System information

Troubleshooting Serial Line Problems 15-315
Serial Lines: show interfaces serial Status Line Conditions
Serial x is up,
line protocol is down
(DTE mode)
• Local or remote router
is misconfigured
• Keepalives are not
being sent by
remote router
• Leased-line or
other carrier
service problem—noisy
line, or misconfigured
or failed switch
• Timing problem
on cable (SCTE
5
not set on CSU/DSU)
• Failed local or
remote CSU/DSU
• 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. Troubleshoot the serial line as
described in the sections “Troubleshooting
Clocking Problems” and “CSU and DSU
Loopback Tests” later in this chapter.
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.
Status Line
Condition Possible Problem Solution