System information

Troubleshooting X.25
Book Title
19-414
• Data—Carries upper-layer data. Its size and format vary, depending on the Layer 3 packet type.
The maximum length of this field is set by agreement between a PSN administrator and the
subscriber at subscription time.
• FCS—Ensures the integrity of the transmitted data.
Layer 1
Layer 1 X.25 uses the X.21 bis physical-layer protocol, which is roughly equivalent to
EIA/TIA-232-C (formerly RS-232-C). X.21 bis was derived from ITU-T Recommendations V.24
and V.28, which identify the interchange circuits and electrical characteristics, respectively, of a
DTE-to-DCE interface. X.21 bis supports point-to-point connections, speeds up to 19.2 kbps, and
synchronous, full-duplex transmission over four-wire media. The maximum distance between DTE
and DCE is 15 meters.
Troubleshooting X.25
This section presents troubleshooting information relating to X.25 connectivity. The “Using the
show interfaces serial Command,” section discusses the use of the show interfaces serial
command in an X.25 environment and describes some of the key fields of the command output.
The remaining sections describe specific X.25 symptoms, the problems that are likely to cause each
symptom, and the solutions to those problems.
Using the show interfaces serial Command
This section describes the information provided by the show interfaces serial exec command in an
X.25 environment. For additional information about the output of the show interfaces serial exec
command, refer to Chapter 15, “Troubleshooting Serial Line Problems,” and the Cisco IOS
Configuration Fundamentals Command Reference.
The show interfaces serial command provides important information useful for identifying
problems in X.25 internetworks. The following fields provide especially important information:
• REJs—Number of rejects
• SABMs—Number of Set Asynchronous Balance Mode requests
• RNRs—Number of Receiver Not Ready events
• FRMRs—Number of protocol frame errors
• RESTARTs—Number of restarts
• DISCs—Number of disconnects
All but the RESTARTs count are LAPB events. Because X.25 requires a stable data link, LAPB
problems commonly cause an X.25 restart event that implicitly clears all virtual connections. If
unexplained X.25 restarts occur, examine the underlying LAPB connection for problems. Use the
debug lapb exec command to display all traffic for interfaces using LAPB encapsulation. The no
form of this command disables debugging output:
[no] debug lapb