Invariably, troubles and misconfigurations creep into networks. New devices get connected and added to a network. Old devices are removed, and something seemingly unrelated breaks. Troubleshooting is really a test in discerning patterns.

My favored method for solving problems is to start with the simplest elements, verifying correct operation and proceeding to the next layer or element until I have isolated the problem element. If you are lucky, you'll know from a symptom where the problem is likely to be, but more often, you'll have to start at the bottom of the networking hierarchy, and verify each other layer.