[c-nsp] Re-pack IOS

Doug McIntyre merlyn at Geeks.ORG
Wed Jul 29 12:52:10 EDT 2009


On Wed, Jul 29, 2009 at 01:19:16PM +0400, Aleksandr Gurbo wrote:
> Thank you, Peter, for you wide answer, but my question is previous.
> I explain. I want unpack image and pack again image, but with better parametres of compression. So I can receive image with size less, then I have. After this manipulations I can upload new compressed image on my 128Mb compact flash.
> On images for 26xx series it is possible re-pack IOS, you can try yourself. I don't know about re-pack operations on images for 28xx/38xx/76xx series, may be new degree of protection or new methods of compression are used.


The reason it hasn't been done is because its not worthwhile. 

You say you want to gain 6MB, but I content its not possible to
compress it that much better.

I unpacked a 2800 image, and repacked it with 'gzip --best' and gained
only 78k. Then I did the same thing with 'bzip2 --best' and gained 1M

While the bzip2 method got you something almost signifigant, unless
you have a super wunder compression algorithm that can do
signifigantly better than 'bzip2 --best' (at least 6 times better), 
you aren't looking at this as viable, let alone the time to develop
the unpacker in IOS bare-metal coding, and implementing it correctly.
bzip2 takes a signifigant longer time to unpack than the current LWZ alg
does as well, increasing boot up time much more. 

The previous answers of either booting off TFTP or buying more flash
which is virtually dirt cheap on the 2800/3800/7600 routers is the
correct answer, even if you didn't like to hear it.


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