A link is a physical circuit segment from one node to an adjacent node. It relies on the physical layer to provide data transmission and reception services, and provides the network layer with functions such as data packet encapsulation and network-layer parameter negotiation. The sender and receiver determine the necessary information for data transmission by exchanging LCP packets.
1. When a router’s WAN interface cannot be used normally, you should first check from the physical layer. Use the display interface command to view interface information, and determine whether the physical layer is normal based on the hardware device status and LCP status shown in the output. Additionally, Serial0 may show as down, administratively down, or standby. “down” indicates that the physical layer is working abnormally, and you should check the physical layer configuration and device issues. “administratively down” indicates that the physical layer has been manually shut down; you can execute the no shutdown command to manually open this port.
2. If the physical layer has not sent lowerup, PPP has not sent an open message, and LCP should be in the initial state. If the physical layer has sent lowerup, PPP has sent an open message and sent a CONFREQ packet, LCP should be in the req-send state. If the physical layer has sent lowerup, PPP has sent an open message and sent both a CONFREQ packet and a CONFACK packet, LCP should be in the ACKSENT state. If the physical layer has sent lowerup but PPP has not sent an open message, LCP should be in the starting state.
3. Execute the commands display interface bri 0 (for BRI interface 0) or display interface serial 1 (for Serial port 1). If the output shows that the LCP protocol has not entered the OPENED state, an LCP problem can be considered. Problems in this area are generally rare. If they occur, you should enable debug ppp packet or debug ppp negotiation. First, check whether packet sending and receiving on the physical interface are normal. If you confirm that the interface packet sending and receiving are normal but a large number of CONFNAK and CONFREJ packets appear, or packets like TERMACK, CODEREJ, and PROTREJ appear, it indicates a negotiation problem. Then, analyze the reasons for the unsuccessful negotiation based on the content of the packet negotiation items.
4. Use the display interface command to view interface information. If the output shows that the LCP protocol has entered the OPENED state but IPCP remains in the Initial state, or LCP enters the OPENED state but then quickly restarts negotiation, an authentication problem can be considered. Because this is a transient state and difficult to observe directly, you can also observe it through debug ppp packet or debug ppp negotiation, and analyze the reason for authentication failure based on the specific content of the packets. Sometimes authentication is configured, but the negotiation item is rejected during the LCP negotiation process, causing LCP to immediately renegotiate after entering the OPENED state.
5. Use the display interface command to view interface information. If the output shows