Troubleshooting Methodologies and Tools

Gathering Symptoms
To determine the scope of the problem gather (document) the symptoms. The figure shows the a flow chart of this process. Each step in this process is briefly described here:

Step 1. Analyze existing symptoms - Analyze symptoms gathered from the trouble ticket, users, or end systems affected by the problem to form a definition of the problem.

Step 2. Determine ownership - If the problem is within your system, you can move onto the next stage. If the problem is outside the boundary of your control, for example, lost Internet connectivity outside of the autonomous system, you need to contact an administrator for the external system before gathering additional network symptoms.

Step 3. Narrow the scope - Determine if the problem is at the core, distribution, or access layer of the network. At the identified layer, analyze the existing symptoms and use your knowledge of the network topology to determine which pieces of equipment are the most likely cause.

Step 4. Gather symptoms from suspect devices - Using a layered troubleshooting approach, gather hardware and software symptoms from the suspect devices. Start with the most likely possibility, and use knowledge and experience to determine if the problem is more likely a hardware or software configuration problem.

Step 5. Document symptoms - Sometimes the problem can be solved using the documented symptoms. If not, begin the isolating phase of the general troubleshooting process.
Use the Cisco IOS commands to gather symptoms about the network. The table in the figure describes the common Cisco IOS commands you can use to help you gather the systems of a network problem.
Although the debug command is an important tool for gathering symptoms it generates a large amount of console message traffic and the performance of a network device can be noticeably affected. Make sure you warn network users that a troubleshooting effort is underway and that network performance may be affected. Remember to disable debugging when you are done.

Questioning End Users

When you question end users about a network problem they may be experiencing, use effective questioning techniques. This way you will get the information you need to effectively document the symptoms of a problem. The table in the figure provides some guidelines and end-user example questions.

Software Troubleshooting Tools
A wide variety of software and hardware tools are available to make troubleshooting easier. These tools may be used to gather and analyze symptoms of network problems and often provide monitoring and reporting functions that can be used to establish the network baseline.

NMS Tools
Network management system (NMS) tools include device-level monitoring, configuration, and fault management tools. The figure shows an example display from the What's Up Gold NMS software. These tools can be used to investigate and correct network problems. Network monitoring software graphically displays a physical view of network devices, allowing network managers to monitor remote devices without actually physically checking them. Device management software provides dynamic status, statistics, and configuration information for switched products. Examples of commonly used network management tools are CiscoView, HP Openview, Solar Winds, and What's Up Gold.

Knowledge Bases
On-line network device vendor knowledge bases have become indispensable sources of information. When vendor-based knowledge bases are combined with Internet search engines like Google, a network administrator has access to a vast pool of experience-based information.
The figure shows the Cisco Tools & Resources page found at This is a free tool providing information on Cisco-related hardware and software. It contains troubleshooting procedures, implementation guides, and original white papers on most aspects of networking technology.

Baselining Tools
Many tools for automating the network documentation and baselining process are available. These tools are available for Windows, Linux, AUX operating systems. The figure shows a screen chapter of the SolarWinds LAN surveyor and CyberGauge software. Baselining tools help you with common baseling documentation tasks. For example they can help you draw network diagrams, help you to keep network software and hardware documentation up-to-date and help you to cost-effectively measure baseline network bandwidth use.

Protocol Analyzers
A protocol analyzer decodes the various protocol layers in a recorded frame and presents this information in a relatively easy to use format. The figure shows a screen capture of the Wireshark protocol analyzer. The information displayed by a protocol analyzer includes, the physical, data link, protocol and descriptions for each frame. Most protocol analyzers can filter traffic that meets certain criteria so that, for example, all traffic to and from a particular device can be captured.

Hardware Troubleshooting Tools
Network Analysis Module
A network analysis module (NAM) can be installed in Cisco Catalyst 6500 series switches and Cisco 7600 series routers to provide a graphical representation of traffic from local and remote switches and routers. The NAM is a embedded browser-based interface that generates reports on the traffic that consumes critical network resources. In addition, the NAM can capture and decode packets and track response times to pinpoint an application problem to the network or the server.

Digital Multimeters

Digital multimeters (DMMs) are test instruments that are used to directly measure electrical values of voltage, current, and resistance. In network troubleshooting, most of the multimedia tests involve checking power-supply voltage levels and verifying that network devices are receiving power.

Cable Testers

Cable testers are specialized, handheld devices designed for testing the various types of data communication cabling. Cabling testers can be used to detect broken wires, crossed-over wiring, shorted connections, and improperly paired connections. These devices can be inexpensive continuity testers, moderately priced data cabling testers, or expensive time-domain reflectometers (TDRs).
TDRs are used to pinpoint the distance to a break in a cable. These devices send signals along the cable and wait for them to be reflected. The time between sending the signal and receiving it back is converted into a distance measurement. The TDR function is normally packaged with data cabling testers. TDRs used to test fiber optic cables are known as optical time-domain reflectometers (OTDRs).

Cable Analyzers
Cable analyzers are multifunctional handheld devices that are used to test and certify copper and fiber cables for different services and standards. The more sophisticated tools include advanced troubleshooting diagnostics that measure distance to performance defect (NEXT, RL), identify corrective actions, and graphically display crosstalk and impedance behavior. Cable analyzers also typically include PC-based software. Once field data is collected the handheld device can upload its data and up-to-date and accurate reports can be created.

Portable Network Analyzers
Portable devices that are used for troubleshooting switched networks and VLANs. By plugging the network analyzer in anywhere on the network, a network engineer can see the switch port to which the device is connected and the average and peak utilization. The analyzer can also be used to discover VLAN configuration, identify top network talkers, analyze network traffic, and view interface details. The device can typically output to a PC that has network monitoring software installed for further analysis and troubleshooting.


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