Information Technology Reference
In-Depth Information
Table 19-1 outlines the problems that might cause this symptom and describes solutions to those
problems.
Table19-1
X.25: No Connections over X.25 Link
Possible Problem
Solution
Link is down.
Use the
show interfaces serial
exec command to determine
whether the link is down. If the link is down, refer to Chapter 15.
Cabling is
incorrect, or the
router hardware is
bad.
1.
Use the
show interfaces serial
exec command to determine
the status of the interface.
2.
If the interface is down, refer to Chapter 15. If the interface
is up but the line protocol is down, check the LAPB
1
state in
the output of the
show interfaces serial
command.
3.
If the LAPB state is not CONNECT, use the
debug lapb
privileged exec command (or attach a serial analyzer) to look
for SABMs being sent and for UA
2
packets being sent in
reply to SABMs
3
. If UAs are not being sent, one of the other
possible problems described in this table is the likely cause.
Caution
: Exercise care when using
debug
commands. Many
debug
commands are processor-intensive and can cause serious
network problems (such as degraded performance or loss of
connectivity) if they are enabled on an already heavily loaded
router. When you finish using a
debug
command, remember to
disable it with its specific
no debug
command (or use the
no
debug all
command to turn off all debugging).
Use
debug
commands to isolate problems, not to monitor normal
network operation. Because the high processor overhead of
debug
commands can disrupt router operation, you should use
debug
commands only when you are looking for specific types
of traffic or problems and have narrowed your problems to a
likely subset of causes.
4.
If the
show interfaces serial
exec command indicates that
the interface and line protocol are up but no connections can
be made, there is probably a router or switch
misconfiguration. Refer to the other possible problems
outlined in this table.
5.
Check all cabling and hardware for damage or wear. Replace
cabling or hardware as required. For more information, refer
to Chapter 3, “Troubleshooting Hardware and Booting
Problems.”
Protocol
parameters are
misconfigured.
Enable the
debug lapb
privileged exec command and look
for SABMs being sent. If no SABMs are being sent, disable
the
debug lapb
command and enable the
debug x25 events
privileged exec command.
1.
Look for RESTART messages (for PVCs
4
) or CLEAR
REQUESTS with nonzero cause codes (for SVCs
5
).
2.
