HOW TO BOOT ROUTER IN LAB

Router goes
through the three steps when booting up

1.  POST (Power
on self test)

2.  Locate and
load IOS

3.  Locate and
run device configuration file

Before we
start exploring this procedure in more deeply, let’s be familiar with the type
of memories used by router. Usually router uses four types of memory.

·         ROM

·         FLASH

·         NVRAM

·         RAM

ROM

ROM ( Read
only memory ) stores four components POST, Bootstrap program, ROMMON mode and
Mini IOS.

POST

POST (
Power on self test ) is a low level diagnostic utility that performs various
tests on hardware components. It verifies that all necessary components are
present and operational. Modular slots are checked in this process for any
hardware change like installing new interface or removing existing interfaces.

Bootstrap

Bootstrap
is the second utility in booting sequence. It controls the search and load
process of IOS. Bootstrap program is responsible for bringing up the router,
finding IOS on all possible locations and loading it in RAM.

ROMMON

ROMMON is a
portable IOS program that allows us to perform various diagnostic tests. This
program is also used for password recovery procedure. It has its own mode (
Check our previous article to know more about Cisco IOS mode) known as ROMMON
mode. Boot sequence follows a conditional rule for this mode. If bootstrap
successes in finding and loading operation of IOS, then boot sequence will not
enter in this mode. Boot sequence will enter in this mode automatically, if it
fails to load IOS in RAM from all possible locations.

You can
manually enter in this mode for diagnostic purpose.
Run reload command from privileged mode to reboot the router. Press + C key
combination ( CTRL Key with C Key ) in first 60 seconds of boot sequence.

Mini-IOS

Mini-IOS is
a fallback utility that contains a stripped down version of IOS. This is used
in critical situations where IOS image in flash is not found. Mini-IOS contains
only IP code that allows you to load IOS from other resources such as TFTP
Server. Cisco IOS mode used by this stripped down IOS utility known as RXBOOT
mode.

Flash

Flash is a
nonvolatile memory. Data store in flash is not lost when you turn the router
off. You could assume flash as hard disk of router. Like data stored in hard
disk remain safe, same as data stored in flash remain safe. Router uses flash
to store IOS image. During the boot process router load IOS file from flash to
ram.

NVRAM

NVRAM is
another permanent memory. Data stored in NVRAM is also remain safe. Router use
NVRAM to store configuration files. NVRAM uses a battery to maintain the data
during the power off stage.

RAM

RAM is a
temporary memory. Information stored in RAM does not remain in power off stage.
Everything in RAM is erased, when you turn off the router. RAM is the fastest
memory among these memories. In a powered on router, RAM contains all the
information required to function the device.

·         During the boot process IOS is
copied and decompressed in RAM from Flash.

·         Same as ISO, running configuration
is also copied in RAM from NVRAM.

·         RAM stores ARP, CDP neighbor,
routing and other tables.

·         Interface input and output buffers
are also stored in RAM.

Now we know
the memories used by router to store the data. It’s time to understand the
process followed by router to boot the device.

Configuration
register value

Configuration
register value is a special register that is used to control the booting
process. You can check current configuration register value with “show
version” command from exec mode .

· It is a
four characters hexadecimal value, that can be changed from privileged mode.

·         Default value is 0x2102.

·         “0x” indicates that
the characters that follow are in hexadecimal.

·         The third character in the
configuration register can modify how the router loads the configuration file.

·         The fourth character in the
configuration register is known as the boot field. Changing this value, will
change booting process.

·         Setting of 0x2100 will
always boot router in ROMMON mode.

·         Setting of 0x2101 will
always boot router in RXBOOT mode.

·         Setting
of 0x2102 to 0x210F will always load first valid IOS from
flash.

·         values
of 2 through F for the fourth character specify other IOS
image files in flash.

·         Router keeps password in
startup-config file that is stored in NVRAM.

·         The setting
of 0x2142 causes the router to ignore the startup-config file in
NVRAM and proceed without a configuration. Thus the setting of 0x2142 is used
for password recovery process of router.

Router
booting process

Router goes
through the following steps during the boot process.

1.  Router is
powered on.

2.  Power on
self test (POST) is performed to check hardware components including memory and
interfaces.

3.  Bootstrap
program is loaded and executed.

4.  Bootstrap
reads configuration register value to determine how the router will boot up.

5.  Depending
on the value of configuration register, bootstrap program finds and loads the
IOS image.

6.  If
bootstrap fails to load IOS from all possible locations it will drop boot
sequence in ROMMON mode for troubleshooting.

7.  If IOS is
loaded, the IOS will try to find and load the configuration.

8.  If
configuration is not presented, system configuration dialog would be launched.

9.  If
configuration is loaded, you would be presented in CLI interface.