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www.petri.co.il/csc_analyzing_boot_process_cisco_ios_router.htm

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Posted by crouse

The IOS startup process
1. Run the POST.
2. Find the IOS.
3. Load the IOS to RAM
4. Find the configuration
5. Load the configuration to RAM

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Posted by waqasahmad

www.petri.co.il/csc_analyzing_boot_process_cisco_ios_router.htm

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Posted by crouse

1. The POST runs.

2. Assuming that there are no critical errors with the POST run, the bootstrap checks the startup-config file in NVRAM for boot system commands. These commands might have been entered by the router admin to override the default behavior, perhaps to load a different IOS for test purposes.

3. Assuming that there are no boot system commands, the router loads the first valid IOS image it finds in the flash memory.

4. If there is no usable IOS in flash, the router will begin broadcasting for a TFTP server in the hopes of finding an IOS it can download and use. Cisco has programmed several preconfigured filenames if you want to set up such a system.

5. If no TFTP server can be contacted, or if no valid IOS is found on one, the router will load the RXBOOT mini-IOS. This IOS has a familiar command set, and some of the features of a full IOS. The main purpose of RXBOOT is to allow you to manually connect to a TFTP server to download a valid IOS to flash. The command prompt for RXBOOT looks like this:
Router(boot)>

6. In the unlikely event that RXBOOT fails, the router will load the ROM Monitor (ROMMON). ROMMON can also connect to a TFTP server, but if your router has dropped into ROMMON all by itself, chances are that you have a pretty serious problem. ROMMON command prompts vary with hardware type.

7. After an IOS is loaded (except for ROMMON), the router looks for the startup-config file in NVRAM. If it is found, it is copied to RAM and renamed running-config.

8. If there is no startup-config file in NVRAM, the router will broadcast for a TFTP server to see if there is a configuration file available for it.

9. If that fails, the router will launch Setup mode.

Note that on most devices, the IOS image is decompressed and copied to RAM to run from there; similarly, the startup-config file is copied from NVRAM and renamed running-config as it is copied into RAM.

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Posted by campbell123

The IOS startup process is actually more complex than it appears. The basic steps are as follows:
1. Run the POST.
2. Find the IOS.
3. Load the IOS to RAM
4. Find the configuration
5. Load the configuration to RAM

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Posted by sagitraz