System information

Troubleshooting X.25
Book Title
19-416
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, “Troubleshooting Serial Line
Problems.”
Incorrect cabling or bad router
hardware
Step 1 Use the show interfaces serial exec command to determine the
status of the interface.
Step 2 If the interface is down, refer to Chapter 15, “Troubleshooting Serial
Line Problems.” 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.
Step 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.
Step 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.
Step 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.”
Misconfigured protocol parameters
Step 1 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.
Step 2 Look for RESTART messages (for PVCs
4
) or CLEAR REQUESTS
with nonzero cause codes (for SVCs
5
).
To interpret X.25 cause and diagnostic codes provided in the debug
x25 events output, refer to the Debug Command Reference.
Step 3 Verify that all critical LAPB parameters (modulo, T1, N1, N2, and k)
and the critical X.25 parameters (modulo, X.121 addresses, SVC
ranges, PVC definitions, and default window and packet sizes)
match the parameters required by the service provider.