When applying a bottom-up approach towards troubleshooting a networking
problem, the examination starts with the physical components of the network and
then is worked up through the layers of the OSI model until the cause of the
problem is identified.
It is a good
approach for a troubleshooter to use when the problem is suspected to be
physical. Most networking problems reside at the lower levels, so implementing
the bottom-up approach will often result in effective results.
The
downside to selecting this approach is that it requires checking of every
device and interface on the network until the possible cause of the problem is
found. It is a requirement to document each conclusion and possibility. The
challenge is to determine which devices to start with.
In many cases,
problems within the first four layers can be determined by entering a
ping or traceroute command. If the
connection is successful, then the cause is likely at the application level.
Otherwise, a closer look at the lower levels will be needed to locate the
problem.
Verify that Internet control message protocol (ICMP) echo
request and echo reply are enabled on the network in order for commands such as
ping and traceroute to work. If
disabled, the failure of a ping or
traceroute command can easily be mistaken for a loss of
connectivity.