PPP and Layer 2 Considerations for Routed and Routing Protocols
Link Control Protocol

Most of the problems that occur with PPP involve the link negotiation. Typically the problems can be classified as follows:

  • Authentication
  • Incorrect encapsulation at the far end
  • Conflicting PPP parameters

The first step in troubleshooting a PPP problem is to check that the appropriate encapsulation is in use with the show interfaces serial command.

Assuming that the correct encapsulation is in use, the next step is to confirm that the Link Control Protocol (LCP) negotiations have succeeded by checking the output for LCP Open. Finally, the Layer 3 component can be verified by checking for Open:IPCP or whatever other Layer 3 protocol is expected.

If one of these elements does not appear correct, obtain a detailed debug of the negotiation process using the command debug ppp negotiation. Figure explains in detail each step in a successful PPP link establishment.

One of the most likely problems with bringing up a link is an authentication failure. This will be covered in the next sections.