So it is sunday morning, the change window just kicked in, you copied the new IOS image to the router, used the ‘boot system’ command as per my previous post, you save your config and reload. All looking good for an early night, but when the router reloads you get a bunch of errors during bootup along the lines of:
% Invalid input detected at '^' marker.
% Incomplete command.
Oh no, you didn’t do you homework, did you check for command differences between the IOS versions? Did you test the current config on the new IOS in a lab prior to the upgrade (yes not always possible), do you have a config backup?
If you don’t have a full config backup you have BIGGER problems. If you remove the first ‘boot system’ command to boot of the working IOS, and write your config, usually all commands that gave errors during that boot-up, will now be LOST since you saved the config and overwrote the startup-config.
So what now?
Appose to freaking out and start dancing like a banshee doing some tribal dance, do the following. ‘Rename’ the NEWLY installed IOS image in Flash, the image specified in the first ‘boot system’ command to something else, and ‘Reload’ WITHOUT SAVING the config. When the router reboots it will attempt to locate the first specified boot system image, but because you renamed it, it can not be loaded. The router will then attempt to boot off the second specified boot system image, the old working IOS image. And happiness is restored.
Routing-Bits 