Troubleshooting guide

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Cisco Broadband Local Integrated Services Solution Troubleshooting Guide
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Chapter 2 Troubleshooting Overview
Troubleshooting Tools
Each outgoing packet can result in one of two error messages:
• A "time exceeded" error message indicates that an intermediate router has seen and discarded the
probe.
• A "port unreachable" error message indicates that the destination node has received the probe and
discarded it, because it could not deliver the packet to an application. If the timer goes off before a
response comes in, the trace command prints an asterisk (*).
The trace command terminates when the destination responds, when the maximum TTL is exceeded, or
when the user interrupts the trace with the escape sequence.
For detailed information on using the trace and extended trace commands, refer to the Cisco BTS 10200
Softswitch Operations Guide.
Hardware Tools
Most Cisco components include LED indicators on the front or rear panels and, in some cases, on both
panels. These LEDs indicate the status of the equipment. The meaning of each LED is described in the
component’s documentation.
In many situations, third-party diagnostic hardware tools can be more useful than commands that are
integrated into the router. For example, enabling a processor-intensive debug command can help to
overload an environment that is already experiencing excessively high traffic levels. Attaching a network
analyzer to the suspect network is less intrusive and is more likely to yield useful information without
interrupting the operation of the router.
This section presents an overview of the various tools that are available for troubleshooting cable
networks that include Cisco devices. Some typical third-party tools used for troubleshooting include
• Volt-ohm meters (VOM), digital multimeters (DMM), and cable testers
• Breakout boxes, fox boxes, bit error rate testers (BERT), and block error rate testers (BLERT)
• Network analyzers and network monitors
• Time domain reflectometers (TDR) and optical time domain reflectometers (OTDR)
• Low-End Cable Test Equipment
• High-End Cable Testers
• Digital Interface Testing Tools
• Network Management Systems
• Simulation and Modeling Tools
Each of these tools has a specific purpose and works at specific OSI reference model layers.
Understanding what each tool can do and which tool is appropriate for each troubleshooting task will
help you become a more efficient network technician.
When troubleshooting, you should start at the physical layer. Use cable testers and other low-level testers
to ensure that there are no problems with the media, such as noise, too much attenuation, improper cable
lengths, improper connectors, and so forth. If the physical layer seems fine, then move up the layers to
the data link layer. You can use a protocol analyzer to check for excessive collisions on Ethernet,
beaconing on Token Ring or FDDI networks, excessive soft errors on Token Ring, and other link-layer
issues. If the data link layer seems fine, check for routing errors or misconfigurations at the network
layer, using a protocol analyzer and Cisco IOS commands. Finally, you can look for upper-layer
problems such as misconfigurations, software bugs, and user errors.