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The Internal Components of a Cisco Router
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T A B L E 7 . 1
Cisco Router Components
Component
Description
Bootstrap
Stored in the microcode of the ROM, the bootstrap is
used to bring a router up during initialization. It will
boot the router and then load the IOS.
POST (power-on
self-test)
Stored in the microcode of the ROM, the POST is used
to check the basic functionality of the router hardware
and determines which interfaces are present.
ROM monitor
Stored in the microcode of the ROM, the ROM monitor
is used for manufacturing testing and troubleshooting.
Mini-IOS
Called the RXBOOT or bootloader by Cisco, the mini-
IOS is a small IOS in ROM that can be used to bring
up an interface and load a Cisco IOS into flash mem-
ory. The mini-IOS can also perform a few other main-
tenance operations.
RAM (random-
access memory)
Used to hold packet buffers, ARP cache, routing
tables, and also the software and data structures that
allow the router to function. Running-config is stored
in RAM, and the IOS can also be run from RAM in
some routers.
ROM (read-only
memory)
Used to start and maintain the router.
Flash memory
Used on the router to hold the Cisco IOS. Flash mem-
ory is not erased when the router is reloaded. It is an
EEPROM created by Intel.
NVRAM (non-
volatile RAM)
Used to hold the router and switch configuration.
NVRAM is not erased when the router or switch is
reloaded.
Configuration
register
Used to control how the router boots up. This value
can be seen with the
show
version
command and
typically is 0x2102, which tells the router to load the
IOS from flash memory.
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