One of the most important skills that a network engineer must learn and develop is troubleshooting. I have designed, configured and managed hundreds of networking projects.
Its very rare to configure a network (especially complex ones) and have everything running as expected from the first shot.
Most of the times a lot of troubleshooting activity needs to take place in order to bring everything up and have your network run flawlessly.
For CCNA/CCNP candidates, the troubleshooting skill is of paramount importance. In the exam, you need to analyze configurations and find what the problem and what the solution is, check debug output data to find what’s going wrong on the router or switch, and in general show your ability to troubleshoot in addition to configuring devices.
Troubleshooting, therefore is an essential skill to learn and to practice. A Cisco CCNA/CCNP engineer will need to perform a lot of it in their career.
Ofcourse, if you are a newcomer in the field of networking you will not have enough time from the very beginning to develop and practice your troubleshooting skills.
Also, you will not be able to “play” around on the live network of your company or of your clients in order to find out how to solve and troubleshoot problems.
So my suggestion is to build a cheap home lab with 1-2 routers and a switch. The troubleshooting skills that you will acquire will be gold knowledge for your career in the future.
Some Good Training Books for Troubleshooting
Here are some books that will help you to enhance your network troubleshooting skills:
Network Maintenance and Troubleshooting Guide
Network Troubleshooting Tools