Performing the Upgrade
An IOS upgrade requires a router reboot, which interrupts service. If all goes well, the router will only be down for a few minutes. If the new IOS image does not work, you must reload the old IOS onto the router and reboot again. If you have an external flash card, or if you have enough room on the router's flash memory to store multiple IOS images, you'll be able to restore the old image fairly quickly. If you don't have those features, you will need to get the old IOS image onto the router either via FTP or SCP, or if your router no longer knows it has network interfaces, via xmodem over a serial cable. Allow yourself at least two hours for this recovery process.
In short, schedule a two-hour (or longer!) window of acceptable ...
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