No matter how well you plan an upgrade, you are eventually going to need to change a router configuration remotely. If you make a mistake and can no longer get to the router because of the change you made, you have to make the embarrassing call to someone to go "hit the power." The power cycle takes the router back to the original starting configuration because your change was never saved. In other words, since you lost connection, you couldn't type "copy run start" to save your changes.

One of the classic mistakes (I know because I have done it myself a number of times) is to incorrectly update an access-list on an interface when you are connected to the device remotely. And suddenly, the Telnet connection is dropped to the router because of a forgotten list entry that would permit your incoming connection.

There is another way. When you are doing something tricky, you can use the following feature of the reload command, which causes the router to reboot in a certain number of minutes. For example, let's tell the router to reboot in three minutes.

MyRouter#reload in 3
   Reload scheduled in 3 minutes
Proceed with reload? [confirm]y

Now, we have three minutes to do what we need to do. Let's say we are applying an access-list to serial0.

MyRouter#config terminal
Enter configuration commands, one per line. End with CNTL/Z. MyRouter(config)#interface serial0
MyRouter(config-if)#ip access-group 110 in
MyRouter(config-if)#^Z
MyRouter#

We made the change and everything still works. (Well, at least our connection wasn't dropped.) Now all we have to do cancel the impending reload with the following command:

MyRouter#reload cancel

Or, if our access-list update did destroy our connection to the router, all we need to do is wait three minutes (plus the router's reload time) before the router is back online. After the reload, the router uses the original saved configuration before our access-list change.