Router Boot Process
typesh
Member Posts: 168
in CCNA & CCENT
Needed some help with understanding the Router Boot process please.
According to the 4 steps Odom lists (Page 426 of ICND1 2nd Edition):
1. Router performs POST
2. Router copies BootStrap program from ROM into RAM and runs the BootStrap program.
3. BootStrap program decides which IOS image to load into RAM. After loading the OS, then BootStrap hands over control of the Router to the newly loaded IOS.
4. IOS finds the startup-config in NVRAM and loads it into RAM
If the config-register value is:
2100 then load ROMMON OS
2101 then load first IOS in Flash
2102 - 210F, then try each boot-system command in the startup-config file until one works, otherwise load the first IOS found in flash.
My confusion is this:
If the config register has a value of 2102 - 210F, then that means the router would have to look into the start-up config file for the boot system commands... however according to steps 3 and 4, the router must already load an IOS before it can access the start-up config file (since it is the IOS that accesses the startup-config file held in NVRAM)
See what I mean...?
According to the 4 steps Odom lists (Page 426 of ICND1 2nd Edition):
1. Router performs POST
2. Router copies BootStrap program from ROM into RAM and runs the BootStrap program.
3. BootStrap program decides which IOS image to load into RAM. After loading the OS, then BootStrap hands over control of the Router to the newly loaded IOS.
4. IOS finds the startup-config in NVRAM and loads it into RAM
If the config-register value is:
2100 then load ROMMON OS
2101 then load first IOS in Flash
2102 - 210F, then try each boot-system command in the startup-config file until one works, otherwise load the first IOS found in flash.
My confusion is this:
If the config register has a value of 2102 - 210F, then that means the router would have to look into the start-up config file for the boot system commands... however according to steps 3 and 4, the router must already load an IOS before it can access the start-up config file (since it is the IOS that accesses the startup-config file held in NVRAM)
See what I mean...?
Comments
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billscott92787
Member Posts: 933
Check another source to understand it.
1. POST. When first powered up, a router will carry out a power-on self-test (POST). Recall that the POST is used to check whether the CPU and router interfaces are capable of functioning correctly.
2. Execute bootstrap to load IOS. After a successful POST, the router will execute the Bootstrap program from ROM. The bootstrap is used to search Flash memory for a valid Cisco IOS image. If one is present, the image is loaded. If an image cannot be found, the router will boot the RxBoot limited IOS version found in ROM.
3. IOS loads configuration file. Once the IOS image is loaded, it will search for a valid startup configuration in NVRAM. If a valid startup configuration file cannot be found, the router will load the System Configuration Dialog, or what is sometimes called setup mode. This mode allows you to perform the initial configuration of the router.
The configuration register comes into play #2, if it is 0x2102 it proceeds with the normal normal boot process.