Router Boot Sequence

A Router is a networking device that forwards data packets between computer networks. Router Boot sequence involves the following memory elements:

• Read-Only Memory (ROM): ROM stores the bootstrap startup program of the router along with the operating system software and other test programs like POST programs (Power On Self Test).
• Flash Memory: Flash memory generally called flash holds the IOS images. The flash content is used by the router at the time of reload. Flash is erasable and reprogrammable ROM.
• Random Access Memory (RAM): RAM stores information such as routing tables and running configuration files. RAM is volatile hence, its content is lost during router power down and reload.
• Non-volatile RAM (NVRAM): NVRAM stores the startup configuration files. It is non-volatile RAM; hence contents are not lost during router power down and reload.

Ports:

Cisco routers have two types of ports: Interfaces and lines. Interfaces connect routers to other devices. Data travels through these ports in the network. Interfaces are identified by their name and number. Some common interfaces are:

• Serial Interface
• Ethernet Interface
• Fast Ethernet Interface

We can configure routers by connecting them with other types of ports called lines. Like interfaces, lines are also identified by line name and number. Some common lines are:

• Console Ports
• Auxiliary Ports
• VTY Ports

Command Line Interface:

IOS provides a command line interface to interact with the Cisco router. The command line interface is first used to configure and manage Cisco devices. It can be accessed through modem, console, and telnet connection. In CLI, we can type a command and execute it.

Router Boot Sequence: 

The series of steps performed by the router during the booting process is called the router boot sequence. The router boot sequence defines the sequence in which the booting process takes place in a router.

Steps of Router Boot Sequence:

1. When the router is turned on it performs the POST (Power On Self Test) program. The POST program tests the present hardware and checks it is operational or not. The POST programs are stored and run from the ROM.
2. The bootstrap program present in the ROM checks the Configuration Register value to find where to load the IOS. The default value of configuration register 0x2102 indicates that the router should load the Cisco IOS Operating System software image from the flash memory and load the startup configuration.
3. The Bootstrap Program looks for and loads the IOS program to the configuration register. This program is also responsible for initializing the hardware and finding the IOS program location and loading the IOS image from the flash memory.
4. If the Bootstrap program does not find the IOS image it will act as ROM Monitor. It supports a command line that is used to perform configuration tasks.
5. The IOS finds the valid configuration file stored in NVRAM. This file is called startup-config.
6. If the Startup configuration(startup-config) is present in NVRAM the router loads the file into RAM and applies the startup-config file. If the file is not present in NVRAM it tries to load a file from TFTP. If no TFTP server responds it enters the Setup mode.
7. When the Startup Configuration is loaded IOS will display CLI mode in user mode. 

Router Booting Process Example:

The router goes through the above steps during the booting process.

• The router is powered on.
• POST is performed which checks the hardware components including memory and interfaces.
• The bootstrap program is loaded and executed.
• Bootstrap reads the configuration register value which identifies how the router will boot up.
• Depending on the value of the configuration register, the bootstrap program finds and locates the IOS image.
• If bootstrap fails to load the IOS it will drop the boot sequence to ROMMON (ROM Monitor) mode for troubleshooting.
• If IOS is loaded it finds and loads the configuration.
• If the configuration is not present, the system configuration dialog would be launched.
• If the configuration is loaded, you would be presented in the CLI interface.