So the boot field, this last value, determines how the router will boot.
That consists of four binary bits which either set on or off a value of 0 means ROM monitor, a value
of 1 means boot into RX boot in the old days or today means boot using the first operating system
in flash.
Setting it to 2 or a greater number,
such as F or E, in other words, anything from 2 to F means normal boot.
So this value is set to 2 or greater means normal boot, so in other words, enables default booting
from flash memory.
You can override that by using the boot system commands in the running-config of a router.
So on a router, you could use the boot system command to specify that a different operating system is
used as an example.
So the router would boot from a TFTP server as an example.
So in other words, on the router, you could use a command such as boot system and tell the router to
boot its operating system or for TFTP server or an FTP server as an example, rather than booting the
operating system from Flash.
When a router boots up, it goes through a series of steps which include loading the Cisco iOS software
from Flash or if specified from somewhere else, such as an FTP server into the RAM of the router,
the router will also load the configuration stored in startup-config and apply that and use it as the running
config.
So here these steps that are takes when booting up, the first thing it does is perform the
power on self-test or post.
This is a series of hardware tests that verify that all the components of a Cisco router are functional, during
the test
the router also determines which hardware is present.
This executes from microcode that is stored in the system
ROM.
So here's our router
and here's our APC, I'm going to do an immediate reboot on the router.
So you can see the router is booting up and then is loading the operating system.
Now it's going to do a power and self-test, but that's not very clear on this router.
So what I'm going to do is show you the power and self-tests on a Cisco switch.
So I'll do a reboot on the Cisco switch
and
you can see the 2950 boot loader is being used.
It's now loading the operating system from Flash
and what I'd like to show you here is notice the power and self-tests that are shown on the switch, so you
can see clearly that power and self-test have been passed on the switch
and then the switch boots up and the interfaces come up. So scrolling back notice power and self-test,
power and self-test
can see the operating systems loaded and then the switch is ready to start. Now something similar is taking
place on a router.
We just don't see it as clearly in the output here
but here's the output when I reloaded the router, you can see ROM monitors initialized the program,
loads are taking place.
The operating system is being decompressed and loaded.
So the first step on a router is to perform a power-on Self-test, the router then loads and runs the
bootstrap code, which is used to perform subsequent events, such as locating the Cisco iOS software
and loading it into RAM and running it after the Cisco iOS software is loaded and running.
The bootstrap code is not used until the next time the router is reloaded or power cycled.
So on this 1841 router, we can see that the bootstrap code is loading the operating system
and here we can see which version of operating system has been loaded into RAM.