A very wise friend once taught me a Cisco configuration command that I should carry around on a laminated card in my wallet:

copy running-config startup-config (or copy run star)
reload in 10

A little background:

I have a high-speed digital line providing my home internet access, and a Cisco 4700M router that sits at my end of that line. The router is powerful enough that I use its packet filters as a crude firewall in front of a few of my machines, and as I add or remove services on those machines, I need to make changes to those filters. As often as not, I’m doing this from someplace outside my home — meaning that I’m (stupidly) making changes to the router’s access rules over the very internet connection to which those access rules apply. And there have been about a dozen instances — this afternoon being one of them — when a typo or mistake on my part has created a rule that essentially shuts down any traffic over the connection… so that all of my servers are essentially deaf to the world, and the only recourse is to come home and fix the mistake via a direct connection to the router.

So now, what’s so good about the reload in 10 command? If you issue that command before trying to make any configuration changes, that sets the router up to reboot itself in 10 minutes. During that 10-minute window, you can make any configuration changes you want, and so long as you don’t save the new config to startup, any problems will be wiped away once the box restarts itself. If you make all the changes you need before 10 minutes passes and you’re happy with the results, you can just issue a reload cancel command, and you’ll be all set.

Now, if only I could follow my own advice…