boot up process and config-register router.

[sarahr202]

Hi every body.

My book says  the value of config-register is stored in nvram.  Let say our config register value is( boot field) is 1, which means  load the ios from first ios file in flash.  Let say we power on our router( cold start) ,following sequence of events take place:

1)  router performs post

2)  If the post is successful, load the boot strap program into RAM.

Here is my problem, in order for bootstrap program to know from where to load ios ,  the boot field  of config-register must be checked which is stored in nvram.  Does bootstrap  check the value of config register from nvram  before instructing router  how to load  ios?

Thanks and Happy new year to all of you.

[Mahesh Gohil · Accepted solution]

Hello Sara,

The answer lies in below link

http://www.ccnaguru.com/router-bootup-process.html

Few details may be useful to you

Booting up the router and locating the Cisco IOS

1. POST (power on self test)
2. Bootstrap code executed
3. Check Configuration Register value (NVRAM) which can be modified using the config-register command

0 = ROM Monitor mode
1 = ROM IOS
2 - 15 = startup-config in NVRAM

4. Startup-config file: Check for boot system commands (NVRAM)
If boot system commands in startup-config
a. Run boot system commands in order they appear in startup-config to locate the IOS
b. [If boot system commands fail, use default fallback sequence to locate the IOS (Flash, TFTP, ROM)?]

If no boot system commands in startup-config use the default fallback sequence in locating the IOS:
a. Flash (sequential)
b. TFTP server (netboot)
c. ROM (partial IOS) or keep retrying TFTP depending upon router model

5. If IOS is loaded, but there is no startup-config file, the router will use the default fallback sequence for locating the IOS and then it will enter setup mode or the setup dialogue.
6. If no IOS can be loaded, the router will get the partial IOS version from ROM

Regards

Mahesh