o back out all changes was coming, and nothing was working. We were installing a new T1 link, which was not a complicated task, but it wasn't going right. The problem turned out to be human error. My instructions clearly stated that the T1 was to be in the port marked 4 in a panel with eight ports. What the documents didn't say was that there were two panels marked the same way, and I was in the wrong panel. I managed to bring down the entire operation's phone system, and for three hours, the company took no calls. This is called "a learning When to Upgrade and Why experience." My hope is that you will learn from mine, but most people need to learn these lessons on their own. Change of default behavior in software Over the years, Cisco has decided (rightfully) that certain commands should be the default behavior of Cisco routers-for example, ip subnet-zero and ip class less. If you designed your network without these features turned on and somehow relied on the fact that they were turned off, you'll be in for a surprise if you upgrade and find that some of your networks are now not functioning. Valid Reasons to Upgrade The following are legitimate reasons to upgrade: To resolve a problem If you have a router that crashes every time you type the show arp command, chan- ces are you've got a bug that can be fixed by upgrading that router. Determining whether you truly have a bug should be a job for tech support, but bug listings are available online and can be searched by anyone with Internet access. Software or hardware is end-of-life Vendors stop supporting products. If your software or hardware is end-of-life, it won't be supported anymore, and if you have any problems, you will not be able to get them resolved. Unsupported hardware is a risk to the ongoing operation of your network and should be upgraded. A new feature is needed Notice I didn't say, "A new feature is wanted." Sometimes a new feature is a ne- cessity. I once worked on a large network with DS3s crossing the globe. We had almost 40 of them. When on