CISCO Router configuration register

Hello,

i want to know what a router do if the configuration register is set to

0x2102.

In some lyrics i found that in this case it prevents the successfull loading of the Cisco IOS from flash memory.

But also i found this explantion:

" The factory-default setting for the configuration register is 0x2102, which indicates that the router should attempt to load a Cisco IOS image from flash memory."

??

So I hope someone can help me with this problem.

Greets

Reply to
Willy the Wisp
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The router will boot up from the start-up config that is in NVRAM, if there is one and will tell it to boot from the first IOS on flash, or the one specified in your start up config. This number can be changed in order to make the router do different things. For example, 0x2101 if you do not want it to boot from the default IOS. 0x2142 will tell the router not to boot up from the startup config, mostly used for doing password breaks. Hope this helps.

Reply to
ttrotter01

The configuration register is a 16-bit register. Each bit has a specific meaning:

15 - Diagnostic mode disabled 14 - Ip broadcast do not have network numbers 13 - Boot default ROM software if network boot fails 12-11 - Console speed is 9600 baud 10 - IP broadcasts with ones 8 - Break disabled 7 - OEM disabled 6 - Ignore configuration disabled 03-00 - Boot file is ... Boots Field Parameters 0x0000 Stays at the system bootstrap prompt 0x0001 Boots system image on EPROM 0x0002-0x000F Specifies a default netboot filename

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Doan

Reply to
Doan

Hi Willy,

Warning: Some Config Register bit values can vary between device MODELS, so it does help to correctly identify the Model for a valid reply. The "default" setting for Cisco Routers is usually 0x2102, but for Cisco Etherswitches it is usually 0x102. Mixing Router default settings with Switch defaults can cause great confusion........;-) I once found such a problem were a layer 2/3 Switch should have used

0x102 but was actually set to 0x2102 and a wrong version IOS was always loaded from FLASH (it always loaded the SECOND IOS in flash regardless of Config statements to the contrary.). There are detailed docs on the Cisco Web site for ALL products, I recommend that you read them for definitive statements.

Assuming you have a Router with a setting of 0x2102, this will cause a Router to boot "normally". From power on, "normally" usually includes - 1. BOOTROM will load and perform simple functional tests, and then check if the BREAK key has been pressed to halt a "normal" boot. If Break HAS been pressed it NOTES THIS, but does not stop at that point! 2. BOOTROM will then read the startup-config to see if BREAK has been disabled. If it has not been disabled and BREAK WAS pressed, BOOTROM then exits to ROMON mode, otherwise it continues. 3. BOOTROM will then read the startup-config to see if a specific VERSION(s) of IOS are defined to be loaded from either TFTP, FTP or FLASH. 4. If there is 1 or more specific version loads requested, then BOOTROM will try them IN ORDER, before falling back on the default of loading the FIRST valid IOS image it finds in FLASH. If it can't find a VALID FLASH image to load, it halts in ROMON (BOOTROM) mode.

That is a very simple overview of the process as config statements can enforce other options (EG TFTP load) before it tries to boot from whatever it finds in FLASH.

I hope this helps..............................pk.

Reply to
Peter

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