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http://sce.uhcl.edu/yang/teaching/.../VPN.ppt

15

Layer 2 Tunneling Protocol

PPP [RFC1661]

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PPP defines an encapsulation mechanism for transporting multiprotocol packets across layer 2 (L2) point-to-point links. That is, a tunneling protocol

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Used to tunnel PPP over a public network using IP

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Typically, a user obtains a L2 connection to a Network Access Server (NAS) using one of a number of techniques (e.g., dialup POTS, ISDN, ADSL, etc.) and then runs PPP over that connection.

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In such a configuration, the L2 termination point and PPP session endpoint reside on the same physical device (i.e., the NAS).

L2TP

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L2TP extends the PPP model by allowing the L2 and PPP endpoints to reside on different devices interconnected by a packet-switched network.

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With L2TP, a user has an L2 connection to an L2TP access concentrator (LAC, e.g., modem bank, ADSL DSLAM, etc.), and the concentrator then tunnels individual PPP frames to the NAS.  (See Fig. 12-1)

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This allows the actual processing of PPP packets to be divorced from the termination of the L2 circuit.

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