[c-nsp] Configuration register 0x2 sanity check

[Dale Shaw]

Hi John,

On Thu, Sep 23, 2010 at 2:13 PM, John Neiberger <jneiberger at gmail.com> wrote:
> The engineer said that on all Cat 4K
> devices, we should be using 0x2 as the configuration register. I told
> her that I normally used 0x2102, but she said not to use that on these
> switches, that 0x2 was the recommended setting on all 4K platforms.
>
> Does that sound right to you all? It seemed overly general to me, but
> apparently this boot issue is a known problem and using a setting of
> 0x2 is a reliable workaround.
>
> Any thoughts?

Breaking it down..

The configuration register contains 16 bits. The 4 least significant
bits (bits 0-3) make up the boot field. If set your config-register to
0x2, you're flipping bit 1 on and leaving all other bits set to 0. All
values from 0x2 to 0xF (inclusive) in the boot field result in the
same behaviour.

(best viewed with a fixed-width font):


             boot
------------|----
000000000000|0000
            |

0000 = 0x0 (does not autoboot)
0001 = 0x1 (boot first image in flash)
0010 = 0x2 (boot based on 'boot system ..' commands)
0011 = 0x3  |
0100 = 0x4  |
0101 = 0x5  |
0110 = 0x6  |
0111 = 0x7  |
1000 = 0x8  |
1001 = 0x9  |
1010 = 0xA  |
1011 = 0xB  |
1100 = 0xC  |
1101 = 0xD  |
1110 = 0xE  |
1111 = 0xF  V

OK, so what does a config-register of 0x2102 do?

0x2102 (hex) = 0010000100000010 (binary).

The 'on' bits (i.e. set to '1') are bits 13, 8, 1

bit 1  = part of the boot field (as explained above)
bit 8  = unused on cat4k
bit 13 = determines how the router responds to a netboot failure (off
by default)

So my take on this is that you're not likely to see much of a
functional difference between 0x2102 and 0x2 -- the important bit
(literally!) is bit 1, which is set/on with 0x2 and 0x2102.

Maybe the cat4k has some special .. features .. that make flipping
bits 8 and 13 on a problem. Did the engineer explain why? Some
platforms behave differently when bit 1 is flipped. The cat4k doesn't
fall back to booting from flash, whereas other platforms do. I can see
a problem if you had bad 'boot system' statements but aside from that,
based on the docs, I don't see an issue.

See: http://www.cisco.com/en/US/partner/docs/switches/lan/catalyst4500/12.2/52sg/configuration/guide/supcfg.html#wp1021899

cheers,
Dale