System information

Troubleshooting Serial Line Problems 15-331
Clocking Problem Solutions
Aborts on one end suggests that the other end is sending bad information or that there is
a line problem.
Note Always refer to the show interfaces serial command output (see Figure 15-1) and log any
changes in error counts or note if the error count does not change.
Clocking Problem Solutions
Table 15-8 outlines suggested remedies for clocking problems, based on the source of the problem.
Table 15-8 Serial Lines: Clocking Problems and Solutions
Possible Problem Solution
Incorrect CSU configuration
Step 1 Determine whether the CSUs at both ends agree on the clock source
(local or line).
Step 2 If the CSUs do not agree, configure them so that they do (usually the
line is the source).
Step 3 Check the LBO
1
setting on the CSU to ensure that the impedance
matches that of the physical line. For information on configuring
your CSU, consult your CSU hardware documentation.
1 LBO = Line Build Out
Incorrect DSU configuration Step 1 Determine whether the DSUs at both ends have SCTE mode
enabled.
Step 2 If SCTE is not enabled on both ends of the connection, enable it.
(For any interface that is connected to a line of 128 kbps or faster,
SCTE must be enabled. If your DSU does not support SCTE, see the
section “Inverting the Transmit Clock” later in this chapter.)
Step 3 Make sure that ones density is maintained. This requires that the
DSU use the same framing and coding schemes (for example, ESF
and B8ZS) used by the leased-line or other carrier service.
Check with your leased-line provider for information on its framing
and coding schemes.
Step 4 If your carrier service uses AMI coding, either invert the transmit
clock on both sides of the link or run the DSU in bit-stuff mode. For
information on configuring your DSU, consult your DSU hardware
documentation.
Cable to router out of specification If the cable is longer than 50 feet (15.24 meters), use a shorter cable.
If the cable is unshielded, replace it with shielded cable.