Possible Causes:

1. Telephone company problem---Line down; line not connected to CSU/DSU
2. Faulty or incorrect cabling
3. Hardware failure (CSU/DSU)

Suggested Actions:

Step 1 Check the LEDs on the CSU/DSU to see if CD is active, or insert a breakout box on the line to check for the CD signal.

Step 2 Verify that you are using the proper cable and interface (see your hardware installation documentation)

Step 3 Insert a breakout box; check all control leads.

Step 4 Contact your leased-line or other carrier service.

Step 5 Swap faulty parts.

Step 6 If you suspect faulty router hardware, change the serial line to another port. If the connection comes up, the previously connected interface has a problem.

1. Local or remote router misconfigured
2. Keepalives not being sent by remote router
3. Leased-line or other carrier service problem---noisy line; misconfigured or failed switch
4. Timing problem on cable (serial clock transmit external [SCTE] not set on CSU/DSU)
5. Failed local or remote CSU/DSU
6. Router hardware failure (local or remote)

Suggested Actions:

Step 1 Conduct CSU/DSU loopback tests. During local loopback, use the show interfaces serial number command to determine whether the line protocol comes up. If the line protocol does come up, it is likely that there is a telephone company problem or that the remote router is down.

Step 2 If the problem appears to be on the remote end, repeat Step 1 on the remote modem, CSU, or DSU.

Step 3 Verify all cabling. Make certain that the cable is attached to the correct interface, the correct CSU/DSU and the correct telephone company network termination point. Use the show controllers EXEC command to determine which cable is attached to which interface.

Step 4 Enable the debug serial interface EXEC command.

Step 5 If the line protocol does not come up in local loopback mode and if the output of the debug serial interface EXEC command shows that the keepalive counter is not incrementing, a router hardware problem is likely; swap router interface hardware.

Step 6 If you suspect faulty router hardware, change the serial line to an unused port. If the connection comes up, the previously connected interface has a problem.

1. Loop exists in circuit. The sequence number in the keepalive packet changes to a random number when a loop is initially detected. If the same random number is returned over the link, a loop exists.

Suggested Actions:

Step 1 Use the write terminal privileged EXEC command to look for any instances of the loopback interface configuration command.

Step 2 If you find an occurrence of the loopback interface configuration command, use the no loopback interface configuration command to remove the loop.

Step 3 If you do not find the loopback interface configuration command, examine the CSU/DSU to determine whether they are configured in manual loopback mode. If they are, disable manual loopback.

Step 4 Reset the CSU or DSU and inspect the line status. If the protocol comes up, no other action is needed.

Step 5 If the CSU or DSU is not configured in manual loopback mode, contact the leased-line or other carrier service for line troubleshooting assistance.

1. Router configuration includes the shutdown interface configuration command
2. Duplicate IP address

Suggested Actions:

Step 1 Check router configuration for the shutdown command.

Step 2 Use the no shutdown interface configuration command to remove the shutdown command.

Step 3 Verify that there are no identical IP addresses using the write terminal privileged EXEC command or the show interfaces EXEC command.

Step 4 If there are duplicate addresses, resolve the conflict by changing one of the IP addresses.

If the line protocol comes up when the CSU/DSU is in local loopback mode, the problem might be on the remote end of the serial connection. If the status line does not change state, there is a possible problem in the router, connecting cable, or CSU/DSU.