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Cisco IOS

IOS (Internetworking Operating System)

IOS is the basic and core software of a switch and router that allocates resources and manages hardware.

Cisco IOS is a proprietary operating system that provides routing, switching, internetworking, and telecommunication services.

Cisco IOS is a proprietary operating system (not just a kernel)

The first IOS was written by William Yeager in 1986.

In simple words, it is the operating system for switches and routers (Layer 2 and Layer 3 devices).

Cisco Switch / Router Internals (Hardware)

Components and Functions

CPU

  • Executes operating system instructions
  • Controls switch/router operations

RAM (Random Access Memory)

  • Contains running configuration
  • Stores routing table
  • Volatile memory (data lost when power is off)

ROM (Read Only Memory)

  • Holds diagnostic software
  • Stores bootstrap program
  • Used during startup process

NVRAM (Non-Volatile RAM)

  • Stores startup configuration
  • Includes:
    • IP address
    • Hostname
    • Passwords
    • Protocol settings

Flash Memory

  • Contains Cisco IOS operating system
  • Non-volatile (not lost when power off)

Switch Interfaces

LAN Interfaces (Data Transfer)

Used for connecting devices to switch/router

  • Ethernet — 10 Mbps
  • Fast Ethernet — 100 Mbps
  • Gigabit Ethernet — 1000 Mbps
  • 10 Gigabit Ethernet — 10,000 Mbps
  • 40 Gigabit Ethernet — 40,000 Mbps
  • 100 Gigabit Ethernet — 100,000 Mbps

Added: These interfaces usually use RJ-45 or fiber ports.

Management Interfaces

Used for switch configuration

Auxiliary Port (AUX)

  • Used for modem connection
  • Rarely used for configuration
  • RJ-45 port

Console Port

Used for initial configuration

  • Console cable connects switch to PC
  • One end: RJ-45 → Switch console port
  • Other end: Serial/USB → PC
  • Required software:
    • HyperTerminal
    • PuTTY
    • TeraTerm

Connecting to a Switch

We can connect using:

  1. Console port
  2. Auxiliary port
  3. Telnet (remote access)

Cisco 2800 is a router series called ISR (Integrated Services Router)

SDM (Security Device Manager)

  • Web-based management tool
  • Allows GUI configuration
  • Accessed through web browser

Switch Boot Process

  1. Switch runs POST (Power On Self Test)
  2. Loads IOS from Flash to RAM
  3. Loads Startup Config from NVRAM to RAM
  4. Startup config becomes Running Config
  5. If no startup config:
    • Switch sends broadcast to find TFTP server
    • If not found → enters Setup Mode

To enter setup mode manually:

Switch# setup

To exit setup mode:

Ctrl + C

Three Main Modes of Switch/Router

1. User EXEC Mode

Switch>
  • Basic monitoring commands
  • Limited access
  • Enter privileged mode:
enable

Exit session:

logout

2. Privileged EXEC Mode

Switch#
  • Full monitoring access
  • Can enter configuration mode

Return to user mode:

disable

3. Global Configuration Mode

Switch(config)#

Enter from privileged mode:

configure terminal

Shortcut:

conf t

Exit to privileged mode:

exit

OR

Ctrl + Z

Basic Commands

Mode Switching

R> enable
R# conf t
R(config)#

Set Hostname

R(config)# hostname R1

Set Enable Password (Plain Text)

R(config)# enable password 123

Set Enable Secret (Encrypted)

R(config)# enable secret 1234

Important: enable secret overrides enable password

Console Password Configuration

R(config)# line console 0
R(config-line)# password 123456
R(config-line)# login

Assign IP Address to Interface

R(config)# interface fastEthernet 0/0
R(config-if)# ip address 192.168.10.1 255.255.255.0
R(config-if)# description Third Floor of ATVI
R(config-if)# no shutdown

Enable Telnet

R(config)# line vty 0 4
R(config-line)# password 12345
R(config-line)# login

Save Configuration

R# write

OR

R# copy running-config startup-config

Show Commands

Set Date and Time

R# clock set hh:mm:ss 25 June 2014

Show IOS version

R# show version

Show running configuration

R# show running-config

Show flash memory

R# show flash

Show IP interfaces

R# show ip interface brief

Show interfaces

R# show interfaces

Show routing table

R# show ip route

Other Important Commands

Reload device

R# reload

Copy startup to running config

R# copy startup-config running-config

Backup IOS to TFTP

R# copy flash tftp

Backup running config to TFTP

R# copy running-config tftp

Show startup config

R# show startup-config

Erase startup config

R# erase startup-config

DHCP Server Configuration

R1(config)# ip dhcp pool NET-POOL
R1(dhcp-config)# network 192.168.1.0 255.255.255.0
R1(dhcp-config)# default-router 192.168.1.1
R1(dhcp-config)# dns-server 192.168.1.5
R1(dhcp-config)# domain-name example.com
R1(dhcp-config)# lease 9

Exclude addresses:

R1(config)# ip dhcp excluded-address 192.168.1.1 192.168.1.5
R1(config)# ip dhcp excluded-address 192.168.1.10

Show DHCP bindings:

show ip dhcp binding

Debugging Commands

Trace route

R# traceroute 192.168.10.1

Enable debugging

R# debug all

Stop debugging

R# undebug all

Show CPU usage

R# show process