The configuration register contains different options, and one of them determines how a router boots.
When we use the show version command and we press spaceball to scroll down, we can see what the configuration
register of the router is configured as at the moment, it's using the default of 0x210
2.
Whenever you see 0x you need to know that the numbers that follow are hexadecimal.
So in other words, we have four hexadecimal values here, so 0x2102 equates in binary
to 0010 0001 followed by 0000 0010.
This last hexadecimal value determines how a router boots, so as an example, on this second router show version
it's also set to the default value.
But what I'll do now is change the configuration register
to a value of 0x2100 and I won't save the config, I'm going to use the command
show version.
So notice at the moment that is the configuration register value, that this is what it's going to be
at next reload. I'm not going to save the config but what I'm going to do is I'm going to power off
that router.
So I'm going to say immediate off in my APC power management system.
So I've powered off router 1,
so now when I press enter, I'm getting no response on the router, what I'll do now is power it on
so you can see the router is booting up, but notice it booted immediately into ROM Monitor mode rom or
read only memory contains ROM monitor mode and the router has now booted into this mode because we set
the configuration register to 0x2100.
The last hexadecimal value is called the boot field and that determines how the router booted it, booted
in this example directly into ROM monitor mode because we configured
the config register as follows, we didn't have to save it,
it's saved immediately and when the routr is booted, it booted straight into a monitor mode.
Now, ROM monitor mode doesn't have the same commands as you get in the Cisco iOS, so notice the output
is very different to what we see in typical Cisco iOS commands.
The conf reg command allows you to specify the configuration register.
So in this example, I'm going to set it back to 2102 and we're told that we need to reset
or power cycle the router to take effect so we can use the reset command or we can simply power cycle
a router.
So in this example, I'll use the reset command and what you'll notice is it doesn't boot directly into
ROM monitor mode it is loading the Cisco operating system.
So ROM MON is initialized
but notice the Cisco iOS image is decompressed and is being loaded by the router.
Because we changed the configuration register
to use normal boot, it's going to boot
the operating system and start up the router as normal, notice here is the Cisco iOS image that the
router is booting and there we go
the router is now booted up.
Interfaces are coming up, OSPF neighbor relationships are being formed in this example, show Flash
confirms the operating system stored in Flash and show version shows us which operating system was booted
from Flash.
Notice the configuration register is back to 0x2102.
The configuration register is a 16-bit number
once again, that resides in the NVRAM of a router
it affects router behavior.
The least significant 4-bits of the configuration register are called the boot field
and this determines how the router locates Cisco iOS software.
The configuration register is 16 bits in size but is displayed once again in hexadecimal.
You'll have access to the configuration register software, which is part of this course
and this allows you to see the various options of the binary bits that make up the configuration register
the default value of the configuration register is once again 0x2102.
We can see that there are 16 bits that make up the configuration register.
The least significant bit is bit-0 and that goes up to bit-15.
So this first number in hexadecimal is the most significant.
These last two contains the least significant 4-bits, which is the boot field.
We can configure the configuration register as follows through the Cisco iOS
but you can change the values. So as an example, if you want to set the speed of the console to 1
15 200 bits per second, rather than the default of 9600 bits per second, you could configure
the configuration registers follows.
Sending it back to default, the speed is now changed back to 9600 bits per second.
If you change it to 2142 that means that bit 6 is set, which means ignore NVRAM.
That means that the router will boot up and not load the startup-config, which is very useful when
doing password recovery.
There are various other options here.
You can also look at the Cisco documentation
which shows you
what the configuration register values mean.
So if you want to see this on a Web page, there's an example of 0x2142 which means ignore nonvolatile
RAM.
So it ignores the configuration and the speed is set to 9600 bits per second.
But otherwise, you can use the software, which you get as part of the course if you want to see what
various options are.
So once again, if we change the speed to 115200, notice these bits are set, bit-12, bit-11,
bit-5
and bit-1.
If we change the speed to 19200,
notice other values are set, bit-12 in this example is not set, but if I check that option, notice
the speed of the console port is changed. For the CCNA course the values that you want to know are
2102, which means normal boot.
If we change it to 2100, that means that the router is going to boot into ROM monitor mode.
2142,
means bypass startup-configuration,
and 2101 means boot into RX boot mode if supported, on older Cisco routers, they had
a mini iOS called RX Boot.
Modern routers don't have that.
A value of 2101 means that the router will boot up using the first operating system in flash.