Imagine you get a call at 2:00 a.m. in the morning. The person on the other end says: "Hi, we are facing a difficult issue that is impacting production services. We suspect it might be network-related. Can you check this for us? For this type of urgent, open-ended question, what would be the first thing you do?" Most of the time, the thing that comes to mind would be: What changed between the time when the network was working until something went wrong? Chances are you would check your monitoring tool and see if any of the key metrics changed in the last few hours. Better yet is if you have received any monitoring alerts from any metric baseline deviation.

Throughout this book, we have been discussing various ways to systematically make predictable changes to our network, with the goal of keeping the network running as...