This
output means that you have a problem with the cable, CSU/DSU, or the serial line.
First, use the show controllers serial [slot/port] command to verify that the cable is present
and recognized by the router.
Next, you might need to troubleshoot the problem with a loopback test.
Follow these steps to perform a loopback test:
Step 1 Set the serial line encapsulation to High-Level Data Link Control (HDLC) and
keepalive to 10 seconds. To do this, use the encapsulation hdlc and keepalive
10 commands in the interface con?¬?guration mode of the interface you are
troubleshooting.
Step 2 Place the CSU/DSU or modem in local-loop mode. Check the device
documentation to determine how to do this. If the line protocol comes up
when the CSU/DSU or modem is in local-loop mode, indicated by a ???line
protocol is up (looped)??? message, this suggests that the problem is
occurring beyond the local CSU/DSU. If the status line does not change
status, a problem could be in the router, connecting cable, CSU/DSU, or
modem. In most cases, the problem is with the CSU/DSU or modem.
Step 3 Execute a ping command to the IP address of the interface you are
troubleshooting while the CSU/DSU or modem is in local-loop mode.
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