As most administrators know, it's very important to have a backup of your router and switch configurations, because...
when a device fails, and they ship you a new one, you may not be able to get the old config from the failed device. You definitely don't want to have to reconfigure the new device from memory, especially if you're under pressure in a critical outage. You're likely to cause more problems than you solve.
There are several ways to save configs now, with the most sophisticated and expensive being full-blown configuration management systems. These are typically part of framework packages like Ciscoworks or HP's Openview. On the other end of the spectrum is simply displaying the configuration in a terminal window and then using the cut and paste features to save it to a text file. What this quick and dirty method lacks is a way to keep track of which config files are most current and a way to share configs easily with other administrators who may need them.
A good solution is to save these files to a central server. For decades you've been able to use the TFTP service and RCP to accomplish this task, but these protocols have quite a few drawbacks, not the least of which is security. But for a few years now, most manufacturers have supported more secure versions, including FTP and SCP, so consider setting up an FTP server with a directory for device configs and be prepared. Also keep in mind that you'll probably be copying files from network devices directly to the server, but until your device is configured, it won't be able to access the server. So you'll either have to copy from the server to your PC and then cut/paste into the replacement router or switch, or you'll need to be prepared to configure the router or switch enough that it can talk to the FTP server to get its config.
For Cisco's IOS, here's a sample command that copies from an FTP server to a router's start-up config:
Router# copy ftp://myacctname:mypassword .168.1.10/savedrouter-confg nvram:startup-config
It's generally better to copy to the startup-config and then reboot, so you don't have any conflicts with your temporary configuration that is only in the running-config.
Tom Lancaster, CCIE# 8829 CNX# 1105, is a consultant with 15 years experience in the networking industry, and co-author of several books on networking, most recently, CCSPTM: Secure PIX and Secure VPN Study Guide published by Sybex.