A physical link connecting two adjacent nodes is called a link. It requires the physical layer to provide data transmission and reception services, as well as functions such as data packet encapsulation and network layer parameter negotiation for the network layer. The sending and receiving ends exchange LCP packets to determine the necessary information for data transmission.
1. Use the display interface command to check interface information. Based on the hardware device status and LCP status in the displayed information, determine whether the physical layer is normal. Additionally, Serial0 may be in a down, administratively down, or standby state. down indicates an abnormal physical layer; you should check the physical layer configuration and device issues. administratively down means the physical layer has been manually shut down; in this case, execute the no shutdown command to manually enable this port.
2. LCP should be in the initial state. If the physical layer sends lowerup, PPP has sent an open message, and a CONFREQ packet is sent, LCP should be in the req-send state. If the physical layer sends lowerup, PPP has sent an open message, and both CONFREQ and CONFACK packets are sent, LCP should be in the ACKSENT state. If the physical layer sends lowerup but PPP has not sent an open message, LCP should be in the starting state.
3. If the LCP protocol is shown as not having entered the OPENED state, consider it an LCP problem. Issues in this area are generally rare. If they occur, enable debug ppp packet or debug ppp negotiation. First, check whether the physical interface is sending and receiving packets normally. If packet transmission is normal but a large number of CONFNAK, CONFREJ packets appear, or packets like TERMACK, CODEREJ, PROTREJ emerge, this indicates a negotiation problem. Then, analyze the reasons for negotiation failure based on the content of the packet negotiation items.
4. If the LCP protocol is shown as having entered the OPENED state, but IPCP remains in the Initial state, or LCP enters the OPENED state and then immediately restarts negotiation, consider this an authentication problem. Since this state is temporary and difficult to observe, you can also observe it through debug ppp packet or debug ppp