Router and Switch IOS Overview

Router and Switch IOS

• IOS (Internetwork Operating System) is a Cisco proprietary operating system for Cisco network devices like routers and switches.
• IOS allows network administrators to interact with Cisco devices through command modes, offering a way to configure settings.
• CLI (Command Line Interface) is used to input and execute commands within the IOS.
• IOS allows configuration changes on interfaces, routers, and switches, and can be used to manage devices remotely.

Router Elements

• ROM (Read-Only Memory) stores the bootstrap program responsible for the router starting up and performing the POST (Power On Self Test) program.
• Flash Memory stores the IOS images, which are used when the router reloads. It is erasable and reprogrammable.
• RAM (Random Access Memory) stores routing tables and running configuration files. It is volatile and loses its information when the router is powered down or reloads.
• NVRAM (Non-Volatile RAM) stores the startup configuration files. It is non-volatile, meaning data is not lost during power down or reload.

Router Ports

• Interfaces connect the router to other devices. These are used to transmit data across the network, and are identified by name and number.
  • Serial interface
  • Ethernet interface
  • Fast Ethernet interface
• Lines provide a way to configure a router by connecting it to other ports.
  • Console port
  • Auxiliary port
  • Virtual Terminal (VTY) ports

Router Command Line Interface (CLI)

• IOS interacts with the router through the CLI, which is used for managing and configuring the device.
• The CLI can be accessed through a modem, console, or Telnet connection.
• User Exec Mode is the initial mode when the router starts up.
• Privilege Exec Mode provides more advanced configuration options and access to commands.
• Global Configuration Mode allows global configurations to be applied to the router.

Router Boot Sequence

• The router boot sequence details the steps involved in starting a router.
• The router begins by performing the POST (Power On Self Test) program, checking the hardware's functionality.
• The bootstrap program, located in ROM, checks the Configuration Register for instructions about where to load the IOS image.
• If the Configuration Register is set to 0x2102 (default), the router loads the IOS image from flash memory and the startup configuration.
• The bootstrap program loads the IOS, initializes the hardware, and locates the IOS image.
• If the bootstrap program cannot find the IOS image, the router functions as a ROM Monitor.
• The IOS finds the valid configuration file in NVRAM called startup-config.
• The router either loads the startup-config into RAM and applies it or attempts to load a file from a TFTP (Trivial File Transfer Protocol) server.
• If no TFTP server responds, the router enters Setup mode.
• After loading the startup configuration, the IOS enters User Mode in the CLI.