System information

Troubleshooting Clocking Problems
Book Title
15-330
When the DCE device uses SCTE instead of its internal clock to sample data from the DTE, it is
better able to sample the data without error even if there is a phase shift in the cable between the
CSU/DSU and the router. Using SCTE is highly recommended for serial transmissions faster than
64 kbps. If your CSU/DSU does not support SCTE, see the section “Inverting the Transmit Clock”
later in this chapter.
Clocking Problem Causes
In general, clocking problems in serial WAN interconnections can be attributed to one of the
following causes:
• Incorrect DSU configuration
• Incorrect CSU configuration
• Cables out of specification (longer than 50 feet [15.24 meters] or unshielded)
• Noisy or poor patch panel connections
• Several cables connected together in a row
Detecting Clocking Problems
To detect clocking conflicts on a serial interface, look for input errors as follows:
Step 1 Use the show interfaces serial exec command on the routers at both ends of the link.
Step 2 Examine the command output for CRC, framing errors, and aborts.
Step 3 If either of these steps indicates errors exceeding an approximate range of 0.5% to 2.0%
of traffic on the interface, clocking problems are likely to exist somewhere in the WAN.
Step 4 Isolate the source of the clocking conflicts as outlined in the following section, “Isolating
Clocking Problems.”
Step 5 Bypass or repair any faulty patch panels.
Isolating Clocking Problems
After you determine that clocking conflicts are the most likely cause of input errors, use the
following procedure will help you isolate the source of those errors:
Step 1 Perform a series of ping tests and loopback tests (both local and remote), as described in
the section “CSU and DSU Loopback Tests” earlier in this chapter.
Step 2 Determine which end of the connection is the source of the problem, or whether the
problem is in the line. In local loopback mode, run different patterns and sizes in the ping
tests (for example, use 1500-byte datagrams). Using a single pattern and packet size may
not force errors to materialize, particularly when a serial cable to the router or CSU/DSU
is the problem.
Step 3 Use the show interfaces serial exec command and determine whether input errors counts
are increasing and where they are accumulating.
If input errors are accumulating on both ends of the connection, clocking of the CSU is
the most likely problem.
If only one end is experiencing input errors, there is probably a DSU clocking or cabling
problem.