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Troubleshooting Virtual Private Networks by Mark Lewis

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Technical Overview of L2F

This chapter focuses on in-depth troubleshooting of L2F. Fast and efficient troubleshooting requires a good knowledge of both L2F and its configuration, and to that end, a brief description of both follows.

As previously described, L2F is a protocol that allows the tunneling of PPP and SLIP frames across a IP backbone between a NAS and a Home Gateway. This chapter focuses on the tunneling of PPP frames because, to a great extent, PPP has superceded SLIP as the remote access protocol of choice.

The protocol itself uses the User Datagram Protocol (UDP), on top of which sits the L2F packet (header, payload, and optional CRC) itself.

Figure 2-2 illustrates the relationship between IP, UDP, and L2F.

Figure 2-2. Overall L2F ...

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