3.4 PPP Compression
3.4.5 Verifying and troubleshooting PPP
This section introduces tools for verifying and troubleshooting a PPP session. One way to determine whether PAP or CHAP authentication was passed is to use the show dialer command. This command must be used to view the progress of asynchronous dialup connections.

If show dialer displays the name of the remote router, it means that you passed PAP or CHAP authentication, as shown in Figure .

You can check the show dialer command on both routers to verify that the name of the other router is displayed. If it is, then you know that PAP or CHAP authentication is working.

If you do not see the name of the other host, you know that something was misconfigured because the authentication failed in at least one direction.

The debug ppp negotiation command is a great tool for troubleshooting the PPP LCP activities such as authentication, compression, and multilink. When the LCP is in OPEN state, the NCP negotiation takes place. In the following output, you can observe that for PPP to work, LCP options must be negotiated before any NCP activities take place. You therefore see the negotiations shown in Figure :

  • CHAP authentication;
  • Compression Control Protocol (CCP);
  • NCP protocols IP, IPX, and Cisco Discovery Protocol (CDP).