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Troubleshooting Virtual Private Networks by Mark Lewis

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L2TPv2 Technical Overview

This section provides an overview of the operation of the L2TP. Note that although voluntary tunnel mode is discussed, the focus remains on compulsory tunnel mode.

L2TP uses two kinds of messages:

  • Control messages— Used in L2TP tunnel and session setup, maintenance, and teardown. Control messages are reliably transmitted in-band over a control connection between the LAC and the LNS. Only one control connection is required for each L2TP tunnel regardless of the number of data sessions.

  • Data messages— Used to transport PPP frames between the LAC and the LNS. Each PPP connection is carried over a separate data session within the L2TP tunnel.

Note that many PPP connections can be carried over a single L2TP tunnel between ...

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