When configuring a Cisco network from scratch, you can get things up and running flawlessly provided that you’ve configured everything properly. But chances are that in your day-to-day work you won’t get to build a network from scratch. You’ll inherit an existing network, and it will likely have some minor misconfigurations. In the worst case, it may even be broken. But even if your network isn’t broken by a misconfiguration, routers and switches do eventually wear out. Ports stop working, IOS images become corrupted, and power supplies fail.
The bottom line is that you’ll eventually encounter various problems that appear to be network-related. Here are some examples: