When you apply the divide-and-conquer approach toward troubleshooting a
networking problem, you select a layer and test in both directions from the
starting layer. Begin the divide-and-conquer approach at a particular layer
based on the user experience level and the symptoms that have been gathered
about the problem. The Cisco IOS command set allows the user to choose an
intermediate level at which to begin troubleshooting and a direction to work
toward. Once the direction of the problem has been identified, work in that
direction until you can identify the cause of the problem.
If you can
verify that a layer is functioning properly, it is typically a safe assumption
that the layers below it are functioning. If a layer is not functioning
properly, gather symptoms of the problem at that layer and work your way
down.