Troubleshooting of router link faults

A link is a physical line from a node to an adjacent node. It needs to provide data sending and receiving services at the physical layer, encapsulate data packets at the network layer, and negotiate network layer parameters, the sender and receiver determine the necessary information in data transmission by sending an LCP package.

1. When a vro shows that the WAN interface cannot be used normally, check the physical layer and run the display interface command to view the interface information, check whether the physical layer is normal based on the status of the hardware device and the LCP in the displayed information. In addition, Serial0 may be down, administratively down, and standby, where down indicates that the physical layer is abnormal, check the physical layer configuration and device problems. administratively down indicates that the physical layer is manually disabled. You can run the no shutdown command to manually open this port.

2. The physical layer does not send lowerup messages, the PPP does not send open messages, and the LCP should be in the initial state. If the physical layer sends lowerup messages, the PPP has sent open messages, the LCP of CONFREQ messages should be in the req-send status. If the physical layer sends lowerup messages, the PPP has sent open messages, and the CONFREQ and CONFACK messages are sent, the LCP should be in the ACKSENT status, if the physical layer sends lowerup messages and the PPP does not send open messages, the LCP should be in the starting status.

3. Execute the preceding command display interface bri 0 (BRI interface 0) or display interface serial 1 (serial Port 1). If the LCP protocol is not in the OPENED status, consider LCP. Generally, this problem rarely occurs. If yes, you should enable debug ppp packet or debug ppp negotiation. First, check whether the packets sent and received by the physical interface are normal. If you confirm that the packets sent and received by the interface are normal, in addition, there are a large number of CONFNAK and CONFREJ packets, or TERMACK, CODEREJ, and PROTREJ packets, which can be described as negotiation problems, analyze the reason for the negotiation failure based on the content of the message negotiation item.

4. Run the display interface command to view interface information. For example, if the LCP protocol enters the OPENED State, and the IPCP is still in the Initial state, or the LCP changes to the OPENED State, the negotiation starts again soon, it can be considered as a verification problem because the status is temporary and not easy to observe. You can also observe it through debug ppp packet or debug ppp negotiation, you can analyze the cause of verification failure based on the specific content of the message. Sometimes verification is configured, but this negotiation item is rejected during LCP negotiation. When LCP enters the OPENED state, it will immediately negotiate again.

5. Run the display interface command to view interface information. For example, if the LCP protocol enters the OPENED State, and the IPCP is in REQ_SEND or ACK_RCVD, check that there are a large number of IPCP packets sent and received by PPP packets, it indicates that there is a problem with router IPCP negotiation. If the IPCP is in the STOPPED status, it may also be because the TERMREQ or CODEREJ of IPCP is received, because the IPCP must negotiate with the parameter as an IP address, and others are optional parameters. Generally, there is a problem with the IP Address Configuration, IPCP negotiation fails.

Sometimes, after aaa-enable is configured on the vro, both LCP and IPCP are in the OPENED State, but soon start LCP negotiation again, because after aaa-enable is configured, billing is performed by default. If no billing server is set, AAA will end up the PPP link.