Troubleshooting PPP

Table 4.6 shows several commands you can use to troubleshoot a PPP connection.

Table 4.6. PPP Troubleshooting Commands

PPP Troubleshooting Command	Explanation
show dialer	This command gives basic information about calls, including successes and attempts for a particular phone number, as well as current status and, if up, what brought a link up.
show ppp multilink	This command gives the current status and configuration of multilink bundles.
debug ppp negotiation	This command shows what PPP interaction is happening during the negotiation phase. To debug successfully, it is best to turn on debugging before bringing up the link.
debug ppp authentication	This debug only shows PAP and CHAP authentication information.
debug ppp packet	This command shows what PPP-oriented packets are being sent.
debug ppp multilink	This command gives packet fragmentation information regarding a multilink bundle. Because the information is almost real time and a link often has traffic, this command is not a command to use frequently in a production environment. Consider CPU utilization before debugging multilink.
debug ppp multilink negotiation	This command gives information about the status of a forming multilink bundle.

debug ppp negotiation

Listing 4.1 is output from a router negotiating link establishment. Notice that when the interface is started, PPP becomes active and the LCP process begins negotiation. The last part of the LCP is authentication where you can see the names of the devices that are trying to talk. Once the devices authenticate, they need to figure out which control programs to use. At the bottom of the display, you'll notice that IP and CDP are the protocols that can be used between the routers.

Listing 4.1 Debugging PPP negotiation

 0:06:37: %LINK-3-UPDOWN: Interface Serial1/1, changed state to up 00:06:37: Se1/1 PPP: Treating connection as a dedicated line 00:06:37: Se1/1 PPP: Phase is ESTABLISHING, Active Open 00:06:37: Se1/1 LCP: O CONFREQ [Closed] id 5 len 15 00:06:37: Se1/1 LCP:    AuthProto CHAP (0x0305C22305) 00:06:37: Se1/1 LCP:    MagicNumber 0x505AB72C (0x0506505AB72C) 00:06:37: Se1/1 LCP: I CONFREQ [REQsent] id 39 len 15 00:06:37: Se1/1 LCP:    AuthProto CHAP (0x0305C22305) 00:06:37: Se1/1 LCP:    MagicNumber 0x5056251B (0x05065056251B) 00:06:37: Se1/1 LCP: O CONFACK [REQsent] id 39 len 15 00:06:37: Se1/1 LCP:    AuthProto CHAP (0x0305C22305) 00:06:37: Se1/1 LCP:    MagicNumber 0x5056251B (0x05065056251B) 00:06:37: Se1/1 LCP: I CONFACK [ACKsent] id 5 len 15 00:06:37: Se1/1 LCP:    AuthProto CHAP (0x0305C22305) 00:06:37: Se1/1 LCP:    MagicNumber 0x505AB72C (0x0506505AB72C) 00:06:37: Se1/1 LCP: State is Open 00:06:37: Se1/1 PPP: Phase is AUTHENTICATING, by both 00:06:37: Se1/1 CHAP: O CHALLENGE id 2 len 25 from "p1r2" 00:06:37: Se1/1 CHAP: I CHALLENGE id 3 len 25 from "p1r3" 00:06:37: Se1/1 CHAP: O RESPONSE id 3 len 25 from "p1r2" 00:06:37: Se1/1 CHAP: I RESPONSE id 2 len 25 from "p1r3" 00:06:37: Se1/1 CHAP: O SUCCESS id 2 len 4 00:06:37: Se1/1 CHAP: I SUCCESS id 3 len 4 00:06:37: Se1/1 PPP: Phase is UP 00:06:37: Se1/1 CDPCP: O CONFREQ [Closed] id 3 len 4 00:06:37: Se1/1 IPCP: I CONFREQ [Not negotiated] id 3 len 10 00:06:37: Se1/1 IPCP:    Address 192.168.1.2 (0x0306C0A80102) 00:06:37: Se1/1 LCP: O PROTREJ [Open] id 6 len 16 protocol IPCP (0x80210103000A0306C0A80102) 00:06:37: Se1/1 CDPCP: I CONFREQ [REQsent] id 3 len 4 00:06:37: Se1/1 CDPCP: O CONFACK [REQsent] id 3 len 4 00:06:37: Se1/1 CDPCP: I CONFACK [ACKsent] id 3 len 4 00:06:37: Se1/1 CDPCP: State is Open 00:06:38: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1/1, changed state to up 

debug ppp authentication

The debug ppp authentication command provides a subset of the data shown in debug ppp negotiation. Only the authentication phase is shown when you use this command. Compare the output in Listing 4.2 to the preceding output.

Listing 4.2 Debugging PPP authentication

 00:08:19: %LINK-3-UPDOWN: Interface Serial1/1, changed state to up 00:08:19: Se1/1 PPP: Treating connection as a dedicated line 00:08:21: Se1/1 PPP: Phase is AUTHENTICATING, by both 00:08:21: Se1/1 CHAP: O CHALLENGE id 3 len 25 from "p1r2" 00:08:21: Se1/1 CHAP: I CHALLENGE id 4 len 25 from "p1r3" 00:08:21: Se1/1 CHAP: O RESPONSE id 4 len 25 from "p1r2" 00:08:21: Se1/1 CHAP: I RESPONSE id 3 len 25 from "p1r3" 00:08:21: Se1/1 CHAP: O SUCCESS id 3 len 4 00:08:21: Se1/1 CHAP: I SUCCESS id 4 len 4 00:08:22: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1/1, changed state to up