Cisco IOS 1 - introduction, show command

What is IOS and How It Works

IOS stands for Cisco's Internetwork Operating System, which is the operating system used by most of Switches and Routers from Cisco (I will refer to switches, but generally, it can also apply to routers). The entire IOS is stored in a single image file with the extension bin and in current switches, it is about 5 MB in size. In the flash memory of the switch, there may also be a web interface stored in the HTML folder.

IOS is a sophisticated and custom-made system. It offers a wide range of configuration options, and if we know what we want to configure, it is not too difficult. The operation of IOS is based on the CLI – Command Line Interface.

To properly use/configure the switch, it is necessary to be familiar with the different types of memory used and know how the switch works with them. The types of memory are described in the next chapter. The startup process of the switch is also important:

• connect the power to the switch
• the microcode from the ROM memory starts
  • if it does not find a valid version of IOS, it starts in the limited RxBoot function
• finds the IOS image (usually in Flash memory) and loads it
• loads and applies the startup-config from NVRAM (this text file contains a sequence of IOS commands that are executed just like when entered through the CLI)
  • if not found, express setup is invoked
• if it exists, the vlan.dat is loaded from Flash memory

If we enter any configuration commands in IOS, they are executed immediately but are only saved to the running-config, which is stored in RAM (at startup, the contents of the startup-config are copied into it). This means they are currently valid, but will be erased after restarting the switch. If we make a configuration that we cannot revert, simply restart the switch, and it will be in the state it was in at the last saved configuration. However, if we want to keep our changes, we must always copy the running configuration to the startup configuration.

Note: To emphasize, all changes that we do not want to lose when restarting the switch must be saved! This is done by copying the running configuration to the startup configuration.

SWITCH#copy running-config startup-config // save
Destination filename [startup-config]? // prompt for name, press ENTER
Building configuration...
[OK]

Types of Memory

Cisco switches and routers use five different types of memory (usually only four are mentioned, but I think the last one is also important).

ROM Memory

ROM memory is read-only and independent of power. It contains processes that are executed during the booting (starting) of the switch. In older versions of switches, it also contained IOS. The ROM memory contains several basic functions:

• POST (Power-on Self Test) - after turning on the switch, this microcode tests the functionality of the main parts (memory, CPU, interfaces)
• Bootstrap Program - initializes booting, loads IOS
• ROM monitor - a special diagnostic mode for troubleshooting
• RxBoot - if a functional IOS is not found, this limited version of IOS is loaded, allowing the correct IOS image to be installed

Flash Memory

Flash is a type of NVRAM (Non-volatile random access memory) that can be written to and retains its contents when power is disconnected. The flash memory primarily stores IOS (there may be multiple versions), a copy of the startup-config in the config.text file, and VLAN information in vlan.dat. Theoretically, we can store anything here. The memory can be listed using dir flash: or show flash:.

NVRAM Memory

Non-volatile random access memory (NVRAM) is similar to Flash in that it is writable and independent of power. It is used to store the startup-config. The memory can be listed using dir nvram:.

RAM Memory

Random Access Memory (RAM) is operational memory. It is fast writable memory that is dependent on power and is erased when the switch is restarted. This memory is typically divided into two parts - the main memory of the processor and shared I/O memory. The main part stores the running configuration (running-config), the running IOS, routing and ARP tables. The shared memory is used as a buffer for storing currently processed packets. Its contents can be displayed using show memory.

External Memory - TFTP

The last type of memory is not part of the device, so it may not be counted here, but it is used for various tasks. A TFTP (Trivial FTP) server is used, which can be run on any computer. Note that this is not FTP, but TFTP. It is most commonly used for backing up or upgrading IOS, but it is also possible to load IOS directly from the TFTP server.

Basic Work with IOS

First, we need to connect to the switch to work in the CLI. We can do this via the console, i.e., with a serial cable through a special port on the switch. This is the basic connection and sometimes the only possible one. Or, if we have already done the basic configuration, via telnet or ssh. For these connections to work, they must be set up, and a login password must always be set. For some tasks, such as basic configuration, we can connect via the web interface or the useful program Cisco Network Assistant. But although it may not seem so at first glance, it is ultimately easier to perform most settings via the CLI. For console connection, we use hyperterminal or putty, which also serves for ssh and telnet.

Entering Commands

All IOS commands can be entered in shorthand, just enter the first characters that uniquely identify the command (i.e., so that no other command starts with these characters in the given context).

The tab key completes the command. Enter the first few letters of the command and press TAB to complete the command if it is unique, or it completes the part common to multiple commands.

Entering ? (question mark) displays a list of commands with a brief description that can be used at the current location. We can also enter the first few letters of the command and a question mark to display a list of commands starting with those letters. Most commands consist of a sequence of keywords; if we enter command ?, we get a list of arguments or keywords that can be entered at that point.

SWITCH#?
Exec commands:
             Session number to resume
  access-enable    Create a temporary Access-List entry
  access-template  Create a temporary Access-List entry
  archive          manage archive files
 --More--
SWITCH#co?
configure  connect  copy

The up arrow and down arrow keys are used to scroll through history, going through previously entered commands.

If data is being displayed on the screen, it scrolls by pages. When a page is displayed, the output stops. Press the SPACE key to display the next page, and the ENTER key to display the next line.

For commands that generate some informational output, we can use the output modifier | (vertical bar) to limit the output.

Displaying MAC addresses on port 16
SWITCH#show mac-address-table | include Fa0/16

If we are in the CLI on a switch, we can connect to another switch by entering one of the commands in user/privileged mode. ssh address, telnet address, connect address, or simply entering the address if it does not match any keyword. The address can be either an IP address or a hostname. To switch back without ending the current session, use Ctrl+Shift+6 and then x.

Canceling Commands

If we enter a command in IOS and send it using the ENTER key, it is immediately applied and saved to the running configuration. If we want to cancel some setting, i.e., revoke a previously entered command, we use the keyword no followed by the same command as for entering. Thus, all settings can be canceled using the no command. Often, it is not necessary to enter all the command parameters. These changes are again applied only to the running configuration.

SWITCH(config-if)#no ip address	// canceling the IP address for the interface

Command Modes

The user interface of IOS is divided into several different modes that allow us to perform different tasks. The main modes are:

• user mode (EXEC) – user EXEC – SWITCH>
  in this mode, we are immediately after logging in and it has only limited commands
• privileged mode (EXEC) – privileged EXEC - SWITCH#
  the default mode for transitioning to other configurations, allows displaying various data
• global configuration mode – global configuration - SWITCH(config)#
  here we configure functions that affect the entire system
• interface configuration – interface configuration - SWITCH(config-if)#
  in this mode, we configure the properties of a specific interface

There are many similar modes like interface configuration. The modes are distinguished by different prompts.

SWITCH> 				// user mode
SWITCH>enable 				// switch to privileged mode
SWITCH#configure  terminal 		// switch to global configuration
SWITCH(config)#interface {name}	// switch to interface configuration
SWITCH(config-if)# 			// interface configuration 

To move from a higher level to a lower one, use the exit command. If we want to move from several higher levels to privileged mode, use Ctrl+Z.

SWITCH(config-if)#exit 
SWITCH(config)# 

Basic Control (Informational) Commands - show

These commands are used to display settings and information about the current item. Most are entered in privileged mode using the show command, which displays information from the running configuration.

General Commands

Displaying the contents of flash memory (two identical commands)
SWITCH#show flash:
SWITCH#dir

Information about the switch and IOS version
SWITCH#show version

Displaying the running configuration
SWITCH#show running-config

Displaying the startup configuration
SWITCH#show startup-config

Information about logging and recent entries
SWITCH#show logging

List of recently entered commands
SWITCH#show history

Displaying information about neighboring Cisco switches using the CDP protocol
SWITCH#show cdp neighbors
SWITCH#show cdp neighbors detail

Information about CPU usage and running processes
SWITCH#show processes

Connections

Information about current telnet connections
SWITCH#show sessions

Information about current ssh connections
SWITCH#show ssh

Information about logged-in users
SWITCH#show users

Information about lines
SWITCH#show line

Interface

Detailed information about the interface
SWITCH#show interfaces 

Detailed information about one interface
SWITCH#show interfaces {interface-name}
SWITCH#show interfaces f0/1

Interface – interfaces are mainly physical ports on the switch and VLANs. Ports are addressed by type, today mainly fastEthernet (just f) and gigabitEthernet (just g), and port number (it can be a combination of numbers, where, for example, the order in the stack, but the last is always the port number).

Information about the port with VLAN details
SWITCH#show interfaces {interface-name} switchport

Displaying a list of physical interfaces with description and status in a clear form
SWITCH#show interfaces status

Brief list of all interfaces with transfer statistics
SWITCH#show interfaces summary

List of statistics for interfaces
SWITCH#show interfaces stats

Brief information about VLANs and port assignments
SWITCH#show vlan

Information about existing trunks
SWITCH#show interfaces trunk

Displaying the CAM table – MAC addresses and ports of communicating devices
SWITCH#show mac-address-table

Displaying the ARP table
SWITCH#show arp

Displaying information about ACL and routing on the interface
SWITCH#show ip interface

Displaying the routing table
SWITCH#show ip route