Cisco IOS and Switch Fundamentals

Definition

Cisco IOS (Internetwork Operating System) is the operating system used on Cisco routers and switches. It manages hardware resources, provides networking services, and allows administrators to configure and monitor network devices.

Cisco IOS

Definition

Cisco IOS (Internetwork Operating System) is the core software that controls Cisco networking devices such as routers and switches.

Key Points

• IOS stands for Internetwork Operating System.

• It is a proprietary operating system developed by Cisco.

• Manages hardware resources such as CPU, memory, and interfaces.

• Provides:

  • Routing
  • Switching
  • Network management
  • Telecommunication services

• The first Cisco IOS was written by William Yeager in 1986.

Explanation

Cisco IOS acts as the bridge between the hardware and the administrator.

It allows administrators to:

• Configure network interfaces
• Create routing tables
• Configure VLANs
• Manage users and passwords
• Troubleshoot network problems

Without IOS, Cisco devices cannot perform networking functions.

Cisco Switch Internals

Definition

A Cisco switch contains several memory components, each responsible for storing different types of information.

Key Points

Memory Summary

Explanation

When a switch starts:

1. ROM runs hardware diagnostics.
2. IOS is loaded from Flash into RAM.
3. Startup configuration is copied from NVRAM into RAM.
4. The configuration in RAM becomes the Running Configuration.

Switch Interfaces

Definition

Switch interfaces are physical ports used for connecting devices or managing the switch.

LAN Interfaces

These ports are used for data communication.

Typical devices connected:

• Routers
• Switches
• Bridges
• Hubs
• PCs
• Servers

Management Interfaces

These interfaces are used to configure the switch.

Console Port

• Used for initial configuration.
• Uses a console cable.
• One side connects to the switch (RJ-45 or USB on newer models).
• The other side connects to the computer.
• Requires terminal software such as:
  • PuTTY
  • Tera Term
  • HyperTerminal (older Windows versions)

Auxiliary (AUX) Port

• Primarily used with a modem for remote management.
• Rarely used today.

Accessing a Cisco Switch

There are several methods to access and configure a Cisco switch.

Note: Modern Cisco devices generally use SSH instead of Telnet because SSH encrypts communication.

Switch Boot Process

Definition

The boot process is the sequence the switch follows when it powers on.

Steps

1. POST (Power-On Self-Test)

The switch checks:

• CPU
• RAM
• Flash
• Interfaces

to ensure the hardware is functioning correctly.

2. Load Cisco IOS

The switch copies the IOS image from Flash Memory into RAM.

3. Load Startup Configuration

The startup configuration is copied from NVRAM into RAM.

After loading, it becomes the Running Configuration.

4. Search for Configuration

If no startup configuration exists:

• The switch attempts to locate a configuration file from a TFTP Server.
• If unsuccessful, it enters Setup Mode.

Setup Mode

Setup Mode is an interactive configuration wizard.

Enter Setup Mode:

setup

Exit Setup Mode:

Ctrl + C

Cisco CLI Modes

Cisco devices use different command modes.

1. User EXEC Mode

Prompt:

Switch>

Purpose:

• Basic monitoring
• Limited commands

Enter Privileged Mode:

enable

Exit:

logout

2. Privileged EXEC Mode

Prompt:

Switch#

Purpose:

• View configurations
• Save configurations
• Reload the device
• Access configuration mode

Return to User Mode:

disable

3. Global Configuration Mode

Prompt:

Switch(config)#

Purpose:

• Configure the entire device.

Enter:

configure terminal

or

conf t

Exit:

exit

or

Ctrl + Z

Basic Cisco Commands

Enter Configuration Mode

Switch> enable
Switch# configure terminal

Set Hostname

Switch(config)# hostname R1

Changes the device name to R1.

Configure Enable Password

Switch(config)# enable password 123

Stores the password in plain text.

Configure Enable Secret

Switch(config)# enable secret 1234

Stores the password in encrypted form.

Enable Secret is preferred because it is encrypted.

Configure Console Password

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

Explanation

Configure an Interface

Switch(config)# interface FastEthernet0/0
Switch(config-if)# ip address 192.168.10.1 255.255.255.0
Switch(config-if)# description Third Floor Office
Switch(config-if)# no shutdown

Explanation

Configure Telnet

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

Explanation

• Enables Telnet access.
• Configures a password for remote login.

Security Note: Telnet sends passwords in plain text. Use SSH for secure remote management whenever possible.

Save Configuration

Switch# write

or

Switch# copy running-config startup-config

Both commands save the running configuration into NVRAM.

Show Commands

Maintenance Commands

Restart the Device

reload

Copy Startup to Running Configuration

copy startup-config running-config

Loads the saved configuration into RAM.

Backup IOS

copy flash tftp

Backs up the IOS image to a TFTP server.

Backup Running Configuration

copy running-config tftp

Saves the running configuration to a TFTP server.

Erase Startup Configuration

erase startup-config

Deletes the saved configuration from NVRAM.

DHCP Server Configuration

Definition

DHCP (Dynamic Host Configuration Protocol) automatically assigns IP configuration to client devices.

Configuration Example

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 Firewall.cx
R1(dhcp-config)# lease 9

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

Explanation

Verify DHCP

show ip dhcp binding

Displays IP addresses leased to clients.

Debugging Commands

Trace Route

traceroute 192.168.10.1

Displays the path packets take to reach the destination.

Enable Debugging

debug all

Shows detailed debugging information.

Disable Debugging

undebug all

Stops all debugging.

CPU Utilization

show processes cpu

Displays CPU usage.

Note: The lecture slide uses show process, but the common Cisco IOS command for CPU utilization is show processes cpu.

Example Configuration

Switch> enable
Switch# configure terminal
Switch(config)# hostname R1
Switch(config)# enable secret Cisco123
Switch(config)# line console 0
Switch(config-line)# password Admin123
Switch(config-line)# login
Switch(config)# interface FastEthernet0/0
Switch(config-if)# ip address 192.168.10.1 255.255.255.0
Switch(config-if)# no shutdown
Switch# copy running-config startup-config

Line-by-Line Explanation

Common Mistakes

• Confusing RAM with NVRAM.
• Forgetting to save the configuration before rebooting.
• Using enable password instead of the more secure enable secret.
• Forgetting to use no shutdown after configuring an interface.
• Assuming Telnet is secure; prefer SSH for remote management.
• Not excluding gateway or server IP addresses from the DHCP pool.
• Forgetting that the running configuration is lost after a reboot unless it is saved to NVRAM.

Short Exam Notes

• Cisco IOS is the operating system for Cisco routers and switches.
• RAM stores the running configuration (lost on power off).
• NVRAM stores the startup configuration.
• Flash Memory stores the IOS image.
• ROM stores the bootstrap program and POST diagnostics.
• Boot Process: POST → Load IOS → Load Startup Configuration → Running Configuration.
• CLI Modes: User EXEC (>), Privileged EXEC (#), Global Configuration ((config)#).
• enable enters Privileged EXEC mode.
• configure terminal enters Global Configuration mode.
• copy running-config startup-config saves the current configuration.
• DHCP automatically assigns IP addresses, subnet masks, gateways, and DNS settings.
• Telnet provides remote access but is not secure; SSH is the recommended alternative.