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The configuration register contains different options, and one of them determines how a router boots. 

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When we use the show version command and we press spaceball to scroll down, we can see what the configuration

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register of the router is configured as at the moment, it's using the default of 0x210

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2.

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Whenever you see 0x you need to know that the numbers that follow are hexadecimal.

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So in other words, we have four hexadecimal values here, so 0x2102 equates in binary

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to 0010 0001 followed by 0000 0010.

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This last hexadecimal value determines how a router boots, so as an example, on this second router show version 

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it's also set to the default value.

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But what I'll do now is change the configuration register

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to a value of 0x2100 and I won't save the config, I'm going to use the command

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show version.

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So notice at the moment that is the configuration register value, that this is what it's going to be

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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

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that router.

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So I'm going to say immediate off in my APC power management system.

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So I've powered off router 1, 

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so now when I press enter, I'm getting no response on the router, what I'll do now is power it on

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so you can see the router is booting up, but notice it booted immediately into ROM Monitor mode rom or

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read only memory contains ROM monitor mode and the router has now booted into this mode because we set

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the configuration register to 0x2100.

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The last hexadecimal value is called the boot field and that determines how the router booted it, booted

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in this example directly into ROM monitor mode because we configured

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the config register as follows, we didn't have to save it,

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it's saved immediately and when the routr is booted, it booted straight into a monitor mode.

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Now, ROM monitor mode doesn't have the same commands as you get in the Cisco iOS, so notice the output

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is very different to what we see in typical Cisco iOS commands.

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The conf reg command allows you to specify the configuration register.

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So in this example, I'm going to set it back to 2102 and we're told that we need to reset

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or power cycle the router to take effect so we can use the reset command or we can simply power cycle

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a router.

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So in this example, I'll use the reset command and what you'll notice is it doesn't boot directly into

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ROM monitor mode it is loading the Cisco operating system.

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So ROM MON is initialized

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but notice the Cisco iOS image is decompressed and is being loaded by the router.

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Because we changed the configuration register

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to use normal boot, it's going to boot

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the operating system and start up the router as normal, notice here is the Cisco iOS image that the

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router is booting and there we go

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the router is now booted up.

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Interfaces are coming up, OSPF neighbor relationships are being formed in this example, show Flash

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confirms the operating system stored in Flash and show version shows us which operating system was booted

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from Flash.

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Notice the configuration register is back to 0x2102.

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The configuration register is a 16-bit number

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once again, that resides in the NVRAM of a router

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it affects router behavior.

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The least significant 4-bits of the configuration register are called the boot field

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and this determines how the router locates Cisco iOS software.

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The configuration register is 16 bits in size but is displayed once again in hexadecimal.

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You'll have access to the configuration register software, which is part of this course

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and this allows you to see the various options of the binary bits that make up the configuration register

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the default value of the configuration register is once again 0x2102.

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We can see that there are 16 bits that make up the configuration register.

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The least significant bit is bit-0 and that goes up to bit-15.

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So this first number in hexadecimal is the most significant.

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These last two contains the least significant 4-bits, which is the boot field.

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We can configure the configuration register as follows through the Cisco iOS

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but you can change the values. So as an example, if you want to set the speed of the console to 1

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15 200 bits per second, rather than the default of 9600 bits per second, you could configure

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the configuration registers follows.

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Sending it back to default, the speed is now changed back to 9600 bits per second.

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If you change it to 2142 that means that bit 6 is set, which means ignore NVRAM.

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That means that the router will boot up and not load the startup-config, which is very useful when 

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doing password recovery.

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There are various other options here.

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You can also look at the Cisco documentation

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which shows you

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what the configuration register values mean.

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So if you want to see this on a Web page, there's an example of 0x2142 which means ignore nonvolatile

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RAM.

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So it ignores the configuration and the speed is set to 9600 bits per second.

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But otherwise, you can use the software, which you get as part of the course if you want to see what

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various options are.

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So once again, if we change the speed to 115200, notice these bits are set, bit-12, bit-11,

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bit-5

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and bit-1.

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If we change the speed to 19200,

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notice other values are set, bit-12 in this example is not set, but if I check that option, notice

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the speed of the console port is changed. For the CCNA course the values that you want to know are

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2102, which means normal boot.

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If we change it to 2100, that means that the router is going to boot into ROM monitor mode.

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2142,

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means bypass startup-configuration,

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and 2101 means boot into RX boot mode if supported, on older Cisco routers, they had

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a mini iOS called RX Boot.

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Modern routers don't have that.

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A value of 2101 means that the router will boot up using the first operating system in flash.