Troubleshooting guide

dialer 1 configuration.
If your ISP is sending the I CONFNAK, look for commands under interface dialer 1 that match the line
following the CONFNAK and remove them. If the Cisco DSL Router is sending the O CONFNAK, add a
command to interface dialer 1 to properly negotiate PPP with your ISP. In the case of the router sending
packets, you may need to call the Cisco TAC to determine which command(s) need to be enabled on the Cisco
DSL Router.
Authentication Failure
An authentication failure occurs when your ISP is unable to authenticate your PPP username or password.
There are two scenarios in which this may occur. The first scenario is an authentication type mismatch, which
is caused when you don't properly configure the router. All the authentication configurations listed in this
document account for both PAP and CHAP authentication types. For configuration flexibility, you should
have both CHAP and PAP configured. If you don't have both configured, you may see output from a debug
ppp command like the following:
Router#debug ppp negotiation
00:34:29: Vi1 LCP:OCONFREQ [REQsent] id 53 Len 15
00:34:29: Vi1 LCP: AuthProto CHAP (0x0305C22305)
!−−− Sending CHAP requests.
00:34:29: Vi1 LCP: MagicNumber 0x01B63483 (0x050601B63483)
00:34:29: Vi1 LCP: I CONFREQ [REQsent] id 252 Len 14
00:34:29: Vi1 LCP: AuthProto PAP (0x0304C023)
!−−− Receiving PAP requests from the service provider.
00:34:29: Vi1 LCP: MagicNumber 0xBC5233F9 (0x0506BC5233F9)
00:34:29: Vi1 LCP: O CONFREJ [REQsent] id 252 Len 8
Router#undebug all
or
Router# debug ppp negotiation
00:45:44: Vi1 LCP: I CONFREQ [Listen] id 141 Len 15
00:45:44: Vi1 LCP: AuthProto CHAP (0x0305C22305)
!−−− Receiving CHAP requests from the service provider.
00:45:44: Vi1 LCP: MagicNumber 0xBC5C7DDC (0x0506BC5C7DDC)
00:45:44: Vi1 LCP: O CONFREQ [Listen] id 255 Len 14
00:45:44: Vi1 LCP: AuthProto PAP (0x0304C023)
!−−− Sending out PAP requests.
Router#undebug all
!−−− Turn off ppp debug.
To correct both authentication mismatch problems, refer to the appropriate PPPoA implementation option
configuration and reconfigure PPP authentication.
The second authentication problem scenario you may encounter is an incorrect PAP username or password.
To determine if this is the problem, issue the command debug ppp negotiation. Assuming your router is
configured for both Challenge Handshake Authentication Protocol (CHAP) and Password Authentication
Protocol (PAP), as the configuration outlined earlier in this guide shows, your ISP may not be using PAP
authentication.
To determine the authentication used by your ISP, check the options in the I CONFREQ packet sent to you
from your ISP. If this packet is followed by an option called AuthProto PAP, you are using PAP. If the I
CONFREQ is followed by an option called AuthProto CHAP, you are using CHAP and should proceed to
How do I know if my CHAP username and password are correct?
Cisco DSL Router Configuration and Troubleshooting Guide