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New Member

Bootsystem flash command

I have a query relating to how the command "Bootsytem flash" works.

When we have multiple IOS images we use this command from which IOS to boot.

This is my understanding of the booting process.

POST--->BOOTSTRAP---->IT LOOKS FOR AN IOS IMAGE FROM THE CONFIG REG--->LOADS THE IOS--->LOOKS FOR NVRAM TO LOAD THE STARTUP CONFIG.

NOw my question here is, when the "bootsystem flash" command is given which is stored in NVRAM, how does it know which IOS to load which is happening before the startup config is loaded from NVRAM??

Awaiting reply.

4 REPLIES

Re: Bootsystem flash command

Hi Navneet,

When router or switch boots and it reaches the stage where it checks the config register value now depending upon the register value it decide its further steps

If you set the boot field value to 0x2 through 0xF and there is a valid "boot system" command stored in the configuration file, the switch router boots the system software as directed by that value.

If you use the boot command without specifying a file or any other boot instructions, the system boots from the default Flash image (the first image in onboard Flash memory). Otherwise, you can instruct the system to boot from a specific Flash image (using the boot system flash filename command), or boot from a network server by sending broadcast TFTP requests (using the boot system filename command), or by sending a direct TFTP request to a specific server (using the boot system filename address command).

Check this link for more details on config-reg values

http://www.cisco.com/univercd/cc/td/doc/product/l3sw/8540/cnfg_gd/ccfg8540.htm#xtocid44294

HTH, if yes please rate the post.

Ankur

New Member

Re: Bootsystem flash command

Hi Ankur,

Thanx for your response. Iam clear now about the bootstrap checking the Config register values to load the IOS image. Now only after u load the IOS image will the router look for a startup file which is in NVRAM. And since that "Bootsystem flash" command is in the startup file, how will load the required IOS, since startup file lookup happens after IOS loading. Hope u got my query.

Awaiting Reply.

Purple

Re: Bootsystem flash command

Hi,

The 'boot system flash' command actually results in the setting of certain environment variables (which are stored separately to the startup-config). These are set when you enter these commands and save the configuration.

When the router boots, it uses these boot variables to determine where to boot from and what config file to use. Therefore, it does not need to read the startup config to determine this.

Pls do rate posts that help.

Paresh

New Member

Re: Bootsystem flash command

Thanx paresh, i got my answer.

Navneet

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