Troubleshooting PPP configuration

PPP authentication always fails, preventing the link from going up.

This problem might occur if the parameters for authentication are incorrect.

• Enable the debugging of PPP, and you can see the information describing that LCP went up upon a successful LCP negotiation but went down after PAP or CHAP negotiation.

• Check the PPP authentication settings at the local and peer ends to make sure that they are consistent. See "Configuring PAP authentication" and "Configuring CHAP authentication" for reference.

Physical link is always down.

Check the following:

• The interface has been brought up.

• The interface has not been administratively shut down.

• LCP negotiation has passed.

Execute the display interface serial command to check the state of the interface. The output can be as follows:

• serial number is administratively down, line protocol is down—Indicates that the interface is shut down by the administrator.

• serial number is down, line protocol is down—Indicates that the interface is not activated or the physical layer has not gone up yet.

• serial number is up, line protocol is up—Indicates that LCP negotiation succeeded.

• serial number is up, line protocol is down—Indicates that the interface is active, but LCP negotiation failed.

Configure an IPv6 address on an interface where PPP encapsulation is enabled but IPv6 is disabled. The PPP link fails IPv6CP negotiation and cannot go up. After enabled with IPv6, the interface still cannot go up.

IPv6CP negotiation cannot succeed when IPv6 is disabled. As IPv6CP does not support renegotiation, IPv6CP negotiation cannot succeed even if you enable IPv6 subsequently.

Do the following:

• Enable IPv6 before configuring an IPv6 address on a PPP link.

• If IPv6CP negotiation fails, re-enable the interface by executing the shutdown command and then the undo shutdown command to re-enable IPv6CP negotiation.