The earlier sections of this chapter explained a general troubleshooting methodology. When going
through this methodology, it is often helpful to approach the problem in a logical manner that leverages
the OSI model. Therefore, Cisco has started backing a model of troubleshooting that does just
that. This model has three distinct approaches: bottom-up, top-down, and divide-and-conquer.
Bottom-Up Troubleshooting Approach
As the name implies, when you use the
bottom-up troubleshooting
approach, you start with the
bottom—the Physical layer of the OSI model—and work your way up to the top—the Application
layer. This approach is used when you suspect the problem is at the Physical layer, or when you
are troubleshooting a complex network problem. In these situations, ensuring that the core components
required for networking are in place can go a long way toward isolating the problem.
The downside to bottom-up troubleshooting is that it can require the checking of each interface
along the path to see if errors are occurring there. Depending on the length of the path from
the end points of the problem, this process can be very time-consuming. In these cases, determining
the most likely culprit based on the symptoms of the trouble can save a lot of time.
Top-Down Troubleshooting Approach
If you suspect that the problem lies in a piece of software, then
top-down troubleshooting
should be used. You start by testing the application and work down the OSI layers to find the
source of the problem. The challenge to this type of troubleshooting is that you need to check
all the user’s network applications in order to find the one that is causing the errors. This is a
potentially time-consuming troubleshooting method if there are a large number of applications
that could be the source of the trouble.
Divide-and-Conquer Troubleshooting Approach
The
divide-and-conquer troubleshooting
approach allows you to select the specific layer (Data
Link, Network, or Transport) of the OSI model in which to begin troubleshooting. You make
your selection based on experience with similar problems in the past, along with the specific
symptoms of the current trouble. After selecting the layer you wish to start with, the next task
is to determine the direction of the problem by determining whether the problem exists at,
above, or below this layer. Most commonly this is done by studying output from the IOS commands
on the router or through analysis of the output of network management tools. Once the
direction of the problem is determined, you continue troubleshooting through the OSI model in
that direction until you isolate the difficulty.
Often you can check the first four layers (Physical through Transport) by using
the
traceroute
command.
Summary
With the complexity of today’s networks, it is important to adhere to a troubleshooting model
to aid in efficiently and effectively isolating and resolving network problems.
Various methods of problem isolation and the troubleshooting method itself help administrators
pinpoint problem areas and foresee future trouble. Troubleshooting skills are gained
through experience. It is unreasonable to expect that you can jump in on your first network failure
and be able to solve it quickly. Experience is the best teacher. Following a problem-solving
model helps you to reach a timely solution to network failures. It helps to know your network,
but the “shooting-from-the-hip” style of troubleshooting is nowhere near as effective as a
methodical and logical process.
Using the three steps of the Cisco troubleshooting model in order is a clear, calculated, and
logical way to make a network run more smoothly. The three methods of problem isolation
(bottom-up, top-down, and divide-and-conquer) are more subjective, and it is up to each individual
to use the appropriate method for the problem that they are facing. It is important to document
changes so you have a trail of what was done on the network. Finally, it’s important to
reverse any network alterations that did not correct the problem.
Exam Essentials
Know the three steps to the Cisco troubleshooting model and the function that each step performs.
The three steps to the Cisco troubleshooting model are gather symptoms, isolate the
problem, correct the problem, and repeat if necessary. Once a problem is resolved, documentation
should be updated.
Know the troubleshooting methodologies and how to use them.
These troubleshooting
methodologies are bottom-up, top-down, and divide-and-conquer. In addition to understanding
them, know when it is most appropriate to use each method.
Be able to apply the Cisco troubleshooting methodology to example situations.
Know how
to apply each step of the troubleshooting model in real-life scenarios. You should be able to
determine what step in a troubleshooting scenario is next in the series, and understand how to
correlate a task with the correct step in the process. 1042