Boot a 2800-router with an USB image

If you would like to boot an ISR router (for example, a 2800) from USB flash, but cannot upgrade the ROMMON, all is not lost - you can specify the USB-based image with the boot system configuration command (for example, boot system usbflash1:c2800nm-advipservicesk9-mz.124-11.T1.bin), but the boot process will take significantly longer (this also applies to any other scenarios where the ROMMON cannot get the image specified in the boot system command):

  • When the router is reloaded, ROMMON reads the router configuration and tries to start the required image
  • If ROMMON cannot load the image specified in the boot system command, it starts the default image (usually whatever is stored in on-board flash)
  • When the first IOS image loads (after being copied and decompressed in most cases), it discovers that it's not the correct image
  • The now-operational IOS image loads the new image in RAM (in our case from usbflash1:), decompresses it and transfers the control to it

2 comments:

  1. Does this work?

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  2. I just found out today, it does not - if you try to the rommon command to show devices (I forget what it is, the help menu is real short though), it's not listed. My config did have that command in there, still nothing. If you are trying to save space by keeping your image on a USB (like I was), you'll get burned sooner or later

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Ivan Pepelnjak, CCIE#1354, is the chief technology advisor for NIL Data Communications. He has been designing and implementing large-scale data communications networks as well as teaching and writing books about advanced technologies since 1990. See his full profile, contact him or follow @ioshints on Twitter.