Boot a 2811 router from an USB Image

In the case of your flash bad but you still want to boot the router.
You can use your pen drive as your temporary boot drive

Prepare pen drive with FAT format (not FAT32)


This link said Cisco router support FAT32

-insert pen drive with size <4GB otherwise formatting will be failed
-check its drive letter
run cmd
>format f: /fs:FAT /q
Copy the router IOS image into it
Put pen drive onto the router
In my 2811, usb port on top is usbflash1 while below that is usbflash0.
Power on the router
Before its booting press Break
rommon1> boot system usbflash1:c2800nm-advipservicesk9-mz.124-11.T1.bin
It will start booting from your pen drive
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

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