Detailed description of the Cisco router startup process and principle of Password Reset

Cisco router startup process:

① After power-on, the router executes POST (BOOT self-check). POST is the microcode stored in the ROM to check whether the hardware of the device can run normally.

② Load and execute boot loader (that is, bootstrap, a piece of code stored in ROM ),

Bootloader searches and loads the System (IOS) and configuration files based on the value of the configuration register.

The configuration register is a 16-bit (Binary) software register in NVRAM. Its value can be expressed as a 4-bit hexadecimal value, such as 0 xAAAA, the last one affects IOS loading.

Start field:

0xAAA0: Enter ROM Monitor mode, or press Ctrl + C to enter this mode during normal router loading.

(Stays at the system bootstrap prompt. Staying on the bootstrap interface during the startup process is equivalent to entering the grub interface during Linux Startup)

0xAAA1: run the default Loading Order directly: Mini TFTP-ROM mode in flash-IOS-ROMMON (whether to enter the mini IOS to see what IOS is stored in ROM)

0xAAA2-0xAAAF: Before executing the default loading sequence, check the boot system settings and load them according to the settings. If no effective system is found for loading according to the command settings, in the default Loading Order.

③ Load the configuration file based on the register value.

The default value is 0x2102. in NVRAM, find startup-config and copy it to RAM, that is, running-config. Use this configuration to run the vro. If NVRAM does not have startup-config, send the broadcast to all interfaces to search for the TFTP host configuration file. If not, start setup mode for configuration.

If the value is 0x2142, the configuration file is not loaded and the setupmode is directly entered. Because the password settings are stored in the startup-config and running-config files, because the configuration file is not loaded, the password is not loaded. You can set a new password Based on the setup session to reset the password.

Note: If you only want to change the password without modifying other configurations, pay attention to the configuration saving method.

When the configuration is replicated from flash, nvram, and tftp to RAM, the data in RAM is merged, that is, the same configuration is replaced and no configuration is added.

When the configuration is copied from RAM to flash, nvram, and tftp, the entire file is replaced.

Therefore, after changing the password, you must first merge the configuration from nvram with that in RAM before saving the entire running-config to NVRAM. Otherwise you will lose the configuration!

Recommended reading:

Cisco asa firewall Basics

Implementation of CISCO-style ACL in Linux

NAT between CISCO and Linux

Small case of CISCO devices about GRE

CISCO Wireless AP Initialization Configuration WPA encryption Diary