CCIE Pursuit Blog

December 14, 2007

Cisco 3725: How To Boot From External CompactFlash (slot0:)

This post was initially going to be an LFU and a rant along the lines of “WTF does Cisco have external CompactFlash on the 3725 if you can’t boot an image from it?”  I had already spent an embarrassing amount of time and rage figuring out that the external CompactFlash on the 3725 was slot0: and not flash:.  Then I tried to boot the image from slot0: (because I did not have room for that image in flash:), but I ran across the following problem:

r6(config)#boot system ?
  WORD   TFTP filename or URL
  flash  Boot from flash memory
  ftp    Boot from a server via ftp
  mop    Boot from a Decnet MOP server
  rcp    Boot from a server via rcp
  rom    Boot from rom
  tftp   Boot from a tftp server

It seems that you can’t boot from slot0:  I even tried to see if there was a slot0: option under “flash”: 

r6(config)#boot system flash ?
  WORD  System image filename

No such luck.  A quick trip to the InterWebs seemed to indicate that this was probably not possible.

Great.  I dropped a Base IP image into flash: and called it a weekend.  I was planning on experimenting with having the 3725 boot it’s image though TFTP (from a 2800’s flash:).  Luckily for me, Ethan Banks came to my rescue:

I did a “boot system flash slot0:/imagename.bin”, it was all good – the router would boot from the image on the external CF card. I think you should be able to do that, too. Too many things are missing in the base IP image, although maybe you can sneak by with it on this lab, like you said.

Here’s another trick with the 3745s I’m using. I do a “write erase” and “reload” to get the router ready for the next lab. That wipes out everything including the boot variable, and so during that first boot up after I wiped its little mind, the 3745 would autoboot the first image it found, a stale one left over on the system flash. To work around this “no bootvar on initial reload” nuisance, I deleted all other bins off of the 3745 system flash. Now when I boot the 3745 after a write erase, the boot variable is still wiped, but there’s only one image it can possibly autoboot – the one I need on the external CF card.

I followed his advice:

r6(config)#boot system slot0:c3725-adventerprisek9-mz.124-10.bin

and after the reload:

r6#sh ver | i IOS|image
Cisco IOS Software, 3700 Software (C3725-ADVENTERPRISEK9-M), Version 12.4(10), RELEASE SOFTWARE (fc1)
System image file is “slot0:c3725-adventerprisek9-mz.124-10.bin

Hellz yeah!!!

Thanks to Ethan my weekend will begin with lab 3 and not swearing and tinkering – although there probably will be plenty of that during IGP redistribution.  🙂



  1. I ran into this same “problem” on a 3725 today. Thanks for the fix. I first tried “boot system slot0” which was accepted but translated to “boot system tftp slot0” in the running config, which failed of course.

    Comment by sod — January 3, 2008 @ 2:23 pm | Reply

  2. Thanks for this which also got me sorted on a s/h 3725 I bought.

    You don’t actually need the boot system command. When I read what you had quoted from Ethan Banks I realised that the router would boot directly from any image in slot0 (without the boot system command) as long as there is no image on the internal flash card (but the empty flash card has to be present in the router or the router doesn’t look at slot0 for some reason).

    This has two advantages. One, the router boots quicker because it loads the image you want directly as opposed to loading the image on the internal flash first. Two, when you upgrade the IOS, you don’t have to remember to change the boot system command to match the new IOS.

    This was all tried with the ROMMON upgraded to the current latest 12.3(10r). I don’t know if the same is true for earlier versions. As the boot behaviour is controlled by the ROMMON this may be important.

    Comment by Tom — September 13, 2008 @ 5:46 am | Reply

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