Follow-the-path is one of the most basic troubleshooting techniques. The approach first discovers the traffic path all the way from source to destination. The scope of troubleshooting is reduced to just the links and devices that are in the forwarding path. The objective is to eliminate the links and devices that are irrelevant to the troubleshooting task at hand. This approach usually complements one of the other approaches.
Substitution
The substitution approach is also called swap-the-component because you physically swap the problematic device with a known, working one. If the problem is fixed, the problem is with the removed device. If the problem remains, the cause may be elsewhere.
In specific situations, this can be an ideal method for quick problem resolution, such as with a critical single point of failure. For example, a border router goes down. In this case, simply replacing the device and restoring service may be more beneficial than troubleshooting the issue.
If the problem lies within multiple devices, you might not be able to correctly isolate the problem.
Comparison
The comparison approach, also called the spot-the-differences approach, attempts to resolve a problem by changing the nonoperational elements to be consistent with the working ones. You compare configurations, software versions, hardware, or other device properties, links, or processes between working and nonworking situations and spot significant differences between them.
The weakness of this method is that it might lead to a working solution without clearly revealing the root cause of the problem.
Educated Guess
The educated guess approach is also called the shoot-from-the-hip troubleshooting approach. This less-structured troubleshooting method uses an educated guess based on the symptoms of the problem. Success of this method varies based on your troubleshooting experience and ability. Seasoned technicians are more successful because they can rely on their extensive knowledge and experience to decisively isolate and solve network issues. With less-experienced network administrators, this troubleshooting method might be too random to be effective.
Guidelines for Selecting a Troubleshooting Method (20.1.4)
To quickly resolve network problems, take the time to select the most effective network troubleshooting method. Figure 20-4 illustrates the methods that could be used when certain types of problem are discovered.
Figure 20-4 Selecting a Troubleshooting Method
For instance, software problems are often solved using a top-down approach, whereas hardware-based problems are solved using the bottom-up approach. New problems may be solved by an experienced technician using the divide-and-conquer method. Otherwise, the bottom-up approach may be used.
Troubleshooting is a skill that you develop by doing it. Every network problem you identify and solve adds to your skill set.
Physical Layer Problems (20.2)
A large proportion of networking problems are related to physical components or problems with the physical layer. Physical problems are concerned mainly with the hardware aspects of computers and networking devices, and the cables that interconnect them. Physical problems do not include the logical (software) configuration of devices.
Common Layer 1 Problems (20.2.1)
Remember, the physical layer (Layer 1) deals with the physical connectivity of the network devices. Some of the more common Layer 1 problems include the following:
- Device power turned off
- Device power unplugged
- Loose network cable connection
- Incorrect cable type
- Faulty network cable
- Faulty wireless access point (AP)
To troubleshoot at Layer 1, you first check that all devices have the proper power supplied and that the devices are turned on. This solution might seem to be obvious, but many times the person reporting the problem may overlook a device that is within the network path from source to destination. Then you ensure that no errors show on any LEDs that display the connectivity status. If you‛re on site, visually inspect all network cabling and reconnect cables to ensure a proper connection. If the problem is with wireless, verify that the wireless access point is operational and that wireless settings are configured correctly.
The Sense of Sight
Vision is used to detect problems such as improperly connected or poorly constructed cables:
- Cables that are not connected
- Cables connected to the wrong port
- Loose cable connections
- Damaged cables and connectors
- Use of the wrong type of cable
Vision also enables you to view the condition and function of various network devices with LEDs.
The Senses of Smell and Taste
Smell can alert you to components that are overheating when you‛re troubleshooting. The smell of burning insulation or components is distinct and is a sure sign that something is seriously wrong.
The sense of taste is directly related to the sense of smell because both use the same receptors. You may also taste the acridness of something burning.
The Sense of Touch
When troubleshooting, you can use touch to feel for overheated components as well as to detect mechanical problems with devices such as cooling fans. These devices usually create a small vibration in the component that can be detected using touch. The absence of this vibration or the presence of excessive amounts of vibration can indicate that the cooling fan has failed or is about to do so.
The Sense of Hearing
Hearing is used to detect major problems such as electrical issues and the proper operation of cooling fans and disk drives. All devices have characteristic sounds, and any change from the normal sounds usually indicates a problem of some sort.