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The Internal Components of a Cisco Router
To configure and troubleshoot a Cisco internetwork, you need to know the major components
of Cisco routers and understand what each one does.
Cisco Router Components
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 determine
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 memory. The mini-IOS can also perform
a few other maintenance 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 most routers expand the
IOS from flash into RAM upon boot. Also known as DRAM.
ROM (read-only memory):
Used to start and maintain the router. Holds the POST and the
Bootstrap program, as well as the mini-IOS.
Flash memory:
Stores the Cisco IOS by default. Flash memory is not erased when
the router is reloaded. It is EEPROM (electronically erasable programmable
read-only memory) created by Intel.
NVRAM (nonvolatile RAM):
Used to hold the router and switch configuration. NVRAM is not
erased when the router or switch is reloaded. Does not store an
IOS. The configuration register is stored in NVRAM.
Configuration register:
Used to control how the router boots up. This value can be
found as the last line of the
show version
command output and,
by default, is set to 0x2102. The default setting tells the router
to load the IOS from flash memory as well as to load the configuration
from NVRAM.
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