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 Operating Cisco IOS Software
 Starting a Router
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 Cisco calls its operating system the Cisco
Internetwork Operating System or Cisco IOS .
 It is the embedded software architecture in all of the
Cisco routers and is also the operating system of the Catalyst switches.
 The Cisco IOS provides the following network services:
 Basic routing and switching functions
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Reliable and secure access to networked resources
Network scalability
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The Cisco IOS software uses a command-line interface (CLI) as the traditional console environment.
This environment is accessible through several methods
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A console uses a low speed serial connection directly from a computer or terminal to the console connection on the router.
A dialup connection using a modem or null modem connected to the router AUX port.
Telnet to the router.
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 The IOS provides a command interpreter service known as the command executive
(EXEC).
 As a security feature the Cisco IOS software separates the EXEC sessions into two access levels.
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 user EXEC mode privileged EXEC mode (enable 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.
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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 accesses all router commands.
This mode can be configured to require a password from the user before accessing it.
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.
When the correct password is entered, the router prompt changes to “#”, indicating that the user is now at the privileged EXEC level.
Entering a question mark ( ?
) at the privileged EXEC level will reveal many more command options than those available at the user EXEC level.
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 To optimize the Cisco IOS software that these various platforms require, Cisco is working to develop many different Cisco IOS software images.
 Each image represents a different feature set that serves the various device platforms, available memory resources, and customer needs.
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 The naming convention for the different Cisco
IOS releases contains three parts:
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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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The Cisco IOS devices have three distinct operating environments or modes:
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ROM monitor
Boot ROM
 Cisco IOS
The startup process of the router normally loads into
RAM and executes one of these operating environments.
The configuration register setting can be used by the system administrator to control the default start up mode for the router.
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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 .
 The ROM monitor cannot be accessed through any of the network interfaces. It can only be accessed by way of a direct, physical connection through the console port .
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 Boot ROM allows write operations to flash memory and is used primarily to replace the
Cisco IOS image that is stored in flash.
 The Cisco IOS image can be modified in boot
ROM by using the copy tftp flash command, which copies an IOS image stored on a TFTP server into the flash memory of the router.
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 The normal operation of a router requires use of the full Cisco IOS image as stored in flash.
 In some devices, the IOS is executed directly from flash. However, most Cisco routers require a copy of the IOS to be loaded into RAM and also executed from RAM.
 Some IOS images are stored in flash in a compressed format and have to be expanded when copied to RAM.
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 To see the IOS image and version that is running, use the show version command, which also indicates the configuration register setting .
 The show flash command is used to verify that the system has sufficient memory to load a new Cisco IOS image.
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 Operating Cisco IOS Software
 Starting a Router
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 A router initializes by loading the bootstrap, the operating system, and a configuration file.
 If the router cannot find a configuration file, it enters setup mode . Upon completion of the setup mode a backup copy of the configuration file may be saved to nonvolatile
RAM (NVRAM).
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 The goal of the startup routines for Cisco IOS software is to start the router operations.
 To do this, the startup routines must accomplish the following:
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Make sure that the router hardware is tested and functional.
Find and load the Cisco IOS software.
Find and apply the startup configuration file or enter the setup mode.
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 When a Cisco router powers up, it performs a power-on self test (POST).
 During this self test, the router executes diagnostics from ROM on all hardware modules.
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 After the POST, the following events occur as the router initializes:
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Step 1 The generic bootstrap loader in ROM executes.
Step 2 The IOS can be found in several places.
Step 3 The operating system image is loaded.
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Step 4 The configuration file saved in NVRAM is loaded into main memory and executed one line at a time.
Step 5 If no valid configuration file exists in
NVRAM, the operating system searches for an available TFTP server. If no TFTP server is found, the setup dialog is initiated.
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 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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 An interface LED indicates the activity of the corresponding interface.
 If an LED is off when the interface is active and the interface is correctly connected, a problem may be indicated.
 If an interface is extremely busy, its LED will always be on.
 The green OK LED to the right of the AUX port will be on after the system initializes correctly .
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 The examples show information and messages displayed during initial bootup.
 This information will vary, depending on the interfaces in the router and the Cisco IOS release.
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 All Cisco routers include a TIA/EIA-232 asynchronous serial console port (RJ-45).
 To connect a PC running terminal emulation software to the console port, use the RJ-45 to RJ-45 rollover cable with the female RJ-45 to DB-9 adapter.
 The default parameters for the console port are
9600 baud, 8 data bits, no parity, 1 stop bit, and no flow control.
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 To configure Cisco routers, the router user interface must be accessed with a terminal or by remote access.
 The user EXEC mode prompt is displayed upon login to a router.
 At the " > " prompt, type enable .
 At the password: prompt, enter the password that has been set with the enable secret command.
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 Two commands can be used to set a password used to access privileged EXEC mode: enable password and enable secret .
 If both commands are used, the enable secret command takes precedence.
 The global configuration mode can only be accessed from the privileged EXEC mode.
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The following are specific modes that can also be accessed from the global configuration mode :
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Interface
Subinterface
Line
Router
Route-map
To return to the user EXEC mode from the privileged EXEC mode, the disable command or exit may be entered.
To return to the privileged EXEC mode from the global configuration mode, type exit or Ctrl-Z . Ctrl-Z may also be used to return directly to the privileged EXEC mode from any submode of global configuration.
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 Typing a question mark ( ?
) at the user EXEC mode prompt or the privileged EXEC mode prompt displays a handy list of available commands.
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 The " ^ " symbol appears at the point in the command string where an incorrect command, keyword, or argument was entered.
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 Figure shows the following information from the show version command:
 IOS version and descriptive information
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Bootstrap ROM version
Boot ROM version
Router up time
Last restart method
System image file and location
Router platform
Configuration register setting
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