Serial x is up, line protocol is up
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—
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This is the proper status line condition. No action is required.
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Serial x is down, line protocol is down
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The router is not sensing a carrier detect (CD) signal (that is, the CD is not active).
The line is down or is not connected on the far end.
Cabling is faulty or incorrect.
Hardware failure has occurred in the channel service unit/data service unit (CSU/DSU).
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Check the CD LEDs to see whether the CD is active, or insert a breakout box on the line to check for the CD signal.
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Verify that you are using the proper cable (see your hardware installation documentation).
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Insert a breakout box and check all control leads.
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Contact your leased-line or other carrier service to see whether there is a problem.
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Swap faulty parts.
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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.
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Serial x is up, line protocol is down
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A local or remote router is misconfigured.
Keepalives are not being sent by the remote router.
A leased-line or other carrier service problem has occurred (noisy line or misconfigured or failed switch).
A timing problem has occurred on the cable.
A local or remote CSU/DSU has failed.
Router hardware (local or remote) has failed.
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Put the line in local loopback mode and use the show interfaces serial command to determine whether the line protocol comes
up.
Note
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If the line protocol comes up, a failed remote device is the likely problem.This solution will only work with High-Level Data
Link Control (HDLC) encapsulation. For Frame Relay (FR) and Point-to-Point Protocol (PPP) encapsulation, a looped interface
will always have the line protocol down. In addition, you may need to change the encapsulation to HDLC to debug this issue.
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If the problem appears to be on the remote end, repeat Step 1 on the remote interface.
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Verify all cabling. Make certain that the cable is attached to the correct interface, the correct CSU/DSU, and the correct
remote termination point.
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Enable the debug serial interface EXEC command.
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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.
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If the line protocol comes up and the keepalive counter increments, the problem is not in the local router.
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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.
Note
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First enable per-interface debugging using the command debug interface serial
x , and depending on the encapsulation, enable the corresponding debug:
For HDLC: debug serial interface For PPP: debug ppp negotiation
For FR: debug frame-relay lmi
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Caution
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Because debugging output is assigned high priority in the CPU process, it can render the system unusable. For this reason,
use debug commands only to troubleshoot specific problems or during troubleshooting sessions with Cisco technical support staff. Moreover,
it is best to use debug commands during periods of lower network traffic and fewer users. Debugging during these periods decreases the likelihood
that increased debug command processing overhead will affect system use.
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Serial x is up, line protocol is up (looped)
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A loop exists in the 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.
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Use the show running-config privileged EXEC command to look for any loopback interface configuration command entries.
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If you find a loopback interface configuration command entry, use the no loopback interface configuration command to remove
the loop.
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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.
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Reset the CSU or DSU, and inspect the line status. If the line protocol comes up, no other action is needed.
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If the CSU or DSU is not configured in manual loopback mode, contact the leased-line or other carrier service for line troubleshooting
assistance.
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Serial x is up, line protocol is down (disabled)
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A high error rate has occurred due to a remote device problem.
A CSU or DSU hardware problem has occurred.
Router hardware (interface) is bad.
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Troubleshoot the line with a serial analyzer and breakout box. Examine the output of show controller T1 or show controller T3
or show controller serial
x depending on whether the SPA is an 8-Port Channelized T1/E1 Serial SPA, 4-Port Channelized T3 SPA, or 4-Port T3/E3 Serial
SPA.
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Loop CSU/DSU (DTE loop). If the problem continues, it is likely that there is a hardware problem. If the problem does not
continue, it is likely that there is a telephone company problem.
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Swap out bad hardware, as required (CSU, DSU, switch, local or remote router).
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Serial x is administratively down, line protocol is down
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The router configuration includes the shutdown interface configuration command.
A duplicate IP address exists.
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Check the router configuration for the shutdown command.
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Use the no shutdown interface configuration command to remove the shutdown command.
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Verify that there are no identical IP addresses using the show running-config privileged EXEC command or the show interfaces
EXEC command.
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If there are duplicate addresses, resolve the conflict by changing one of the IP addresses.
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