Authentication failures can make a perfectly functional link appear faulty, and given the ease
with which one can mis-enter a password or username, it is one of the most common issues. The
debug ppp authentication command is useful for resolving these problems.
Examine the following output from the debug session. The ISDN BRI attempted to connect,
but the challenge failed and the link was disconnected immediately. The second packet attempted
to restore the link (response id 8) and also failed. This type of output points to either a username
or password problem—in this case, the password was incorrect:
Router#debug ppp authentication
01:54:14: %LINK-3-UPDOWN: Interface BRI0:1, changed state to up.
01:54:14: BR0:1 PPP: Treating connection as a callout
01:54:14: BR0:1 PPP: Phase is AUTHENTICATING, by both
01:54:14: BR0:1 CHAP: O CHALLENGE id 7 len 27 from "Router"
01:54:14: BR0:1 CHAP: I CHALLENGE id 7 len 24 from "Top"
01:54:14: BR0:1 CHAP: O RESPONSE id 7 len 27 from "Router"
01:54:14: BR0:1 CHAP: I FAILURE id 7 len 25 msg is "MD/DES
compare failed"
01:54:15: %ISDN-6-DISCONNECT: Interface BRI0:1
disconnected from 18008358661 , call lasted 1 seconds
01:54:15: %LINK-3-UPDOWN: Interface BRI0:1, changed state to down.
01:54:18: %LINK-3-UPDOWN: Interface BRI0:1, changed state to up.
01:54:18: BR0:1 PPP: Treating connection as a callout
01:54:18: BR0:1 PPP: Phase is AUTHENTICATING, by both
01:54:18: BR0:1 CHAP: O CHALLENGE id 8 len 27 from "Router"
01:54:18: BR0:1 CHAP: I CHALLENGE id 8 len 24 from "Top"
01:54:18: BR0:1 CHAP: O RESPONSE id 8 len 27 from "Router"
01:54:18: BR0:1 CHAP: I FAILURE id 8 len 25 msg is "MD/DES
compare failed"
01:54:19: %ISDN-6-DISCONNECT: Interface BRI0:1 disconnected
from 18008358661 , call lasted 1 seconds
01:54:19: %LINK-3-UPDOWN: Interface BRI0:1, changed state to down.
01:54:22: %LINK-3-UPDOWN: Interface BRI0:1, changed state to up.
The debug ppp authentication command is most helpful in troubleshooting password
problems. As shown in the preceding example, the message I FAILURE id 8 len 25 msg
is "MD/DEScompare failed” is a clear indication that the administrator should look at the
password settings.