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When a router boots up, it searches for a Cisco iOS image in a specific sequence, it will search for

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the Cisco iOS based on what's configured in the configuration register, whether there's an iOS in flash,

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if a TFTP server is available or it will use ROM monitor.

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So when the router boots up, the first thing it checks is the configuration register

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boot field, if the boot field is set to 0 in hexadecimal, it will load ROM monitor.

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Remember, the configuration registers 16 binary bits by default it would be 0x2102 the

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last

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hexadecimal value specifies the boot field, so if that's set to 0, it'll go into ROM monitor mode.

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If it's set to 2 or greater

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in other words, 2.3.4.5.6.7.8 and so forth up to F

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it will check for boot commands and load the Cisco iOS image as specified in the boot command.

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If no boot command has been configured, it will boot an image in flash and typically it will boot the

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first iOS image found in Flash unless you specify a boot command telling it to boot off a different operating

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system.

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If no files are available in flash, it will look for an iOS image on a TFTP server.

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So it'll try and load a Cisco iOS image from a TFTP server

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but if none is available, then it will load ROM monitor.

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Now on this router

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it has multiple files in flash, so it has this operating system and this operating system in flash.

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Notice the difference in the name, the one that was loaded is the one with _2 in the name

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that was loaded from Flash.

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The reason why that operating system was loaded is the configuration register is set to the default.

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So the boot field is set to 2

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but in the running configuration, there is a boot command telling the router to boot from that second

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operating system.

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So there are two files in flash, this one and this one

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but the router was configured to boot off the second image here's the full show run command notice here's

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the command in the running-config telling the router which image to boot from.

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So show version shows that image was loaded by the router. On this router

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however, there are no boot commands, so show-run pipe include boot doesn't show any boot commands

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specifying that the router should boot from some other operating system.

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Show version shows which operating system was loaded

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and we can see that the configuration registers using the default value, so the router loaded the first

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operating system that it found in Flash, 

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in other words, this operating system.

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So you can specify with boot commands

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which operating system should be loaded by the router, but by default, it will simply use the first

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operating system found in Flash unless you override that with the boot system command. You could

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as an example, specify that it boot from a TFTP server or FTP server as an example.

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So how does Cisco routers load iiOS configuration files? When the operating system is loaded and started,

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the router needs to be configured with IP addresses and other values to make it of any use.

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So it will typically look for the startup-config in NVRAM

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if there's no startup-config in NVRAM, it will either begin an auto-install or enter the setup

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utility.

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So when the router boots up, it looks for a startup-config in NVRAM.

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If it exists, it will load that startup-config.

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If there's no startup-config it looks for an active link to another router and then tries to load

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a configuration from a TFTP server using a protocol called slop.

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If that doesn't work, it will prompt for the initial configuration dialog.