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• As with a computer, a router or switch cannot function without an operating system.
• The Cisco Internetwork Operating System (IOS)
• Also now used in the Catalyst switches
• IOS provides
– Basic routing & switching functions
– Access to networked resources
– Network scalability
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• The command line interface (CLI) is accessed through a console session.
• A console uses a low speed serial connection directly from a computer or terminal to the console connection on the router.
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• The Cisco CLI uses a hierarchical structure.
• This structure requires entry into different modes to accomplish particular tasks.
• Each configuration mode is indicated with a distinctive prompt and allows only commands that are appropriate for that mode.
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• The IOS provides a command interpreter service known as the command executive (EXEC).
• After each command is entered, the EXEC validates and executes the command.
• As a security feature the Cisco IOS software separates the EXEC sessions into two access levels.
• These levels are user EXEC mode and privileged EXEC mode.
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• The user EXEC mode allows only a limited number of basic monitoring commands.
• This is often referred to as a view only mode.
• The user EXEC level does not allow any commands that might change the configuration of the router.
• The user EXEC mode can be identified by the > prompt.
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• The privileged EXEC mode provides access to all router commands.
• This mode can be configured to require a password.
• Configuration and management commands require that the network administrator be at the privileged EXEC level.
• Global configuration mode and all other more specific configuration modes can only be reached from the privileged EXEC mode.
• The privileged EXEC mode can be identified by the # prompt.
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• To access the privileged EXEC level from the user EXEC level, enter the enable command at the > prompt.
• If a password is configured, the router will then ask for that password.
• For security reasons, a Cisco network device will not show the password that is entered.
• When the correct password is entered, the router prompt will change to # .
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• Cisco continues to develop different IOS software images to optimize the Cisco IOS software that these various platforms require. Each image represents a different feature set that serves the various device platforms, available memory resources, and customer needs.
• Use the Cisco Software Advisor to obtain current Cisco
IOS software information.
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• The naming convention for the different Cisco IOS releases contains three parts:
– The platform on which the image runs
– The special features supported in the image
– Where the image runs and whether it has been zipped or compressed
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• One of the main considerations when selecting a new IOS image is compatibility with the router flash and RAM memory.
• Use the show version command on the Cisco device to check the current image and available flash.
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IOS version
Boot ROM version
Router up time
Last restart method
• Platform
• Feature set
• Location to be used for loading the IOS
System image file
& location
Number & type of interfaces on the router
Configuration register setting

Version 3.1
This command will show all of the Cisco
IOS image files – not just the one that the router booted from.
Might compare to a directory listing
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• The ROM monitor performs the bootstrap process and provides low-level functionality and diagnostics. It is used to recover from system failures and to recover a lost password.
• It can only be accessed by way of a direct, physical connection through the console port.
• When the router is running in boot ROM mode, only a limited subset of the Cisco IOS feature set is available.
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When a Cisco router powers up, it performs a power-on self test
(POST).
Step 1
Step 2
Hardware check
Bootstrap loader in ROM executes
Step 3 Operating system loads
Step 4
The configuration file is loaded if one is found in NVRAM
Step 5
No configuration is in NVRAM, it looks for TFTP server, then
SETUP from console
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• Setup mode is not intended to be used to enter complex protocol features in a router.
• The purpose of the setup mode is to permit administrators to install a basic configuration for routers when a configuration cannot be obtained from another source.
• In the setup mode, default answers appear in square brackets [ ] following the question.
• Press the Enter key to use these defaults.
• During the setup process, Ctrl-C can be pressed at any time to terminate the process.
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• The router will enter setup mode when:
– The contents of NVRAM have been erased with the
“erase start” command
– When the router is “out of the box” and has not been initially configured
– After deleting the backup configuration and reloading the router
• You can make the router enter setup mode by entering:
Router#setup
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•This green light means that the system is on and has initialized properly.
•An interface LED indicates the activity of the corresponding interface.
•A problem may be indicated if an LED is off
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Rollover cable
DB-9-to-RJ-45 on the PC serial port
Router
Console port
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• To enter commands and configure a Cisco router, a user must log into the router to access the user interface.
• For security purposes, a Cisco router has two levels of access.
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• To enter privileged EXEC mode, type enable at the > prompt.
• If a password has been set, enter it at the password: prompt.
• Two commands can be used to set a password used to access privileged EXEC mode:
- enable password password
- enable secret password
• If both commands are used, the enable secret command takes precedence.
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• Once into the privileged EXEC mode, it becomes the “jumping off place to go to other configuration modes:
Router>enable
Password: xxxxxx
Router#configure terminal
Router(config)#interface serial 0
Router(config-if)#exit
Router(config)#line con 0
Router(config-line)#login
Router(config-line)#exit
Router(config)#exit
Router#disable
Router>
Ctrl-Z or end may be used at any time to return to privileged mode from anywhere
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Router >
Router #
User mode
Privileged mode
Router(config)#
Router(config-line)#
Global configuration mode
From the global configuration mode, you can enter specific configuration modes
Router(config-if)# Interface configuration mode
Line configuration mode
Router(config-router)# Router configuration mode
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• ?
shows available commands for a particular mode
• There are different commands for user and privileged, and even different commands for global mode.
• show ?
Would give a list of show commands
• s?
would give all commands that begin with the letter “S”
• If you can’t remember the command but know what it starts with, use the ?
after the first letter or two.
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• The --More-- prompt indicates that there are multiple screens of output.
• When a --More-- prompt appears, press the Spacebar to view the next available screen.
• To display just the next line, press the Return or Enter key.
• Press any other key to return to the prompt.
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<Ctrl><A>
<Ctrl><E>
<Esc><B>
<Ctrl><F>
<Ctrl><B>
Move to beginning of line
Move to the end of the line
Move back one word
Move forward one character
Move back on character
<Esc><F> Move forward one word
$ Sign means that the line has scrolled to the left
To disable enhanced editing mode, type terminal no editing at the privileged EXEC mode prompt.
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Version 3.1
Maximum number of commands in the history buffer is 256
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