640-407 | Cisco Certified Network Associate

Booting IOS

There are several ways to boot the CISCO IOS. Typically IOS is booted from flash memory. Flash memory is a type of memory that has persistence. In other words, you can save information to the flash memory and it will be retained even after turning the router off. However, it is also possible to boot IOS from another source, such as a TFTP server or from ROM. Further, you can actually copy an IOS image from a TFTP server into flash memory and vice versa. This provides a backup mechansim for your IOS.

The command used is simply the copy command. Just enter copy flash tftp to copy the IOS image from flash to a TFTP server. You will then be prompted for the name or IP of the remote tftp server, the source file name, and the destination file name. You can also do the reverse, copying from the TFTP to flash memory by using the command copy tftp flash.

This process merely makes a copy of the image. But it would be possible to actually have your server boot from a different location than flash memory. For instance you may need to recover by booting from your backup copy of IOS on an TFTP server. This is accomplished using the boot system command.

Consider the following commands:

- boot system flash {filename}
- boot system tftp {filename} {tftp server address}
- boot system rom

It should be noted, that you can specify multiple boot options. This provides fall back capability in case one option fails. If each of the commands above were entered, we would actually have specified 3 alternatives for booting IOS. The router would first attempt to boot from flash. If that failed it would try the tftp server, and as a last resort it would boot from the image on ROM.

To remove any of the boot options, just precede the command with "no", for isntance "no boot system flash {filename}".

It should also be noted that some routers don't entirely load the IOS into RAM after booting. This is done to conserve memory. As a result, less commonly used portions of IOS are left in flash memory and called when needed. This also results in flash memory being put into "read only" mode, thus preventing something like copying from a tftp server into flash memory. The trick to getting around this, and that you should be aware of, is to boot the router from ROM. The flash memory is now not being used and hence is in read/write mode, ready for being updated.

Copyright 1999, W. Brian Talbert. All Rights Reserved