- Unless you’re really savvy about the inner workings
of your car systems, you’ll take it to someone to figure out
what’s wrong when a problem arises
to get back on the road quickly.
It's the same deal with Cisco Networking Devices.
You need to know all of their major components,
as well as what they do and why
and how they all work together.
The more solid your understanding,
the better equipped you'll be to configure and troubleshoot
a Cisco internetwork.
First let's talk software.
Stored in the microcode of the ROM,
the Bootstrap is used to bring a router up
during initialization.
It boots the router up and then loads the IOS.
Also stored in the microcode of the ROM is the POST
or power-on self-test.
The POST is used to check the basic functionality
of the router hardware
and determines which interfaces are present.
Again stored in the micro code of the ROM,
the ROM Monitor is used for manufacturing, testing,
and troubleshooting as well as running a mini-IOS
when the IOS in flash fails to load.
Called the RX Boot 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.
Next let's chat hardware.
RAM or Random Access Memory holds packet buffers,
ARP cache, routing tables and also the software
and data structures that allow the router to function.
The running config is stored in RAM,
and most routers expand the IOS
from flash into RAM upon boot.
On the other hand, ROM or Read-Only Memory
starts and maintains the router.
As I mentioned, the bootstrap, POST and mini-IOS
are stored in ROM.
Flash memory stores the Cisco IOS by default
and is not erased when the router is reloaded.
It is EEPROM or electronically erasable programmable
read-only memory created by Intel.
Finally, the Configuration Register is used to control
how the router boots up.
This value is displayed in the last line
of output with the show version command.
By default, it is set to 0x2102 which tells the router
to load the IOS from flash memory
and load the configuration from NVRAM.
These basic components are found in routers,
switches and many other devices.