System information

Troubleshooting Using the show interfaces serial Command
Book Title
15-316
Serial x is up,
line protocol is down
(DCE
6
mode)
• Missing clockrate
interface configuration
command
• DTE device does not
support or is not
set up for SCTE
mode (terminal
timing)
• Failed remote
CSU or DSU
• Failed or incorrect
cable
• Router hardware
failure
Step 1 Add the clockrate interface configuration
command on the serial interface.
Syntax:
clock rate bps
Syntax Description:
• bps—Desired clock rate in bits per second:
1200, 2400, 4800, 9600, 19200, 38400,
56000, 64000, 72000, 125000, 148000,
250000, 500000, 800000, 1000000,
1300000, 2000000, 4000000, or 8000000.
Step 2 Set the DTE device to SCTE mode if possible.
If your CSU/DSU does not support SCTE, you
might have to disable SCTE on the Cisco
router interface. Refer to the section “Inverting
the Transmit Clock” later in this chapter.
Step 3 Verify that the correct cable is being used.
Step 4 If the line protocol is still down, there is a
possible hardware failure or cabling problem.
Insert a breakout box and observe leads.
Step 5 Replace faulty parts as necessary.
Serial x is up,
line protocol is up
(looped)
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.
Step 1 Use the show running-config privileged exec
command to look for any loopback interface
configuration command entries.
Step 2 If you find a loopback interface configuration
command entry, 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 line 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.
Serial x is up,
line protocol is down
(disabled)
• High error rate due to
telephone company service
problem
• CSU or DSU hardware
problem
• Bad router hardware
(interface)
Step 1 Troubleshoot the line with a serial analyzer
and breakout box. Look for toggling CTS
7
and
DSR
8
signals.
Step 2 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.
Step 3 Swap out bad hardware as required (CSU,
DSU, switch, local or remote router).
Status Line
Condition Possible Problem Solution