L2TP Question

gojericho0gojericho0 Senior MemberMember Posts: 1,059 ■■■□□□□□□□
I was reading some of the Network+ technotes regarding the Remote Access Protocols and came across this:
L2TP operates at the Data-Link layer of the OSI-model and uses UDP port 1701.

Could someone explain to me how this protocol operates at the Data-Link layer, but also uses the Transport/App Layer as well because it kind of confused me?

Does this mean the whole L2TP packet only provides application data to the p2p device (vpn concentrator, router/firewall) that is receiving it? Then discarded like a normal frame on its way to a different network?



  • tonygtonyg Junior Member Member Posts: 7 ■□□□□□□□□□
    According to http://en.wikipedia.org/wiki/Layer_2_Tunneling_Protocol L2TP actually works at the session layer, layer 5.

    As for me, I am sticking with Layer 2. That is all I need to know for testing purposes.

    L2TP acts like a data link layer (layer 2 of the OSI model) protocol for tunneling network traffic between two peers over an existing network (usually the Internet).

    L2TP is in fact a layer 5 protocol session layer, and uses the registered UDP port 1701.

    The entire L2TP packet, including payload and L2TP header, is sent within a UDP datagram. It is common to carry Point-to-Point Protocol (PPP) sessions within an L2TP tunnel. L2TP does not provide confidentiality or strong authentication by itself. IPsec is often used to secure L2TP packets by providing confidentiality, authentication and integrity. The combination of these two protocols is generally known as L2TP/IPsec (discussed below).

    The two endpoints of an L2TP tunnel are called the LAC (L2TP Access Concentrator) and the LNS (L2TP Network Server). The LAC is the initiator of the tunnel while the LNS is the server, which waits for new tunnels. Once a tunnel is established, the network traffic between the peers is bidirectional. To be useful for networking, higher-level protocols are then run through the L2TP tunnel. To facilitate this an L2TP session (or call) is established within the tunnel for each higher-level protocol such as PPP. Either the LAC or LNS may initiate sessions. The traffic for each session is isolated by L2TP, so it is possible to set up multiple virtual networks across a single tunnel. MTU should be considered when implementing L2TP.

    The packets exchanged within an L2TP tunnel are categorised as either control packets or data packets. L2TP provides reliability features for the control packets, but no reliability for data packets. Reliability, if desired, must be provided by the nested protocols running within each session of the L2TP tunnel. Cisco has the software patent related to L2F and L2TP [1]. It has been assigned U.S. Patent 5,918,019 in the United States
  • gojericho0gojericho0 Senior Member Member Posts: 1,059 ■■■□□□□□□□
    I also found this site...I think it is considered layer 2 because it encapsulates (tunnels) at layer two then gets sent back up the TCP stack to add the higher layers. The part I do not understand now is if a normal packet is encapsulated by L2TP and then by IPsec wouldn't the packet size be much larger than any MTU to handle?


    Would the entire packet have to be sent in fragments?[/url]
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