The operating system used by Cisco routers is called the Internetwork Operating System most commonly known as IOS. Depending on the type and model of a router, a specified IOS image can only be installed.

It is very difficult to understand the output produced by a cisco router during an IOS boot-up sequence. With this, I will enumerate and discuss each line.

System Bootstrap, Version 12.1(3r)T2, RELEASE SOFTWARE (fc1)

Cisco 1841 (revision 5.0) with 114688K/16384K bytes of memory.

This two lines state the version of the system bootstrap (like bios),  hardware model and memory capacity of the device. This performs the POST (Power on Self Test). And execute the bootstrap loader from the ROM.

Self decompressing the image :########################################################################## [OK]

At this stage the bootstrap locates the IOS image from the FLASH and load the operating system.

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Cisco IOS Software, 1841 Software (C1841-ADVIPSERVICESK9-M), Version 12.4(15)T1,

RELEASE SOFTWARE (fc2)

Technical Support: http://www.cisco.com/techsupport

Copyright (c) 1986-2007 by Cisco Systems, Inc.

Compiled Wed 18-Jul-07 04:52 by pt_team

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Cisco 1841 (revision 5.0) with 114688K/16384K bytes of memory.

Processor board ID FTX0947Z18E

M860 processor: part number 0, mask 49

2 FastEthernet/IEEE 802.3 interface(s)

191K bytes of NVRAM.

63488K bytes of ATA CompactFlash (Read/Write)

Cisco IOS Software, 1841 Software (C1841-ADVIPSERVICESK9-M), Version 12.4(15)T1,

RELEASE SOFTWARE (fc2)

Technical Support: http://www.cisco.com/techsupport

Copyright (c) 1986-2007 by Cisco Systems, Inc.

Compiled Wed 18-Jul-07 04:52 by pt_team

 At this stage, the bootstrap locates the startup configuration file in the NVRAM. If found, it is copied to the RAM as the running configuration file.

— System Configuration Dialog —
Continue with configuration dialog? [yes/no]: no

If no startup configuration file present in the NVRAM, the question appears above will be shown.

In summary, there are four major phases for the bootup process.

1.  Perfoming the POST

2. Loading the bootstrap

3. Locating and loading the IOS image

4. Locating and loading the startup configuration file