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All contents copyright © 2003 Cisco Systems, Inc. All rights reserved. | |
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Overview |
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2.1 |
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Operating Cisco IOS Software | |
| 2.1.1 |
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The purpose of Cisco IOS Software |
As with a computer, a router or switch
cannot function without an operating system. 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. Without an operating system, the
hardware does not have any capabilities. The Cisco IOS provides the
following network services:
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2.1 |
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Operating Cisco IOS Software | |
| 2.1.2 |
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Router user interface |
| The Cisco IOS software uses a command-line
interface (CLI) as the traditional console environment. The IOS is a core
technology that extends across most of the Cisco product line. Its operation
details may vary on different internetworking devices.
This environment is accessible through several methods. One way to access the CLI is 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. Another way to access a CLI session is by use of a dialup connection using a modem or null modem connected to the router AUX port. Neither of these methods require that the router have any network services configured. Another method of accessing a CLI session is to Telnet to the router. To establish a Telnet session to the router, at least one interface must be configured with an IP address, and virtual terminal sessions must be configured for login and passwords.
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2.1 |
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Operating Cisco IOS Software | |
| 2.1.3 |
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Router user interface modes |
| The Cisco command-line interface (CLI) uses
a hierarchical structure. This structure requires entry into different modes
to accomplish particular tasks. For example, to configure a router
interface, the user must enter interface configuration mode. From interface
configuration mode, all configurations entered apply only to that particular
interface. Each configuration mode is indicated with a distinctive prompt
and allows only commands that are appropriate for that mode.
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. The privileged EXEC mode is also known as enable mode. The following are the features of the user EXEC mode and privileged EXEC mode:
To access the privileged EXEC level from the
user EXEC level, enter the
enable command at the “>”
prompt.
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2.1 |
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Operating Cisco IOS Software | |
| 2.1.4 |
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Cisco IOS software features |
| Cisco provides IOS images for devices
spanning a wide range of network product platforms.
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. Although there are numerous IOS images for different Cisco device models and feature sets, the basic configuration command structure is the same. The configuration and troubleshooting skills acquired on any one device apply across a wide range of products. The naming convention for the different Cisco
IOS releases contains three parts:
Specific IOS features can be selected using the Cisco Software Advisor. The Cisco Software Advisor is an interactive tool that provides the most current information and allows the selection of options that meet network requirements. One of the main considerations when selecting a new IOS image is compatibility with the router flash and RAM memory. In general, the newer the release and the more features that it provides, the more memory it requires. Use the show version command on the Cisco device to check the current image and available flash. The Cisco support site has tools available to help determine the amount of flash and RAM required for each image. Before installing a new Cisco IOS software
image on the router, check to see if the router meets the memory
requirements for that image. To see the amount of RAM, issue the
show version
command:
This line shows how much main and shared memory is installed in the router. Some platforms use a fraction of DRAM as shared memory. The memory requirements take this into account, so both numbers have to be added together to find the amount of DRAM installed on the router. To find out the amount of flash memory, issue the show flash command:
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2.1 |
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Operating Cisco IOS Software | |
| 2.1.5 |
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Operation of Cisco IOS Software |
The Cisco IOS devices have three distinct
operating environments or modes:
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. 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. When the router is running in boot ROM mode, only a limited subset of the Cisco IOS feature set is available. 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. 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. 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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2.2 |
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Starting a Router | |
| 2.2.1 |
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Initial startup of Cisco routers |
| 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).
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:
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. These diagnostics verify the basic operation of the CPU, memory, and network interface ports. After verifying the hardware functions, the router proceeds with software initialization. After the POST, the following events occur as the router initializes:
Setup is not intended as the mode for entering complex protocol features in the router. The purpose of the setup mode is to permit the administrator to install a minimal configuration for a router, unable to locate a configuration from another source. In the setup mode, default answers appear in square brackets [ ]
following the question.
When the configuration process is completed in setup mode, the following options will be displayed:
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2.2 |
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Starting a Router | |
| 2.2.2 |
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Router LED indicators |
| Cisco routers use LED indicators to provide
status information. Depending upon the Cisco router model, the LED
indicators will vary.
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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2.2 |
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Starting a Router | |
| 2.2.3 |
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Examining the initial router bootup |
The examples in Figures
-
show information and
messages displayed during initial bootup. This information will vary,
depending on the interfaces in the router and the Cisco IOS release. The
screens displayed in this graphic are for reference only and may not exactly
reflect what the screen displays on the console.
In Figure
In Figure
In Figure
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2.2 |
|
Starting a Router | |
| 2.2.4 |
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Establishing a HyperTerminal session |
| All Cisco routers include a TIA/EIA-232
asynchronous serial console port (RJ-45). Cables and adapters are needed to
connect a console terminal to the console port. A console terminal is an
ASCII terminal or PC running terminal emulation software such as
HyperTerminal. 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. The console port does not support hardware flow control. Take the following steps to connect a terminal to the console port on the router:
Figure
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2.2 |
|
Starting a Router | |
| 2.2.5 |
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Logging into the router |
| To configure Cisco routers, the router user
interface must be accessed with a terminal or by remote access. When
accessing a router, a user must login to the router before any other
commands are entered.
For security purposes, the router has two levels of access to commands:
The user EXEC mode prompt is displayed upon
login to a router.
To access the full set of commands, privileged EXEC mode must be entered. At the ">" prompt, type enable. At the password: prompt, enter the password that has been set with the enable secret command. 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. Once the login steps have been completed, the prompt changes to a "#" indicating the privileged EXEC mode has been entered. The global configuration mode can only be accessed from the privileged EXEC mode. The following are specific modes that can also be accessed from the global configuration mode:
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 sub-mode of global configuration.
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2.2 |
|
Starting a Router | |
| 2.2.6 |
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Keyboard help in the router CLI |
| Typing a question mark (?)
at the user EXEC mode prompt or the privileged EXEC mode prompt displays a
handy list of available commands.
Notice
the "--More--"
at the bottom of the sample display. The screen displays many lines at one
time. The "--More--"
prompt at the bottom of the display indicates that multiple screens are
available as output. Whenever a "--More--"
prompt appears, the next available screen can be viewed by pressing the
space bar. To display just the next line, press the Return or
Enter key. Press any other key to return to the prompt.
To access privileged EXEC mode, type
enable
or the abbreviation ena.
This might cause the router to prompt the user for a password if one has
been set. If a "?"
(question mark) is typed at the privileged EXEC mode prompt, the screen
displays a longer list of commands than is available at the user EXEC mode
prompt. Screen output varies, depending on Cisco IOS software level and router configuration. If a user wants to set the router clock but
does not know the necessary command, the help function can be used to check
for the correct command.
The task is to set the router clock. Assuming the command is not known, proceed using the following steps:
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2.2 |
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Starting a Router | |
| 2.2.7 |
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Enhanced editing commands |
| The user interface includes an enhanced
editing mode that provides a set of editing key functions that allows the
user to edit a command line as it is being typed. The key sequences
indicated in Figure
can be
used to move the cursor on the command line for corrections or changes.
Although enhanced editing mode is automatically enabled with the current
software release, it can be disabled if it interferes with the interaction
of written scripts. To disable enhanced editing mode, type
terminal no editing
at the privileged EXEC mode prompt.
The editing command set provides a horizontal scrolling feature for commands that extend beyond a single line on the screen. When the cursor reaches the right margin, the command line shifts ten spaces to the left. The first ten characters of the line cannot be seen, but a user can scroll back and check the syntax at the beginning of the command. To scroll back, press Ctrl-B or the left arrow key repeatedly until the beginning of the command entry is reached. Ctrl-A will return a user directly to the beginning of the line. In the example shown in Figure
Screen output varies, depending on Cisco IOS software level and router configuration. Ctrl-Z is a command used to back out of configuration mode. This will return the user to the privileged EXEC mode prompt.
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2.2 |
|
Starting a Router | |
| 2.2.8 |
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Router command history |
The user interface provides a history or
record of commands that have been entered. This feature is particularly
useful for recalling long or complex commands or entries. With the command
history feature, the following tasks may be completed:
The command history is enabled by default and
the system records ten command lines in its history buffer. To change the
number of command lines the system records during a terminal session, use
the terminal history size
or the history size
command.
To recall commands in the history buffer beginning with the most recent command, press Ctrl-P or the up arrow key repeatedly to recall successively older commands. To return to more recent commands in the history buffer, after recalling commands with Ctrl-P or the up arrow, press Ctrl-N or the down arrow key repeatedly to recall successively more recent commands. When typing commands, as a shortcut, the unique characters may be entered for a command. Press the Tab key, and the interface will finish the entry. When the typed letters uniquely identify the command, the Tab key simply acknowledges visually that the router has understood the specific command that was intended. On most computers additional select and copy functions are available. A previous command string may be copied and then pasted or inserted as the current command entry.
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2.2 |
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Starting a Router | |
| 2.2.9 |
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Troubleshooting command line errors |
Command line errors occur primarily from
typing mistakes. If a command keyword is incorrectly typed the user
interface provides error isolation in the form of an error indicator (^).
The "^" symbol appears at the point in the command string where an
incorrect command, keyword, or argument was entered. The error location
indicator and interactive help system allow the user to easily find and
correct syntax errors.
% Invalid input detected at "^" marker. The caret symbol (^) and help response indicate an error at 93. To list the correct syntax, enter the command up to the point where the error occurred and then enter a question mark (?):
Enter the year using the correct syntax and press Return to execute the command.
If a command line is entered incorrectly, and the enter key is pressed, the up arrow can be pressed to repeat the last command. Use the right and left arrow keys to move the cursor to the location where the mistake was made. Then type the correction that needs to be made. If something needs to be deleted, use the <backspace> key.
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2.2 |
|
Starting a Router | |
| 2.2.10 |
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The show version command |
| The
show version
command displays information about the Cisco IOS software version that is
currently running on the router. This includes the configuration register
and the boot field settings.
Figure
Use the show version command to identify router IOS image and boot source.
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Summary |
An understanding of the following key points
should have been achieved:
|
Web Links
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 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.
. The
system indicates that additional information needs to be provided to
complete the command. 