NVRAM - how does it work

Discussion in 'Cisco' started by Oli, Mar 6, 2008.

  1. Oli

    Oli Guest

    I am looking forward for a book or a webpage which is explaining how the
    NVRAM of a cisco router works - in detail.
    I'm already able to configure a cisco router. ;-)
    I just want to understand detailed the mechanism of the boot system
    commands.

    Greetings from germany

    Oli
    Oli, Mar 6, 2008
    #1
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  2. Oli <> writes:
    >I am looking forward for a book or a webpage which is explaining how the
    >NVRAM of a cisco router works - in detail.
    >I'm already able to configure a cisco router. ;-)
    >I just want to understand detailed the mechanism of the boot system
    >commands.


    Its some Flash memory that gets memory mapped into the address space
    of the processor and read by ROMMON as it boots.

    The memory maps are very different per platform.

    Maybe get the dynamips emulator, and study how it starts up an IOS image?
    Doug McIntyre, Mar 6, 2008
    #2
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  3. Oli

    Oli Guest

    Doug McIntyre schrieb:
    > Oli <> writes:
    >> I am looking forward for a book or a webpage which is explaining how the
    >> NVRAM of a cisco router works - in detail.
    >> I'm already able to configure a cisco router. ;-)
    >> I just want to understand detailed the mechanism of the boot system
    >> commands.

    >
    > Its some Flash memory that gets memory mapped into the address space
    > of the processor and read by ROMMON as it boots.
    >
    > The memory maps are very different per platform.
    >
    > Maybe get the dynamips emulator, and study how it starts up an IOS image?
    >

    Does it mean, that every configuration part like "Boot system" can be
    found in a defined E/O register and is simple activated with a kind of
    flag while the configration is entered?
    I'm thinking because the most of the configuration file ist read after
    the IOS has been loaded.

    I don't know the dynamips emulator. I will take a look at it.

    Thanks a lot so far

    Oli
    Oli, Mar 7, 2008
    #3
  4. Oli <> writes:
    >Doug McIntyre schrieb:
    >> Oli <> writes:
    >>> I am looking forward for a book or a webpage which is explaining how the
    >>> NVRAM of a cisco router works - in detail.
    >>> I'm already able to configure a cisco router. ;-)
    >>> I just want to understand detailed the mechanism of the boot system
    >>> commands.

    >>
    >> Its some Flash memory that gets memory mapped into the address space
    >> of the processor and read by ROMMON as it boots.
    >>
    >> The memory maps are very different per platform.
    >>
    >> Maybe get the dynamips emulator, and study how it starts up an IOS image?
    >>

    >Does it mean, that every configuration part like "Boot system" can be
    >found in a defined E/O register and is simple activated with a kind of
    >flag while the configration is entered?
    >I'm thinking because the most of the configuration file ist read after
    >the IOS has been loaded.


    Depends on the platform..

    Switches tend to have an environmental variable for the boot drive
    that will get updated when you do the 'boot system' in the config.
    On the very rare occassion, this can get messed up and you may have to
    set it manually (at least I've had to on two older switches). ROMMON
    environmental variables get stored in yet another area of flash:
    You can see these from ROMMON on switches.

    Routers don't do this, but instead seem to parse the full config for
    the 'boot system' commands and act appropriately. Again, poke around
    in ROMMON and you won't find the boot env variable the same as in a switch.
    Doug McIntyre, Mar 7, 2008
    #4
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