Routers and Routing Basics CCNA 2
Chapter 2
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Introduction to Routers
Operating the Cisco IOS CLI
The Cisco IOS CLI
Router EXEC Modes
Router Configuration Modes
Cisco IOS Fundamentals
Starting, Configuring, Accessing, and Using a Router
Initial Booting of a Router
Creating an Initial Configuration Using Setup Mode Accessing a Router
Using a Router CLI
Summary
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Operating the Cisco IOS CLI
• Cisco puts all the important software features of a router
into one large OS, called Cisco IOS.
• The IOS provides all the core features of a router,
including the following:
– Controlling the sending and receiving of packets in the router’s
physical interfaces
– Storing packets in RAM until the outgoing interface is available
to forward the packet
– Routing (forwarding) packets
– Dynamically learning routes using routing protocols
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The Cisco IOS CLI
• The router’s user interface is called the command-line
interface (CLI).
• The CLI is not graphical - instead, it is a text-based user
interface.
• The CLI is powerful because it unlocks thousands
of router commands and options, allowing the
Administrator to make choices not available on the GUI.
• The IOS CLI can be accessed using a terminal emulator.
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Three Methods to Access a Router CLI
■ Router console port—Uses
cables between the PC and
the router console port
■ Router auxiliary (aux)
port—Uses modems and a
telephone circuit between the
PC and the router aux port
■ Telnet—Uses the IP network
between the PC and the
router
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Terminal Emulator (HyperTerminal)
Accessing a Router
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Router EXEC Modes
• The IOS CLI has many different areas or parts, called
modes.
• The term EXEC mode comes from command executive.
• To provide a basic level of security, IOS creates two
different levels of EXEC mode by default: user EXEC
mode and privileged EXEC mode.
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Comparing User and Privileged EXEC
Modes
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Navigating to and from User and Privileged
(Enable) Modes in a Router
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Minimum Requirements for Configuring Two
Routers
The routers simply need to be configured with the correct
IP addresses and masks, along with a routing protocol.
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Configuration Process and Configuration
Modes
1. The configure terminal EXEC command moves the user from
privileged EXEC mode into global configuration mode.
2. The interface command moves the CLI from global configuration mode
into interface configuration mode.
3. The router rip command moves the CLI into a routing protocol
configuration mode, as signified by a prompt ending in (config-router)#.
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Cisco IOS Features
■ Cisco manufactures many different models of router hardware, with
each model being a member of a router platform, series, or family.
■ Some IOS images have fewer features and some have more
features, to provide flexible pricing.
■ Cisco updates the software over time with new features and bug
fixes. Cisco creates new IOS images for each new version of IOS
software.
■ Some IOS images are compressed to save storage space, and some
are not.
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Cisco IOS Filename Example
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Storing Cisco IOS in Flash Memory and
RAM
Before installing a new IOS into a router, you need to check
to see if the flash memory in the router has enough space
to hold the new IOS (R1 has 8192 KB of flash memory).
Output from the show flash Command
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Finding the Amount of Router RAM Using
the show version Command
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Two Alternative OSs
• Two OSs, or operating environments, are called
– ROM Monitor (ROMMON) and
– boot ROM
• The main uses of ROMMON mode are as follows:
– To provide a low-level debugging tool that is available even
when flash memory has been erased or corrupted
– To provide a means of putting a normal IOS file into flash
memory in case flash memory is erased or becomes corrupted
– To provide a means to recover or reset router passwords
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Two Alternative OSs
(Continued)
The other alternative OS is called the boot ROM IOS, or
sometimes RX-boot mode
– Boot ROM software resides in the ROM chips
– Unlike ROMMON, the boot ROM software is an IOS
image, although a very basic one
– The purpose of this basic IOS is to allow an engineer to
load new IOS images into flash memory when the image
cannot be loaded due to other issues
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Key Features and Facts About Alternative
OSs
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Initial Booting of a Router
• The process of doing the overhead work to make the
computer ready to use is called booting, or the boot
process, or rebooting the computer.
• When booting, the router generates messages about the
boot process and sends them out the console port.
• Router output during the boot process
–
–
–
–
–
The version of IOS being loaded
The number of interfaces
The types of interfaces
The amount of NVRAM
The amount of flash memory
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Initial Booting of a Router
(Continued)
•
Besides the information in the boot messages sent to the
console, the router also gives some indication about the
boot process through the LEDs on the back of the router.
•
When interfaces are sending a lot of traffic, the LEDs
might even appear to be on all the time
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Initial Booting of a Router
(Continued)
Router takes four steps when booting:
– performs a power-on self test (POST)
– loads a bootstrap program
– loads an IOS image into RAM and switches from using
the bootstrap software to using the IOS software
– loads an initial configuration into RAM, called the
startup configuration
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Loading the Bootstrap Program from ROM
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Finding the Version of the Bootstrap
Program
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Alternative Sources for IOS Image
The bootstrap program
has three main alternatives
from which to get the IOS image:
A. Flash memory
B. An external TFTP server
C. ROM
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Configuration Register
• Cisco provides two tools with which to tell the router
which IOS image to load.
• The first tool is called the configuration register, or
simply config register.
• The config register is a 16-bit value stored in the router,
typically listed as a four-digit hexadecimal number, like
0x2102.
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Configuration Register
(Continued)
The last hex digit of the config register is called the boot
field, and the boot field tells the bootstrap program what
software to load, as follows:
■ Boot field of hex 0—Load ROMMON
■ Boot field of hex 1—Load boot ROM IOS
■ Boot field of hex 2 through F—Load an IOS from
another location, typically from flash memory
If a router initializes when the boot field is 0 or 1, the router
knows exactly what to do.
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boot system Command
If the boot field is any other value, the router must use
another tool, called the boot system configuration
command:
1. Attempt to load the IOS image listed in the first boot system
command found in the router’s configuration file.
2. If the attempt to load that IOS image fails, attempt to load the
IOS image listed in the next boot system command found in
the configuration file.
3. If an IOS image has not been successfully loaded after all
boot system commands have been tried, or if no boot system
commands exist, load the first IOS image found in flash
memory.
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Loading the Startup Configuration into RAM
Configuration loaded into RAM is called the running-config file because
it is the configuration actually used by the IOS that is executing, or running,
at the current time.
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Startup-config File Locations
• The router can choose to ignore the startup-config file in NVRAM, and
starts with no configuration (password recovery).
• The router can be configured to load an initial configuration from a
TFTP server, based on the existence of boot config commands in the
NVRAM startup-config file.
• The router can search for an available TFTP server by sending
broadcasts and try to find an appropriate configuration file on the
server.
• The router can load the startup-config file in NVRAM which is the
default.
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Creating an Initial Configuration Using
Setup Mode
• Setup mode is another tool for configuring routers.
• In setup mode is a method by which the router sends a
series of questions to the console.
• The router builds an initial Configuration based on the
answers, temporary stores it in RAM (running-config file)
and can save it permanently in NVRAM (startup-config file).
• Setup mode is reachable in two ways:
– enter the setup command from privileged mode, or
– boot the router when NVRAM is empty.
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Empty NVRAM
NVRAM might be empty for the following reasons:
■ The router was just unpacked from its shipping box and has not yet been
configured.
■ A user issued a privileged mode EXEC command such as erase startupconfig or write erase, both of which erase the contents of the startup-config
file in NVRAM.
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Setup Mode
• If a router completes the initialization process with no
configuration loaded, the router cannot route any
packets.
• The router gives the network engineer an
opportunity to use setup mode to easily configure the
router.
• To do so, the router poses a question to the
engineer by sending a message to the console.
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Setup Mode
(Continued)
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Setup Mode
(Continued)
• The router asks questions about some of the basic
things that can be configured on a router, and the
engineer’s answers to those questions tell the router
what configuration commands to create.
• The router often suggests a default answer, shown in
brackets.
• The user can always quit the process by pressing Ctrl-C
at any point in the process.
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The Three Options at the End of the Setup
Process
■ Option 0—Ignore all the answers that were given in setup mode, and
give the user a user mode command prompt. As a result, the router
will still not have any configuration.
■ Option 1—The user answered some of the questions incorrectly, so
ignore all the answers given in setup mode. Start setup again so that
the user can answer correctly this time.
■ Option 2—Use the answers given in setup mode, creating a startupconfig file in NVRAM and an identical running-config file in RAM.
(The user also sees a new user mode command prompt.)
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Cabling and Accessing a Router Console
Port
Step 1 Connect a rollover cable to
the router console port. (The
console port is typically an RJ-45
jack, and the rollover cable also
typically has an RJ-45 connector.)
Step 2 Connect a converter to the
other end of the rollover cable.
Since many PC serial ports use a
DB-9 serial port connector, so an
RJ-45-to-DB-9 converter would
be used.
Step 3 Connect the DB-9 end of the
converter to the serial port on the
PC.
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Emulator Settings
The emulator settings must be as follows to match
the default settings on the router console port:
■ 9600 bps
■ 8 data bits
■ No parity
■ 1 stop bit
■ No flow control
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Common Terminal Emulators for Different
OSs
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CLI Modes as Implied by Different
Command Prompts
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Command Prompts Used by Different CLI
Modes
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Router Password Basics
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Example Password Prompting by a Router
• When configured to require passwords, the router prompts
the user to supply the passwords.
• Router does not display the password that is entered, so the
example simply lists a string of the letter z to represent the
password being entered.
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enable secret and enable password
Commands
• The two global configuration commands that can
configure the enable password are enable password
pw and enable secret pw, where pw represents the text
of the password.
• Together, these two configuration commands define what
IOS expects to be entered when the
user enters the enable command and the router prompts
the user for a password.
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enable secret and enable password
Commands (Continued)
■ If only one of the two commands (enable secret or enable
password) is configured, IOS expects the user to enter the password
as defined in that single configuration command.
■ If both the enable secret and enable password commands are
configured, the router expects the password as defined in the enable
secret command. In this case, the route ignores the password defined
in the enable password command.
■ If neither the enable secret nor enable password command is
configured, the behavior varies. If the user is at the console, the router
automatically allows the user access to enable mode. If the user is not
at the console, the router rejects the enable command.
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Using a Router CLI
This section focuses on five main topics:
–
–
–
–
How to find commands available from the CLI
How to find the available options on a known command
How to retrieve an old command so you can repeat it
How to edit the command line to change something
about the command
– How to interpret the automatic help that occurs when
you mistype a command
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Finding IOS Commands Using ?
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Getting Help for the Options on a Command
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Getting Help for the Options on a Command
(Continued)
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Commands and Key Combinations Related
to Command Recall
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CLI Enhanced Editing Feature
The best way to check out and understand enhanced
editing mode is to use the CLI of a real router.
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Error Messages When a Command Is
Mistyped
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The show version Command
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Cross-reference List for Used Example
■ IOS version and descriptive information—Version
12.3(11)T3
■ Bootstrap ROM version—12.3(8r)T8
■ Router uptime—20 minutes
■ Last restart method—Returned to ROM by power-on
■ System image file and location—flash:c1841advipservicesk9-mz.123-11.T3.bin
■ Router platform/series/model—Cisco 1841
■ Amount of RAM—117760K/13312K bytes
■ Amount of NVRAM—191K bytes
■ Amount of flash memory—31744K bytes
■ Configuration register setting—0x2102
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Summary
• Cisco IOS is the OS that runs inside Cisco routers.
• Cisco IOS software provides a command-line interface (CLI) to allow
users to access, use, and control IOS. A terminal emulator can be
used to access the CLI, using one of three main methods of
connecting to the router:
– Using cabling to the router’s console port
– Using a modem to dial in to the router’s aux port
– Using Telnet over an IP network
• Access via any of these three methods can be password protected
using the console, auxiliary, and Telnet passwords, respectively.
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Summary
(Continued)
• The CLI includes a wide variety of modes. Two of these modes, user
EXEC mode and privileged (enable) EXEC mode, allow the user to
enter commands.
• Configuration mode allows the user to enter configuration
commands, which are commands that tell IOS what parameters to
use when routing packets.
• Users can navigate between modes using several commands. The
user begins in user mode, moving to privileged mode by using the
enable command. From there, the configure terminal command
moves the user into global configuration mode.
• IOS supports a large variety of features to help the user navigate the
CLI.
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