Understanding 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. Table 2.1 describes the major Cisco
router components.
TABLE 2 . 1
Cisco Router Components
Component Description
Bootstrap Stored in the microcode of the ROM, the bootstrap brings a router up
during initialization. It will boot the router and then load the IOS.
Power-on self-test
(POST)
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 open an interface and load a Cisco IOS into
flash memory. The mini-IOS can also perform a few other maintenance
operations.
Random-access
memory (RAM)
Used to hold packet buffers, ARP cache, routing tables, and the software
and data structures that allow the router to function. The running-config
file is stored in RAM, and most routers expand the IOS from flash into
RAM upon boot.
Read-only memory
(ROM)
Used to start and maintain the router. Holds the POST and the bootstrap
program, as well as the mini-IOS.
Flash memory Used to store the Cisco IOS by default. Flash memory is not erased when
the router is reloaded. It is electronically erasable programmable readonly
memory (EEPROM) created by Intel.
Nonvolatile RAM
(NVRAM)
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 file 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 it is set to
0x2102, which tells the router to load the IOS from flash memory as well
as to load the configuration from NVRAM.