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Using Cisco Internetwork Operating System (IOS) Software
This Study Guide provides helpful tips for understanding and configuring Cisco IOS software using the command-line interface (CLI). Here are two views you can use.
1. Link to Cisco Web page
"Using the Command-Line Interface"
2. Link to open/download Adobe PDF file "Using the Command-Line Interface"
This Web page contains the following summary sections for your IOS use:
Understanding Command Modes
Summary of Main Command Modes
Getting Help
Example: How to Find Command Options
Using the No and Default Forms of Commands
Saving Configuration Changes
Searching and Filtering Output of show and more Commands
Understanding Command Modes
The Cisco IOS user interface is divided into many different modes. The commands available to you at any given time depend on which mode you are currently in. Entering
a question mark (
?) at the system prompt allows you to obtain a list of commands available for each command mode.
When you log in to the Cisco IOS software, you begin in user mode, often called EXEC mode. Only a limited subset of the commands are available in EXEC mode. To
have access to all commands, you must enter privileged EXEC mode. Normally, you must enter a password to enter privileged EXEC mode. From privileged mode, you can enter
any EXEC command or enter global configuration mode. Most of the EXEC commands are one-time commands, such as
show commands, which show important status information, and
clear commands, which clear counters or interfaces. The EXEC commands are not saved when the networking device reboots.
The configuration modes allow you to make changes to the running configuration. If you later save the configuration to the startup configuration, these
commands are stored when the networking device reboots. To enter the various configuration modes, you must start at global configuration mode. From global
configuration mode, you can enter interface configuration mode, subinterface configuration mode, and a variety of protocol-specific modes.
ROM monitor mode is a separate mode used when a networking device running
Cisco IOS software cannot boot properly. If your networking device does not find a valid system image when it is booting, or if its
configuration file is corrupted at startup, the system might enter ROM monitor mode.
Summary of Main Command Modes
Table 1 below summarizes the main command modes of the Cisco IOS software.
Command Mode |
Access Method |
Prompt |
Exit Method |
User EXEC |
Log in |
Router> |
Use the logout command |
Privileged EXEC |
From user EXEC mode, use the enable EXEC command. |
Router# |
To exit back to user EXEC mode, use the disable command.
To enter global configuration mode, use the configure terminal privileged EXEC command. |
Global configuration |
From privileged EXEC mode, use the configure terminal privileged EXEC command. |
Router(config)# |
To exit to privileged EXEC mode, use the exit or end command or press Ctrl-Z.
To enter interface configuration mode, use an interface configuration command. |
Interface configuration |
From global configuration mode, enter by specifying an interface with an interface command. |
Router(config-if)# |
To exit to global configuration mode, use the exit command.
To exit to privileged EXEC mode, use the exit command or press Ctrl-Z.
To enter subinterface configuration mode, specify a subinterface with the interface command. |
Subinterface configuration |
From interface configuration mode, specify a subinterface with an interface command. |
Router(config-subif)# |
To exit to global configuration mode, use the exit command.
To enter privileged EXEC mode, use the end command or press Ctrl-Z. |
ROM monitor |
From privileged EXEC mode, use the reload EXEC command. Press the Break key during the first 60 seconds while the system is booting. |
> |
To exit to user EXEC mode, use the continue command. |
Getting Help
Entering a question mark
? at the system prompt displays a list of commands available for each command mode.
You can also get a list of keywords and arguments associated with any command by using the context-sensitive help feature.
To get help specific to a command mode, a command, a keyword, or an argument, use one of the following commands:
Command |
Purpose |
help |
Obtains a brief description of the help system in any command mode. |
abbreviated-command-entry? |
Obtains a list of commands that begin with a particular character string.
(No space between command and question mark.) |
abbreviated-command-entry <Tab> |
Completes a partial command name. |
? |
scriptLists all commands available for a particular command mode. |
command ? |
Lists the keywords or arguments that you must enter next on the command line.
(Space between command and question mark.) |
Example: How to Find Command Options
This section provides an example of how to display syntax for a command. The syntax can consist of optional
or required keywords and arguments. To display keywords and arguments for a command, enter a question mark (
?)
at the configuration prompt, or after entering part of a command followed by a space. The Cisco IOS software displays a list of keywords and
arguments available along with a brief description of them. For example, if you were in global configuration mode,
typed the command
arap, and wanted to see all the keywords or arguments that may
be entered next on the command line, you would type
arap ?.
Table 2 shows examples of how you can use the
? to assist you in entering commands. The table steps you through configuring a
serial interface IP address on a Cisco router.
Table 2 How to Find Command Options
Command |
Comment |
Router> enable
Password: <password>
Router# |
Enter the enable command and password to access privileged EXEC commands.
You are in privileged EXEC mode when the prompt changes to
Router#. |
Router# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)# |
Enter the configure terminal privileged EXEC command to enter global configuration mode.
Shot cut command: config t.
You are in global configuration mode when the prompt changes to
Router(config)#. |
Router(config)# interface serial ?
<0-6> Serial interface number
Router(config)# interface serial 4 ?
/
Router(config)# interface serial 4/ ?
<0-3> Serial interface number
Router(config)# interface serial 4/0
Router(config-if)# |
Enter interface configuration mode by specifying the serial interface that you want to configure using the
interface serial global configuration command.
Enter a ? to display what you must enter next on the command line.
In this example, you must enter the serial interface slot number and port number, separated by a back slash.
You are in interface configuration mode when the prompt changes to Router(config-if)# |
Router(config-if)# ? |
Interface configuration commands: |
... |
ip |
Interface Internet Protocol config commands |
keepalive |
Enable keepalive |
lan-name |
LAN Name command |
llc2 |
LLC2 Interface Subcommands |
load-interval |
Specify interval for load calculation for an interface |
|
|
locaddr-priority |
Assign a priority group |
logging |
Configure logging for interface |
loopback |
Configure internal loopback on an interface |
mac-address |
Manually set interface MAC address |
mls |
mls router sub/interface commands |
mpoa |
MPOA interface configuration commands |
mtu |
Set the interface Maximum Transmission Unit (MTU) |
netbios |
Use a defined NETBIOS access list or enable name-caching |
no |
Negate a command or set its defaults |
nrzi-encoding |
Enable use of NRZI encoding |
ntp |
Configure NTP |
Router(config-if)# |
|
|
Enter a ? to display a list of all the interface configuration commands available
for the serial interface. This example shows only some of the interface configuration commands that are available. |
Router(config-if)# ip ? |
Interface configuration commands: |
access-group |
Specify access control for packets |
accounting |
Enable IP accounting on this interface |
address |
Set the IP address of an interface |
authentication |
authentication subcommands |
bandwidth-percent |
Set EIGRP bandwidth limit |
broadcast-address |
Set the broadcast address of an interface |
cgmp |
Enable/disable CGMP |
directed-broadcast |
Enable forwarding of directed broadcasts |
dvmrp |
DVMRP interface commands |
hello-interval |
Configures IP-EIGRP hello |
helper-address |
Specify a destination address for UDP broadcasts |
hold-time |
hold-time |
... |
|
Router(config-if)# ip |
|
|
Enter the command that you want to configure for the interface. In this example, the ip command is used.
Enter a ? to display what you must enter next on the command line.
This example shows only some of the interface IP configuration subcommands that are available. |
Router(config-if)# ip address ?
negotiated IP Address negotiated over PPP
Router(config-if)# ip address |
Enter the subcommand that you want to configure for the interface. In this example, the address subcommand is entered.
Enter a ? to display what you must enter next on the command line. In this
example, you must enter an IP address or the negotiated keyword.
Because a carriage return (<cr>) is not displayed, it indicates that you must enter more keywords or arguments to complete the command. |
Router(config-if)# ip address 172.16.0.1 ?
A.B.C.D IP subnet mask
Router(config-if)# ip address 172.16.0.1 |
Enter the keyword or argument you want to use. In this example, the 172.16.0.1 IP address is entered.
Enter a ? to display what you must enter next on the command line. In this example, you must enter an IP subnet mask.
Because a <cr> is not displayed, it indicates that you must enter more keywords or arguments to complete the command. |
Router(config-if)# ip address 172.16.0.1 255.255.255.0 ?
secondary Make this IP address a secondary address
<cr>
Router(config-if)# ip address 172.16.0.1 255.255.255.0 |
Enter the IP subnet mask. In this example, the 255.255.255.0 IP subnet mask is entered.
Enter a ? to display what you must enter next on the command line. In this example, you can enter the secondary keyword or press Enter.
Because a <cr> is displayed, it indicates that you can press Enter to complete the command. |
Router(config-if)# ip address 172.16.0.1 255.255.255.0
Router(config-if)# |
In this example, Enter is pressed to complete the command. |
Using the No and Default Forms of Commands
Almost every configuration command also has a
no form. In general, use the
no form to disable a function. Use the command without the keyword
no to reenable a
disabled function or to enable a function that is disabled by default. For example, IP routing is enabled by default.
To disable IP routing, use the
no ip routing command and specify
ip routing to reenable it.
The Cisco IOS software command reference publications provide the complete syntax for the configuration commands and describe
what the
no form of a command does.
Configuration commands can also have a
default form. The
default form of a command
returns the command setting to its default. Most commands are disabled by default, so
the
default form is the same as the
no form. However, some commands are enabled by default and
have variables set to certain default values. In these cases, the
default command enables the command and sets
variables to their default values. The Cisco IOS software command reference publications describe what the
default
form of a command does if the command is not the same as the
no form.
Saving Configuration Changes
Enter the
copy system:running-config nvram:startup-config command to save your configuration changes to your startup
configuration so that they will not be lost if there is a system reload or power outage. For example:
Router# copy system:running-config nvram:startup-config
Building configuration...
It might take a minute or two to save the configuration. After the configuration has been saved, the following output appears:
[OK]
Router#
On most platforms, this task saves the configuration to NVRAM. On the Class A Flash file system
platforms, this task saves the configuration to the location specified by the CONFIG_FILE environment variable. The CONFIG_FILE variable defaults to NVRAM.
Searching and Filtering Output of show and more Commands
In Cisco IOS Release 12.0(1)T or later, you can search and filter the output for
show and
more commands.
This functionality is useful when you need to sort through large amounts of output, or if you want to exclude output that you do not need to see.
To use this functionality, enter a
show or
more command followed by the "pipe" character (
|),
one of the keywords
begin,
include, or
exclude, and an expression that you want to search or filter on:
command | begin | include | exclude} regular-expression
The following is an example of the show interface command in which you want the output to only include lines where the expression "protocol" appears:
Router# show interface | include protocol
FastEthernet0/0 is up, line protocol is up
Serial4/0 is up, line protocol is up
Serial4/1 is up, line protocol is up
Serial4/2 is administratively down, line protocol is down
Serial4/3 is administratively down, line protocol is down
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