Using the Cisco Command-Line Interface and Configuring Cisco IOS Software Notes

Accessing the Cisco Catalyst Switch Command-Line Interface (CLI)

• Command-Line Interface (CLI) Overview     * Cisco utilizes the CLI concept for its router products and the majority of its Catalyst LAN switch products.     * Definition: The CLI is a text-based interface where users—typically network engineers—enter text commands and press the Enter key.     * Logic Flow: Pressing Enter transmits the command to the switch. The switch executes the command and, in certain instances, replies with status messages detailing the results.     * Alternative Interfaces: Cisco Catalyst switches also support non-CLI methods for monitoring and configuration, such as a web-based interface.

• Cisco Catalyst Switch Families     * The Cisco Catalyst brand encompasses a diverse range of switch series or families.     * Series/Family Characteristics: Each series contains specific models sharing similar features, internal components, and price-versus-performance trade-offs.     * 2960 Series Positioning: Cisco positions the $2960$ series as full-featured, low-cost wiring closet switches for enterprises. These are primarily utilized as access switches.

• Monitoring Switch Status via LEDs     * Switch hardware includes several Light Emitting Diodes (LEDs) for status and troubleshooting during initial power-on and ongoing operations.     * Most Cisco Catalyst switches feature an LED for every physical Ethernet interface.     * Specific LED Descriptions (Example: Catalyst 2960):         * SYST (System): Indicates overall system status.             * Off: Switch is not powered on.             * On (green): Switch is powered on and operational (Cisco IOS has successfully loaded).             * On (amber): System has power but is not functioning correctly.         * RPS (Redundant Power Supply): Indicates the status of the backup power supply.         * STAT (Status): If lit green, it indicates that each port LED displays that specific port’s status.             * STAT Mode Port LED Off: Link is not working or is shut down.             * STAT Mode Solid Green: Link is operational, but no traffic is currently being transmitted.             * STAT Mode Flashing Green: Link is operational and traffic is passing over the interface.             * STAT Mode Flashing Amber: Port is currently blocked by spanning tree.         * DUPLX (Duplex): If lit green, port LEDs indicate the duplex mode. On/Green indicates full duplex; off indicates half duplex.         * SPEED: If lit green, port LEDs indicate the speed of the individual ports:             * Off: $10\,Mbps$             * Solid Green: $100\,Mbps$             * Flashing Green: $1\,Gbps$         * MODE Button: A physical button used to cycle the meaning of LEDs through three states: STAT, DUPLX, and SPEED.         * Port LED: Individual LEDs whose meanings change based on the mode selected by the MODE button.

Methods for Accessing the Cisco IOS CLI

• Cisco Internetwork Operating System (IOS)     * Definition: The operating system software required by Cisco switches to implement and control logic and functions.     * Function: Controls switch performance and behavior while defining the human-centric CLI.     * Interaction: A terminal emulation program accepts user text. Upon pressing Enter, the emulator sends text to the switch, which processes it as a command and sends a response back.

• Comparison of Access Methods     * Console:         * A physical port built specifically for CLI access.         * Requires a physical connection between a PC and the switch console port.         * Requires terminal emulation software on the PC.     * Telnet and Secure Shell (SSH):         * These methods utilize the IP network where the switch resides.         * They require software on the user device but rely on the existing TCP/IP network rather than a dedicated console cable.

• Cabling the Console Connection     * Components: Physical console port on the switch, physical serial port on the PC, and a compatible cable.     * Older Connections: Used a PC serial port (typically a D-shell $9$-pin connector called a DB-$9$) connected to an RJ-$45$ console port on the switch.     * USB Adapters: Standard USB to RS232 DB9 Serial Adapter cables can connect console cables to modern USB ports on PCs.     * Rollover Cable Pinouts: The standard console cable uses rollover pinouts with eight wires. Pin $1$ rolls to pin $8$, pin $2$ to pin $7$, pin $3$ to pin $6$, and so on.     * Modern Console Ports: Newer routers and switches include USB ports for console access, allowing a standard USB cable to connect from a PC USB port directly to the switch’s USB console port.

• Terminal Emulator Configuration for Console     * The terminal emulator treats all data as text and must match the switch's default console port settings:         * Speed: $9600\,bits/second$         * Flow Control: None (No hardware flow control)         * Data Format: $8$-bit ASCII         * Parity: None         * Stop Bits: $1$

• Network-Based Access (Telnet and SSH)     * Telnet:         * A TCP-based application layer protocol using well-known port $23$.         * Follows a client-server model (Telnet client on PC, Telnet server on switch).         * Requirement: Switch must have an IP address configured, and the network between the PC and switch must be operational.         * Security Flaw: Telnet sends all data, including usernames and passwords, as clear-text.     * Secure Shell (SSH):         * A TCP-based protocol using well-known port $22$.         * Security: Encrypts all message contents to prevent packet capturing and password theft.         * Encryption: Uses public key cryptography to exchange a shared session key.         * Requirement: Requires both a password and a username for login.

CLI Modes and Navigation

• Password Security Levels     * By default, only console access is allowed. When Telnet or SSH are enabled, password security must be configured.     * Lines:         * Console Line: Referred to as line console 0.         * Virtual Terminal (vty) Lines: Switches support $16$ concurrent Telnet/SSH sessions, referenced as line vty 0 15.

• EXEC Modes     * User EXEC Mode (User Mode):         * Allows the user to view status without changing settings (‘look around but not break anything’).         * Prompt: hostname followed by > (e.g., Switch>).     * Enable Mode (Privileged EXEC Mode/Privileged Mode):         * A more powerful mode supporting a superset of commands, including disruptive ones like reload (reboots IOS).         * Prompt: hostname followed by # (e.g., Switch#).     * Transitioning: Use the enable command to enter and disable to exit back to user mode.

• CLI Help Features     * ?: Displays all available commands in the current mode.     * help: Provides text describing how to use the help system (does not list commands).     * Command?: Shows the first parameter options for a command.     * com?: Lists all commands starting with the characters "com".     * command parm?: (No space) Lists all parameters starting with those characters.     * command parm1 ?: (With space) Lists all next parameters with brief explanations.     * Tab Key: Completes a partially typed parameter or command. If it does nothing, the string is ambiguous (matches multiple possibilities).

• Keyboard Shortcuts for Editing and Recall     * Up arrow / Ctrl-P: Displays the most recently used command (Previous).     * Down arrow / Ctrl-N: Scrolls forward to more recent commands (Next).     * Left arrow / Ctrl-B: Moves cursor backward without deleting (Back).     * Right arrow / Ctrl-F: Moves cursor forward without deleting (Forward).     * Backspace: Moves cursor backward and deletes characters.     * Ctrl-A: Moves cursor to the absolute start of the current command.     * Ctrl-E: Moves cursor to the absolute end of the current command.     * Ctrl-R: Redisplays the command line (clears screen clutter from system messages).     * Ctrl-D: Deletes a single character.     * Ctrl-Shift-6: Interrupts the execution of the current command.

Monitoring and Configuration Commands

• The show and debug Commands     * show: The most popular IOS command. It provides a "photograph" or snapshot of the current status of almost every IOS feature.     * debug: Acts like a "movie" or ongoing monitor. It asks the switch to monitor processes and send log messages to the user as events occur.     * terminal monitor: A command used in enable mode to force log/debug messages to appear in a Telnet or SSH terminal session (IOS does not do this by default over IP).     * Reload impacts: A switch reload disables all currently active debug options.

• Configuration Mode     * Logic: Commands entered here update the active configuration immediately upon pressing Enter.     * Global Configuration Mode: Entered via configure terminal from enable mode.     * Configuration Submodes (Contexts):         * Interface Mode: prompt changes to (config-if)#. Example: interface fastethernet 0/1.         * VLAN Mode: prompt changes to (config-vlan)#. Example: vlan 10.         * Console Line Mode: prompt changes to (config-line)#. Example: line console 0.         * VTY Line Mode: prompt changes to (config-line)#. Example: line vty 0 15.     * Exiting Modes:         * exit: Moves back one level in the hierarchy.         * end or Ctrl-Z: Exits configuration mode entirely and returns to enable mode (#).

Switch Memory and File Management

• Four Main Types of Memory     1. RAM (DRAM): Working storage. Holds the running-config (active configuration).     2. ROM: Stores the bootstrap/boothelper program used at power-on to find and load the IOS image into RAM.     3. Flash Memory: Stores fully functional Cisco IOS images. It is the default boot location and can store backup configuration files.     4. NVRAM (Nonvolatile RAM): Stores the startup-config file used during switch initialization or reload.

• Managing Configuration Files     * Persistence: Changes made in config mode are only in RAM. Loss of power loses these changes unless they are copied to NVRAM.     * The copy Command: copy {from-location} {to-location}.         * copy running-config startup-config: Saves active configuration to NVRAM.         * copy startup-config running-config: Merges the NVRAM file into the active RAM configuration (does not replace).         * copy tftp running-config: Moves configuration from a TFTP server into RAM.     * Erasing Configuration:         * write erase         * erase startup-config         * erase nvram

• Initial Configuration (Setup Mode)     * Triggered automatically if the startup-config in NVRAM is empty during boot initialization.     * The switch asks if the user wants to enter setup mode; if "yes," it prompts the administrator with a series of questions for basic parameters.     * IOS builds a configuration from these answers, saves it to NVRAM, and loads it into RAM.

• The show version Command Information     * Cisco IOS software version.     * Switch uptime (time since last load).     * Reason for the last reload.     * Inventory of interfaces (Number of Fast Ethernet and Gigabit Ethernet interfaces).     * Switch model number.

Command Reference Summary

• Configuration Commands:     * hostname {name}: Sets the switch host name and prompt prefix.     * login: (Context: Line) Tells IOS to prompt for a password when logging in.     * password {value}: (Context: Line) Defines the password required by the login command.

• EXEC Mode Management:     * enable: Move from user to enable mode.     * disable: Move from enable to user mode.     * quit: Disconnects the user from the CLI session.     * no debug all / undebug all: Disables all active debugging.     * show running-config: Displays the current configuration in RAM.     * show startup-config: Displays the saved configuration in NVRAM.