CISCO Router configuration register

Discussion in 'Cisco' started by Willy the Wisp, Apr 28, 2006.

  1. 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
     
    Willy the Wisp, Apr 28, 2006
    #1
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  2. Willy the Wisp

    Guest

    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.
     
    , Apr 28, 2006
    #2
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  3. Willy the Wisp

    Doan Guest

    On 28 Apr 2006, wrote:

    > 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.
    >

    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

    http://www.cisco.com/en/US/products...note09186a008022493f.shtml#config-reg-meaning

    Doan
     
    Doan, Apr 28, 2006
    #3
  4. Willy the Wisp

    Peter Guest

    Hi Willy,

    > i want to know what a router do if the configuration register is set to 0x2102.


    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.


    > " 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."


    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.


    --
    Peter from Auckland.
     
    Peter, Apr 28, 2006
    #4
  5. Willy the Wisp

    theapplebee

    Joined:
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    Messages:
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    Location:
    USA
    Last edited: Apr 17, 2010
    theapplebee, Nov 4, 2009
    #5
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