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.