Chapter 14: Network Troubleshooting
Core Idea
This chapter focuses on the establishment of a plan for troubleshooting a network, including
physical activity, software solutions, and a hierarchy of steps needed to correct any network
problems.
Instructor Focus
The focus of this topic is on creating a hierarchy of steps for reliably troubleshooting a network,
identifying the problem, implementing a solution, and testing the repair effort on the network.
This is the final chapter of content in the book. It should serve as a culmination of the topics
Exam Objectives Covered by this Chapter/Lesson
x 5.3 Given a scenario, troubleshoot common wired connectivity and performance issues
x 5.5 Given a scenario, troubleshoot common network service issues
Terminology
Make an effort to define the following terms for the designated areas. After defining these,
calling on the students for repeat definitions as you progress through the slide deck will help
them retain the terminology.
Troubleshooting Issues:
x Trouble ticket
x Open
x Short
x Decibel loss
x Maximum transmission unit (MTU)
x Black-hole router
Focus Questions
x What are the elements in a structured troubleshooting model?
x What common physical layer troubleshooting issues might you encounter?
x What potential Layer 2 issues are you most likely to face when troubleshooting a network
containing Ethernet switches?
x Aside from routing protocol troubleshooting, what Layer 3 troubleshooting issues are
common in a routed network?
x How do characteristics unique to wireless networks impact your troubleshooting of a
network containing wireless access points?
Icebreaker Discussion
Pose one or both of the following questions:
1. What are some likely causes of network interruption or failure?
In addition to the security issues discussed in the last chapter, a number of causes can lead
to failure of a network link or network device. These can include software bugs, traffic
2. Why is it important to establish a hierarchy of steps for troubleshooting networks?
Discussion Points to cover.
a. An essential thing to consider in this question is the time investment involved in
identifying the problem on the network. Randomly trying solutionsis as bad as
acting too thoroughly, too quickly (ie, working too long on the wrong solution).
b. Despite the fact that the causes and solutions are vastly different, it is important to
proceed with troubleshooting by going with minimal effort testing first. This
discussion should be a general conversation about why procedure is important in
Icebreaker Activity
1. Display this sample network.Traffic from the PC on the left cannot reach the PC on the right.
What steps should be taken to determine where the connection is not working?
This should be a discussion exercise in establishing the steps that are most involved and
some easy ways to gather information on why the connection is not working between PC1
and PC2. There are a variety of methods for determining the problem, but you should guide
the discussion such that the less-involved steps are tried first. For example, you should at
Main Activities
This should be a discussion led by the Slide set. The following are thoughts that could be expressed as
tips to students.
1. Establishing a Troubleshooting Plan
The goal of a troubleshooting hierarchy is to localize the problem as quickly and efficiently as
possible. Some steps can be taken in software quickly that may not yield results but cost little
effort to test and may save a significant amount of time if successful. The best approach to
SW1
SW2
R1
troubleshooting is the use of a structured troubleshooting methodology; this is an outline of
steps that should be taken once an issue is known. Keep in mind that issues may come from
users experiencing a problem or from network reporting software or logs on the system. When
multiple issues are competing for attention, the one with the highest impact on operations
should be investigated first (keeping in mind that the other issues may stem from the same root
problem as well).
Note that this is a linear structure, not a hierarchical one. A hierarchal structure might use a flowchart
that starts with a definition of the problem, but by using a series of yes or no decision points, it can
quickly rule out what is working from what is not working. For example, a user claims to not be able to
login to the network. However, others are able to do so. The flowchart in this case would eliminate the
login server as the problem and focus on this specific user. By doing this, it would eliminate the time
consuming attempt to troubleshoot a server when the problem is really the physical cable connector.
(1) Define the Problem:
This is the initial phase of troubleshooting, in which a problem is identified by any one of a variety of
sources (such as a user experiencing difficulty, errors in a log that are cause for concern, significant
events from an SNMP server, and so on). In this initial phase, you may interview the user who identified
(2) Hypothesize the Probable Cause:
This is the phase in which you will try to identify the cause of the problem. This can be the most
time-consuming phase and will require a measure of logic and intuition. The quicker you can
(3) Test Hypothesis:
Before proceeding with the test, make sure that your identified hypothesis would lead to the
(4) Create an Action Plan:
This is where you will plan your actions to solve the problem on the network. This can be a set of
changes in configuration, resetting routing equipment, or even changing out faulty equipment.
(5) Implement Action Plan:
If the action plan is one you are permissioned for, this is the phase in which you will actually
(6) Verify Problem Resolution:
After you have completed the implementation phase of the action plan, use this time to verify
(7) Create a Post-Mortem Report:
A post-mortem report should detail the problem detected, the channel by which it was
detected, the symptoms of the problem, the underlying cause, and the actions taken to resolve
it. This is a useful and worthwhile document because it can be used to correct similar issues in
the future or it can be used to determine what changes were made if any future issues arise
from the implemented solution. A great number of issues are repeatedly seen by technicians
Topic: Non Linear Troubleshooting
Consider the following topology. Client A is unable to reach the Server 1 machine. You will find this
figure and a companion Troubleshooting Scenario in Chapter 14 of the text. You are welcome to refer to
it while we discuss this.
Insert Figure 14-6 Data Link Layer Troubleshooting
No clients can reach Server1, but Client A is the one to first report the problem. If the only information
you have as a technician is that Client A cannot reach Server 1’s IP Address, where do you start? You
might consider replacing all the cables between the two, but that could potentially be a lot of extra
work, and it might not solve the problem. You could also restart the Server 1 machine, but you would
be cutting off a lot more clients while it rebooted. It also might not solve the problem. The answer is to
use hierarchal isolation. The hierarchy in this case is use of the TCPIP model.
x Instead of starting at the bottom (the physical layer cables), or at the top (the server
application), start in the middle.
x You issue the ping from Server 1, and you get no replies from either SW1 or SW2
x You issue the ping from Client A to both SW1 and SW2 and the replies are successful!
o Using this isolation technique, what is left to check?
The physical cable from Server1 to SW2
The Data Link connection from Server 1 to SW2
The Network layer TCPIP settings on Server 1.
The server has a firewall in place to prevent pings (very likely)
Possibly the server application itself could still be a problem, but since there is
Network+ Simulator Matching Activities
x Identifying Troubleshooting commands to Use for Various Tasks
x Reordering Troubleshooting Steps
x Troubleshooting VLAN Client Connection Issues
Quiz Questions
You can ask the following questions either in class for a group discussion or for individual practice. If
you ask them for individual practice, review in class to make sure everyone properly understands
the content.
1. Identify and describe several common problems at Layer 1 of a network.
There are a variety of answers to this, but the following are common issues:
Cable failures or faults: Cables can wear out with use or may be affected by environmental
2. Identify and describe several common problems at Layer 2 of a network.
There are a variety of answers to this, but the following are common issues:
Power failure: Most devices on a network do not have redundant power supplies, so power
3. Identify and describe several common problems at Layer 3 of a network.
There are a variety of answers to this, but the following are common issues:
Mismatched maximum transmission unit (MTU): The MTU sets the maximum packet size; if this
does not match between sending and receiving, it can cause packets to be fragmented or errors
4. Identify and describe several problems with wireless components of a network.
There are a variety of answers to this, but the following are common issues:
RFI: Wireless devices are subject to radio frequency interference (RFI) from outside devices or
devices that use the same channel.