When PPP is configured, the link layer protocol cannot change to Up.
Run the display this interface command in the interface view to check the status of the LCP. If the status is opened, it indicates that the LCP negotiation succeeds; otherwise, it indicates that the LCP negotiation fails.
The possible causes that the PPP authentication fails are as follows:
The interfaces at both ends of the PPP link must be configured with the same parameters and authentication modes. In this manner, the LCP negotiation succeeds.