Status Line Condition Possible Problem Solution
Serial x is up,
line protocol
is up
---
This is the proper status line condition. No action required.
Serial x is down,
line protocol is
down (DTE
mode)
Typically indicates that
the router is not sensing
a CD signal (that is,
CD is not active).
a.. Telephone company
problem---Line is
down or line is not
connected to
CSU/DSU
b.. Faulty or incorrect
cabling
c.. Hardware failure
(CSU/DSU)
Step 1 Check the LEDs on the CSU/DSU to see whether 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 hard- ware installation documentation).
Step 3 Insert a breakout box and check all control leads.
Step 4 Contact your leased-line or other carrier service to see whether
there is a problem.
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.
Serial x is up,
line protocol is down (DTE mode)
a.. Local or remote router
is misconfigured
b.. Keepalives are not
being sent by
remote router
c.. Leased-line or
other carrier
service problem---noisy
line, or misconfigured
or failed switch
d.. Timing problem
on cable (SCTE
not set on CSU/DSU)
e.. Failed local or
remote CSU/DSU
f.. Router hardware
failure (local or
remote)
Step 1 Put the modem, CSU, or DSU in local loopback mode and use the
show
interfaces serial command to determine whether the line protocol comes up.
If the line protocol comes up, a telephone company problem or a failed
remote router is the likely problem.
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.
Caution: 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.
Serial x is up,
line protocol is down (DTE mode)
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 the line protocol comes up and the keepalive counter
increments, the problem is not in the local router.
Step 7 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.
Serial x is up,
line protocol is down (DCE mode)
a.. Missing clockrate
interface configuration
command
b.. DTE device does not
support or is not
set up for SCTE
mode (terminal
timing)
c.. Failed remote
CSU or DSU
d.. Failed or incorrect
cable
e.. 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, 25, 50, 80,
100, 130, 200, 400, or 800.
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.
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.
