System information

Troubleshooting Using the show interfaces serial Command
Book Title
15-320
Serial Lines: Troubleshooting Serial Line Input Errors
Table 15-5 describes the various types of input errors displayed by the show interfaces serial
command (see Figure 15-1), possible problems that might be causing the errors, and solutions to
those problems.
Table 15-5 Serial Lines: Troubleshooting Serial Line Input Errors
Possible Problem Solution
The following problems can result in
this symptom:
• Faulty telephone company
equipment
• Noisy serial line
• Incorrect clocking configuration
(SCTE not set)
• Incorrect cable or cable too long
• Bad cable or connection
• Bad CSU or DSU
• Bad router hardware
• Data converter or other device
being used between router and
DSU
Note: Cisco strongly recommends against the use of data converters when
you are connecting a router to a WAN or serial network.
Step 1 Use a serial analyzer to isolate the source of the input errors. If you
detect errors, it is likely that there is a hardware problem or a clock
mismatch in a device that is external to the router.
Step 2 Use the loopback and ping tests to isolate the specific problem
source. For more information, see the sections “Using the trace
Command” and “CSU and DSU Loopback Tests” later in this
chapter.
Step 3 Look for patterns. For example, if errors occur at a consistent
interval, they could be related to a periodic function such as the
sending of routing updates.
Input Error Type
(Field Name) Possible Problem Solution
CRC errors
(CRC)
CRC errors occur when
the CRC calculation
does not pass (indicating
that data is corrupted)
for one of the following
reasons:
• Noisy serial line
• Serial cable is too long
or cable from the
CSU/DSU to the
router is not shielded
• SCTE mode is not
enabled on DSU
• CSU line clock is
incorrectly configured
• Ones density problem
on T1 link (incorrect
framing or coding
specification)
Step 1 Ensure that the line is clean enough for
transmission requirements. Shield the
cable if necessary.
Step 2 Make sure the cable is within the
recommended length (no more than 50
feet [15.24 meters], or 25 feet [7.62
meters] for T1 link).
Step 3 Ensure that all devices are properly
configured for a common line clock. Set
SCTE on the local and remote DSU. If
your CSU/DSU does not support SCTE,
see the section “Inverting the Transmit
Clock” later in this chapter.
Step 4 Make certain that the local and remote
CSU/DSU are configured for the same
framing and coding scheme as that used
by the leased-line or other carrier
service (for example, ESF/B8ZS).
Step 5 Contact your leased-line or other carrier
service and have it perform integrity
tests on the line.