunnel. However, inside the tunnel there is no Layer2. Despite it creates one subnet, theare are no MAC addresses. Correct me if I wrong. And no other Layer 2 protocols may be used inside.
    – Алдар
    Commented Mar 22, 2015 at 8:13
  • I'm afraid I don't fully understand your comment. I want to say the tunnel itself acts as a layer 2 in that layer 3 packets may be encapsulated by L2TP. The OSI model does not specify any addressing for layer 2, and not all protocols that are modeled best by layer 2 even use addresses (HDLC for example). Again, don't get too hung up on the model. It's only a model. If you want to more deeply understand L2TP or PPTP, it's better to get a sense of what is encapsulated inside those packets and how those packets are encapsulated in turn.
    – Todd Wilcox
    Commented Mar 22, 2015 at 14:02
  • This might help: en.wikipedia.org/wiki/Tunneling_protocol - Read the "Technical Overview" section.
    – Todd Wilcox
    Commented Mar 22, 2015 at 14:04
  • Just picking a nit: the OSI model preceded the development of the OSI protocols, and was intended to be general and not specific to the OSI protocol suite. Also, there is layer-jumbling even in OSI protocols, such as the use of OSI Connection-oriented network service to act as a link layer under the OSI connectionless network protocol. So, even in OSI, the layer model isn't pure.
    – Jeff Learman
    Commented Mar 16, 2020 at 15:56

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