Cisco IOS Basics

Cisco IOS Basics

The following are some key points that summarize the IOS basics:

Types of Memory: There are different types of memory on a Cisco device:

·         ROM: The Read-Only Memory (ROM) on a Cisco device is like the ROM on a computer in the sense that it stores the POST and the boot loader program. The boot loader program is responsible for locating the IOS.

·         Flash: The flash memory is used to store the Cisco IOS.

·         RAM: RAM is used to store things  like the routing table on a router, or the MAC address table on a switch. It is also used to store the running-config. RAM is also known as volatile RAM

·         NVRAM: Non-volatile RAM (NVRAM) is used to store the startup-config, which is copied to the running-config on bootup after the IOS is loaded.

IOS The Boot Process: 

·         POST: The first thing that occurs when a Cisco device boots up is the POST routine, which is responsible for performing a self diagnostic to verify everything is functioning on the router or switch.

·         Locate IOS: After the POST, The configuration register is checked to find from where the ios have to be loaded.If IOS in flash memory it will be loaded into RAM.If IOS is not present in the flash it will check TFTP server or ROM

·         Startup-config applied: After the IOS is loaded into memory, the bootloader program then locates the startup-config and applies it to the device.If there is no start-up config,the router goes to intial setup mode.

Configuration Modes: When making changes to the Cisco device, there are a number of different configuration modes, and each change is made in a specific configuration mode. The following summarizes the major configuration modes:

·         User Exec: When you connect to a Cisco device, the default configuration mode is user exec mode. With user exec mode, you can view the settings on the device but not make any changes. You know you are in user exec mode because the IOS prompt displays a ">".

·         Privilage mode : In order to make changes to the device, you must navigate to privilage mode.Privilage mode displays with a "#" in the prompt.You can control access to privilage mode using password.

·         Global Config: Global configuration mode is where you go to make global changes to the router, such as the hostname. To navigate to global configuration mode from privilage mode, you have to type config terminal, where you will be placed at the "(config)#" prompt.

·         Sub Prompts: You can navigate to a number of different sub prompts from global configuration, such as the interface prompts to modify settings on a specific interface and the line prompts to modify the different ports on the device.

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