Default Cisco Router Boot-up sequence/process

Cisco Router can boot-up Cisco IOS from below locations:

  1. Flash memory
  2. TFTP server
  3. ROM (not full Cisco IOS)

Cisco Router Default Boot Sequence/Process for locate Cisco IOS:

  1. NVRAM
  2. Flash (sequential)
  3. TFTP server (network boot)
  4. ROM (partial IOS)

Note: Cisco boot system commands can be used to specify the primary IOS source and fallback sequences.

Complete Cisco Router Boot-up  sequence and locating the Cisco IOS:

1. POST (power on self test) and  Bootstrap code executed

Cisco Router boot process/Sequence

 

 

 

The POST is a process is used to test the Router hardware. After POST, the bootstrap program is loaded. 

2. Locate Cisco IOS  Software and Loads it into RAM of Cisco Router

The bootstrap program locates the Cisco IOS software and loads it into RAM. Cisco IOS files can be located in one of three places: flash memorya TFTP server, or another location indicated in the startup configuration file. The Cisco IOS software loads from flash memory by default. The configuration settings must be changed to load from one of the other locations.

I). Checks Configuration Register value (NVRAM) which can be modified using the config-register command from Global configuration mode on Cisco Router.

  • 0 = ROM Monitor mode
  • 1 = ROM IOS
  • 2 – 15 = startup-config in NVRAM

II). Checks Startup-config file for boot system commands (NVRAM)

a). If boot system commands in startup-config

  • Run boot system commands in order they appear in startup-config to locate the IOS
  • [If boot system commands fail, use default boot-up sequence to locate the IOS (Flash, TFTP, ROM)?]

b).If no boot system commands in startup-config use the default boot-up sequence in locating the Cisco IOS:

  • Flash (sequential)
  • TFTP server (netboot)
  • ROM (partial or mini IOS) or keep retrying TFTP depending upon router model

Fig 2. Cisco Router boot process/Sequence

 

 

 

 

 

 

Note: If  Cisco IOS Software is not located or corrupted from either from boot system commands and default boot-up sequence, Router will enter into the ROMmonitor mode or Rommon mode.

3. Locate and execute the startup configuration file.

If the Cisco IOS software is loaded, the bootstrap program searches for the startup configuration file in NVRAM. This file contains the previously saved configuration commands and parameters, including interface addresses, routing information, passwords, and other configuration parameters. If startup configuration file located in NVRAM, it will loads in to RAM as  running configuration.

 Fig 3. Cisco Router boot process or sequence

 

 

 

 

 

 

If there is no startup-config file, the Cisco Router will use the default fallback or boot-up sequence for locating the  Startup-config  and then it will enter setup mode or the setup dialogue.

  • NVRAM
  • TFTP server
  • Setup Mode

Cisco Boot System Commands:

Cisco-Router(config)# boot system flash IOS filename   – boot from FLASH memory
Cisco-Router(config)# boot system tftp IOS filename tftp server ip address   – boot from a TFTP server
Cisco-Router(config)# boot system rom   – boot from system ROM

Configuration Register Command in Cisco Router:

Cisco-Router(config)# config-register 0x10x (where that last x is 0-F in hex)

When the last x is:
0 = boot into ROM Monitor mode
1 = boot the ROM IOS
2 – 15 = look in startup config file in NVRAM

 

The fallowing table describes the Cisco Router boot sequence manipulation with  Configuration register value and Boot system commands.

Boot Filed Value             Function
0×0 Loads ROMMON and ignores boot system commands.
0×1 Loads IOS from ROM and ignores boot system commands.This is also known as RXBOOT mode
0×2-0xF If used with the no boot command, the first IOS file in Flashmemory is loaded; if that fails, the router broadcasts looking for an IOS on a TFTP server. If that fails, IOS from ROM is loaded.
0×2-0xF If used with the boot system ROM command, IOS from ROM isloaded.
0×2-0xF If used with the boot system flash command, the first filefrom Flash memory is loaded.
0×2-0xF      If used with the boot system flash file_name command,IOS with the specified file_name is loaded from Flash memory.
0×2-0xF If used with the boot system tftp file_name 10.1.1.1command, IOS with the specified file_name is loaded from theTFTP server.
0×2-0xF If used with multiple boot system commands, an attemptoccurs to load IOS based on the first boot command inconfiguration. If that fails, the second boot command is used,etc., until an IOS is loaded successfully.

 

 

Cisco Router Boot-up Sequence chart:

Cisco Router Boot sequence

Fig. Cisco Router Boot Sequence

 

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  1. Gopi says:

    Hi
    Thanks for explaining the Router boot-up sequence in detail. I understand fully where is the start-up config, running config, mini IOS, IOS located in Cisco Router.

    :)

  2. Prabha says:

    Hi admin,
    You have explained most of the things but IOS loading depends upon Configuration Register value which is 0×2102 by default which means that by default load IOS from flash but we can load it from TFTP server as well depending upon boot sytem global configuration commands.

    As far as Mini IOS is concerned that was used in old 2500 series routers not in new ISRs and was kept in ROM.

    regards
    Prabha

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