Cisco IOS Cookbook, 2nd Edition

Debugging Dial Backup

Problem

Your dial backup is not behaving properly and you want to debug it to isolate and resolve the problem.

Solution

The most common reasons for failed dial backup calls are incorrect dial strings and PPP authentication problems. You can easily diagnose both of these problems with this command:

Router1#debug ppp authentication

Here is another useful command for diagnosing problems with dialer configurations:

Router1#debug dialer

Discussion

When you use CHAP authentication with PPP, as we have done throughout this chapter, it is relatively easy to debug most common problems. We like to use the debug ppp authentication command because it pinpoints the most frequent problems:

Jun 28 14:04:05.211: BR0/0:1 PPP: Phase is AUTHENTICATING, by both
Jun 28 14:04:05.211: BR0/0:1 CHAP: O CHALLENGE id 1 len 33 from "Router1"
Jun 28 14:04:05.211: BR0/0:1 AUTH: Started process 0 pid 60
Jun 28 14:04:05.235: BR0/0:1 CHAP: I CHALLENGE id 35 len 33 from "dialhost"
Jun 28 14:04:05.235: BR0/0:1 CHAP: O RESPONSE id 35 len 33 from "Router1"
Jun 28 14:04:05.267: BR0/0:1 CHAP: I SUCCESS id 35 len 4
Jun 28 14:04:05.271: BR0/0:1 CHAP: I RESPONSE id 1 len 33 from "dialhost"
Jun 28 14:04:05.271: BR0/0:1 CHAP: O SUCCESS id 1 len 4

This output shows the PPP Authentication challenge and response handshake between two routers. This is one of the first places to look when you have dialup sessions that refuse to connect. In this particular trace, the router Router1 has dialed to dialhost. Upon connection, ...