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Setting Boot Variable

I must upgrade IOS on a cisco 3750.

There is not enough room in the flash for 2 IOS versions, so I have to delete the existing IOS and then download the new one into flash.

Then I have to set the boot variable to boot from the new image.

Here is the question: Do I have to save the config (do a "wr mem") after changing the boot variable and before rebooting the switch to make sure the boot variable is set?

I am asking because I would rather not save the config. I need to disable TACACS so a field tech can get into the switch and actually do the upgrade. Id rather the config return to normal after the reboot without having to re-enable TACACS, change the fallback passwords, etc.

Any advice?



New Member

Re: Setting Boot Variable

Hi I read your posting and I can answer a couple of your questions, When I upgrade IOS on my switches I do set the boot variable and do a write memory, by doing this when you enter the reload command the switch will reload without asking if you want to save the configuration. Now if you don't do the write memory after changing the boot variable the switch will ask if you want to save the config before the reload, in your case I would answer “no”. I am not quite following you on the TACACS config, if you do a “show config” are your TACACS setting displayed? if so they are already saved in the start-up config. You can upgrade you IOS and then reload the router at that time you can do a erase startup-config and return the switch back to the default settings. I would suggest working on your IOS upgrade first then the password issue and TACACS issue afterwards. HTH


Re: Setting Boot Variable

OK, here goes.

The switch has TACACS configured.

Someone without a TACACS account must access the switch locally and do the actual code upgrade.

So I am going to remove TACACS to allow him in.

He will download the IOS, change the boot variable (I guess this isnt really necessary because there will only be one IOS in the switch anyway), and then reboot.

If he does set the boot variable, will he HAVE to do a "wr mem" for it to take?

Id rather he NOT do a "wr mem", so that when he does reboot, TACACS will be reinstated and I wont have to apply it again.



Re: Setting Boot Variable

By the way, you may be wondering why I dont just do the code upgrade myself....

Since I will have to delete the existing IOS, leaving the switch with nothing, if the download fails, or the switch reboots because someone unplugged it or we lose power, etc, I do not want to have it get stuck in ROMMON. With fixed configuration switches, it is a BIG pain in the butt to fix. You have to do an xmodem file transfer through the console port and it will take you about 2 hours.

If someone does the code upgrade locally, it minimizes the risk for obvious reasons.


Re: Setting Boot Variable

"Do I have to save the config (do a "wr mem") after changing the boot variable and before rebooting the switch to make sure the boot variable is set?"

you can check the boot variables before realod whether it is for new image or old.


Re: Setting Boot Variable

No you don't have to do a wr mem to save the boot variable , it gets changed in nvram as soon as you hit the enter key . You can verify by entering the new boot string and then doing a show bootvar before doing any write mem.


Re: Setting Boot Variable

Thank you, Mr. Glen Grant.


Glen, do you think my precaution of having the code upgraded locally instead of over the WAN -- given the circumstances (read above) -- is reasonable or a bit exaggerated?

Would like to hear some people's thoughts and/or experiences with this.

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