Saturday, March 8, 2008

Router Boot up Process


When we apply a command
Router # show version
Output: …………………………configuration Register 0x2102

When we turned on the device the first thing they do is to read the configuration register value and this value by default is 0x2102.

Normally the best place from where we should run our operating system is FLASH, then TFTP and at last ROM. Why we should run from FLASH because our full operating system is kept there and after this why from TFTP not from ROM, because we kept our operating system backup here and at last when we need ios and both FLASH are erased and we also don’t have backup so then the best place from where we should run is ROM (here we have baby ios).

Now our heart says that the boot up sequence will be like see Fig A, first it will read the register value and then point towards Flash to load the operating system and then to the NVRAM to load the configuration files. But this is not the case here because; suppose FLASH becomes corrupt then system will not respond.

NOTE: Why the system read the register value first?

There is a concept called Cold Stating, which states that when system start the command will move to a small memory and than to main micro-processor.
Now the correct sequence and diagram of the router boot up process is given see Fig B, in which after reading the register value it will point to the NVRAM and there we identify the boot up sequence say, Boot System FLASH. After loading the IOS it will return to the NVRAM.

Now suppose the FLASH becomes corrupt then? So after reading the register value it will point to NVRAM and there we identified the boot up sequence that if FLASH not found then boot from TFTP and it this is also corrupt or we haven’t took the backup then boot from Rom see Fig c.

6 comments:

nayyares said...

Excellent...it would be rather good if you add more technical detail...but its an excellent effort.

Thanks

Sohail Akhtar said...

Ya sure sir i will....as i have just start the course and studied very little....so from time to time i will something more...once agian thnx

Khush Dil Khan said...

Good yar........
As you know I am not familiar with these things
but still i am able to find what is there which you want to explain.
once again well done and carry on .....

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