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Does a router use bootp to load an IOS image?

I've read that if the "boot system" commands are not found in the startup-config file, the router can look for an IOS image in flash, and if it's not there, then the router will try to get the IOS image from a TFTP server. So does the router use bootp, rarp or dhcp to connect to the TFTP server?

There's a flowchart here:

It basically says Are there boot system commands in the config file? ---> No ---> Is there a valid image in the default Flash device? ---> No ---> The router tries to boot from the network using the default image name.

So, I'm just asking how the router gets to the TFTP server if there are no boot system commands in the config file and there's no IOS in flash? Thanks!


Re: Does a router use bootp to load an IOS image?

the router will only attempt to load an image from a TFTP server if it is configured to do so.

by default, a router will look for the boot system command for its image. if it is not there or the command does not exist, it looks in its default file system, usually bootflash: for the image.

if the image is not found with the boot system command or in bootflash: then the router loads into ROMMON mode.

the router CANNOT load an imgage from a TFTP server without it first being configured to load an image from a TFTP server, using the 'boot system tftp....' command.

please see the following link for more 'boot system' command information:

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Re: Does a router use bootp to load an IOS image?

Suppose the 3 boot system commands (flash, tftp and rom) were in the config file, and the router rebooted normally because the IOS was in flash. Suppose now someone accidentally removes the IOS from flash, and also the boot system commands from the config, saves it to startup config, and reloads the router. So you are saying the router will try to look for a TFTP server, because it was configured to do so in the past? Do you know if the config register 0x2124 is of any importance?

I have seen a screen output example on the web where someone removed the flash IOS and removed his boot system lines, the router reboots, asks him to enter an IP address for an interface, and the router finds the TFTP server and grabs the IOS image.

Re: Does a router use bootp to load an IOS image?

Hello Brandon,

not sure if that is what you mean, but if you would remove the IOS from flash, the router would boot into ROMMON anyway, and you would have to manually load an IOS before the router could attempt to look anywhere for a valid configuration file. Do you have the URL of the screen output you are referring to ?



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Re: Does a router use bootp to load an IOS image?

Hello! Thanks for responding!

Here is the output someone posted:

What do you think about the first Cisco link I posted? Do you think the flow chart is in error? I also have a Cisco textbook that says the same thing. "if no boot system commands, and if no IOS in flash, router goes to TFTP..."

I also wonder what 0x2124 does? There's not much about it on the net, except for a Cisco page that mentions "netboot". Don't know if that is significant.

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