| Now that you have reviewed the list of symptoms and the checklist of potential problems associated with each symptom, walk step by step through the directions for how to handle each item on the checklist. The following sections offer step-by-step instructions for troubleshooting these serial link problems: -
Cable faults causing the loss of the Carrier Detect (CD) -
Faulty cabling -
Incorrect Interface configurations -
Keepalive problems -
Circuit is in loopback mode -
Interface has not been placed in shutdown state -
Line speed overloads the router's capacity -
Interface speed is higher than the line speed can support Cable Faults Causing Loss of the Carrier Detect To troubleshoot this problem, use the following steps: -
Check the CD, RX, and TX on the CSU/DSU to verify the circuit is transmitting and receiving data. -
Contact the local service provider to check for problems and for help in troubleshooting the problem. Faulty Cabling To troubleshoot this problem, use the following steps: -
Make sure that you are using the proper cable for the equipment being used. -
Use a breakout box or other cable tester to verify the throughput on the cables. -
Replace the faulty cables. Incorrect Interface Configurations To troubleshoot this problem, use the following steps: -
Use the show running-config command to verify the proper configuration. -
Verify the proper configuration for the interfaces, both on the problem device and on the device at the opposite end of the link. -
Use the show interface command (discussed previously in this chapter) to verify the proper configuration of the interface, including the encapsulation type being used. Keepalive Problems To troubleshoot this problem, use the following steps: -
Use the show interface command to verify that keepalives are being sent. -
Use the debug serial interface command to view the keepalive sending process. The output for this command is shown in the next section, "Serial Link Troubleshooting Commands." -
Verify that sequence numbers are incrementing properly. -
If the sequence numbers are not incrementing, you can run loopback tests at the local or the remote ends of the circuit.  | If you still cannot get the sequence numbers to increment, even when the CSU/DSU is in loopback, you most likely have a hardware or cabling problem where the faulty hardware resides on either the router or the CSU/DSU. |
Circuit Is in Loopback Mode To troubleshoot this problem, use the following steps: -
Use the show interface command to check the interface configuration. -
Use the no command to disable any loopback entries in the interface configuration. An example would be to the no interface loopback 0 command used in global configuration mode. -
Check the CSU/DSU to make sure that it is not in loopback mode. -
Check with the local service provider for possible problems and to make sure that the circuit is not in loopback mode. Interface Hasn't Been Placed in Shutdown State To troubleshoot this problem, use the following steps: -
Use the show interface command to verify the interface has not been manually shutdown. -
Check to make sure that the IP address is not being used on another interface. -
In interface configuration mode for the interface having problems, use the no shut command. Line Speed Overloads the Router's Capacity To troubleshoot this problem, use the following steps: -
Use the hold-queue command to reduce input queue size. This command allows you to buffer data before it is sent on the link. -
Increase the output queue's size on the other interfaces. Interface Speed Is Higher Than the Line Speed To troubleshoot this problem, use the following steps: -
Implement Quality of Service (queuing) to control traffic. More on queuing can be found in Appendix C, "Policy Networking." -
Increase the output queue size. -
Reduce the broadcast traffic that utilizes the link, if possible.  |
