Cisco’s IOS

Cisco’s IOS

The Cisco inter-network operating system (IOS) is the kernel of Cisco routers as well as all current catalyst switches. A kernel is the elemental, indispensable part of an operating system that allocates resources and manages tasks like low-level hardware interfaces and security.
The Cisco IOS is a proprietary kernel that provides routing, switching, inter networking, and telecommunication features.

The Cisco router IOS is software is responsible for:

·         Carrying network protocol and functions
·         Connecting high-speed traffic between devices
·         Adding security to control access and stop unauthorized network use
·         Providing scalability for ease of network growth and redundancy
·         Supplying network reliability for connecting to network resources.

You can access the Cisco IOS through the console port of a router or switch, from a modem into the auxiliary (aux) port on a router, or even through Telnet and secure shell (SHH). Access to the IOS command line is called an EXEC session.
Bring up a switch

First bring up Cisco device, it will run a power-on-self-test (POST). Upon passing that, the machine will look for and then load the Cisco IOS from flash memory if an IOS file is present, then expand it to RAM. Flash memory is electronically erasable programmable read-only memory an EEPROM. The next step is for the IOS to locate and load a valid configuration known as the startup-config that will be stored in nonvolatile RAM (NVRAM).  
If a valid startup-config isn’t found in NVRAM, switch will enter setup mode, giving you a step-by-step dialog to help configure some basic parameter on it.

--- System Configuration Dialog ---

Would you like to enter the initial configuration dialog? [yes/no]: yes

At any point you may enter a question mark '?' for help.
Use ctrl-c to abort configuration dialog at any prompt.
Default settings are in square brackets '[]'.

Basic management setup configures only enough connectivity
for management of the system, extended setup will ask you
to configure each interface on the system

Would you like to enter basic management setup? [yes/no]: yes
Configuring global parameters:

You can exit setup mode at any time by pressing Ctrl+C.

Here are three different variants of the operating system (O.S): 

IOS XE, IOS XR and Nexus OS. IOS XE runs on enterprise-level Cisco ISRs, Aggregation Services Routers and Cisco Catalyst switches. 

IOS XR runs on Cisco's service provider products, like its Carrier Routing System routers. Nexus OS runs on Cisco's Nexus family of data center switches.

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