Command-Line Interface (CLI)

I sometimes refer to the CLI as “Cash-Line Interface” because if you can create advanced configurations on Cisco routers and switches using the CLI, then you'll get the cash!

Entering the CLI

After the interface status messages appear and you press Enter, the Router> prompt will appear. This is called user exec mode (user mode), and it's mostly used to view statistics, but it's also a stepping stone to logging in to privileged mode.

You can only view and change the configuration of a Cisco router starting in privileged exec mode (privileged mode), which you can enter with the enable command.

Here's how:

Router>enable
Router#

You now end up with a Router# prompt, which indicates that you're in privileged mode, where you can both view and change the router's configuration. You can go back from privileged mode into user mode by using the disable command, as shown here:

Router#disable
Router>

At this point, you can type logout from either mode to exit the console.

Router>logout

Router con0 is now available
Press RETURN to get started.

In the following sections, I am going to show you how to perform some basic administrative configurations.

Overview of Router Modes

To configure from a CLI, you can make global changes to the router by typing configure terminal (or config t for short), which puts you in global configuration mode and changes what's known as the running-config. A global command (a command run from global config) is set only once and affects the entire router.

You can type config from the privileged-mode prompt and then just press Enter to take the default of terminal, as shown here:

Router#config
Configuring from terminal, memory, or network [terminal]? [press enter]
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#

At this point, you make changes that affect the router as a whole (globally), which is why it's called global configuration mode. To change the running-config—the current configuration running in dynamic RAM (DRAM)—you use the configure terminal command, as I just demonstrated.

Here are some of the other options under the configure command:

Router(config)#exit or press cntl-z
Router#config ?
  memory             Configure from NV memory
  network            Configure from a TFTP network host
  overwrite-network  Overwrite NV memory from TFTP network host
  terminal           Configure from the terminal
  <cr>

I'll go through these commands in Chapter 5, “Managing a Cisco Internetwork.”

CLI Prompts

It's really important that you understand the different prompts you can use when configuring a router. Knowing these well will help you navigate and recognize where you are at any time within configuration mode. In the following sections, I'll demonstrate the prompts that are used on a Cisco router and discuss the various terms used. (Always check your prompts before making any changes to a router's configuration!)

I won't go into every different command prompt offered because doing that would be reaching beyond the scope of this book. Instead, I'll describe all the different prompts you'll see throughout this chapter and the rest of the book. These command prompts really are the ones you'll use most in real life anyway; plus, they're the ones you'll need to know for the exam.

Interfaces

To make changes to an interface, you use the interface command from global configuration mode.

Router(config)#interface ?
  Async               Async interface
  BVI                 Bridge-Group Virtual Interface
  CDMA-Ix             CDMA Ix interface
  CTunnel             CTunnel interface
  Dialer              Dialer interface
  FastEthernet        FastEthernet IEEE 802.3
  Group-Async         Async Group interface
  Lex                 Lex interface
  Loopback            Loopback interface
  MFR                 Multilink Frame Relay bundle interface
  Multilink           Multilink-group interface
  Null                Null interface
  Port-channel        Ethernet Channel of interfaces
  Serial              Serial
  Tunnel              Tunnel interface
  Vif                 PGM Multicast Host interface
  Virtual-PPP         Virtual PPP interface
  Virtual-Template    Virtual Template interface
  Virtual-TokenRing   Virtual TokenRing
  range               interface range command
Router(config)#interface fastEthernet 0/0
Router(config-if)#

Did you notice that the prompt changed to Router(config-if)#? This tells you that you're in interface configuration mode. And wouldn't it be nice if the prompt also gave you an indication of what interface you were configuring? Well, at least for now you'll have to live without that prompt information, because it doesn't provide that. One thing is for sure: you really have to pay attention when configuring a router!

Subinterfaces

Subinterfaces allow you to create logical interfaces within the router. The prompt then changes to Router(config-subif)#.

Router(config-if)#interface f0/0.1
Router(config-subif)#

Line Commands

To configure user-mode passwords, use the line command. The prompt then becomes Router(config-line)#.

Router#config t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#line ?
  <0-337>   First Line number
  aux       Auxiliary line
  console   Primary terminal line
  tty       Terminal controller
  vty       Virtual terminal

The line console 0 command is known as a major command (also called a global command), and any command typed from the (config-line) prompt is known as a subcommand.

Routing Protocol Configurations

To configure routing protocols such as RIP and EIGRP, you'll use the prompt Router(config-router#).

Router#config t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#router rip
Router(config-router)#version 2
Router(config-router)#

Defining Router Terms

Table 4.1 defines some of the terms I've used so far.

TABLE 4.1 Router terms

Editing and Help Features

You can use the Cisco advanced editing features to help you configure your router. If you type in a question mark (?) at any prompt, you'll be given a list of all the commands available from that prompt.

Router#?
Exec commands:
  access-enable    Create a temporary Access-List entry
  access-profile   Apply user-profile to interface
  access-template  Create a temporary Access-List entry
  archive          manage archive files
  auto             Exec level Automation
  bfe              For manual emergency modes setting
  calendar         Manage the hardware calendar
  cd               Change current directory
  clear            Reset functions
  clock            Manage the system clock
  cns              CNS agents
  configure        Enter configuration mode
  connect          Open a terminal connection
  copy             Copy from one file to another
  crypto           Encryption related commands.
  ct-isdn          Run an ISDN component test command
  debug            Debugging functions (see also ‘undebug’)
  delete           Delete a file
  dir              List files on a filesystem
  disable          Turn off privileged commands
  disconnect       Disconnect an existing network connection
 -More-

Plus, at this point you can press the spacebar to get another page of information, or you can press Enter to go one command at a time. You can also press Q (or any other key, for that matter) to quit and return to the prompt.

Here's a shortcut: to find commands that start with a certain letter, use the letter and the question mark with no space between them.

Router#c?
calendar  cd        clear    clock
cns   configure connect  copy
crypto    ct-isdn


Router#c

By typing c?, I received a response listing all the commands that start with c. Also notice that the Router#c prompt reappears after the list of commands is displayed. This can be helpful when you have long commands and need the next possible command. It would be pretty lame if you had to retype the entire command every time you used a question mark!

To find the next parameters of a command string, type the first command and then a question mark.

Router#clock ?
  read-calendar    Read the hardware calendar into the clock
  set              Set the time and date
  update-calendar  Update the hardware calendar from the clock
Router#clock set ?
  hh:mm:ss Current Time
Router#clock set 11:15:11 ?
  <1-31>  Day of the month
  MONTH   Month of the year
Router#clock set 11:15:11 25 april ?
  <1993-2035> Year
Router#clock set 11:15:11 25 april 2011 ?
  <cr>
Router#clock set 11:15:11 25 april 2011
*April 25 11:15:11.000: %SYS-6-CLOCKUPDATE: System clock has been
updated from 18:52:53 UTC Wed Feb 28 2011 to 11:15:11 UTC Sat April 25 2011, configured from console by cisco on console.

By typing the clock ? command, you'll get a list of the next possible parameters and what they do. Notice that you should just keep typing a command, a space, and then a question mark until <cr> (carriage return) is your only option.

If you're typing commands and receive

Router#clock set 11:15:11
% Incomplete command.

you'll know that the command string isn't done yet. Just press the up arrow key to redisplay the last command entered, and then continue with the command by using your question mark.

And if you receive the error

Router(config)#access-list 110 permit host 1.1.1.1
                                            ^

% Invalid input detected at ‘^’ marker.

you've entered a command incorrectly. See that little caret—the ^? It's a very helpful tool that marks the exact point where you blew it and entered the command incorrectly. Here's another example of when you'll see the caret:

Router#sh serial 0/0/0
              ^
% Invalid input detected at ‘^’ marker.

This command looks right, but be careful! The problem is that the full command is show interface serial 0/0/0.

Now if you receive the error

Router#sh ru
% Ambiguous command: “sh ru”

it means there are multiple commands that begin with the string you entered and it's not unique. Use the question mark to find the command you need.

Router#sh ru?
rudpv1 running-config

As you can see, there are two commands that start with show ru.

Table 4.2 lists the enhanced editing commands available on a Cisco router.

TABLE 4.2 Enhanced editing commands

Another cool editing feature I want to show you is the automatic scrolling of long lines. In the following example, the command typed had reached the right margin and automatically moved 11 spaces to the left (the dollar sign [$] indicates that the line has been scrolled to the left):

Router#config t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#$110 permit tcp host 10.1.1.1 171.10.10.10 0.0.0.0 eq 23

You can review the router-command history with the commands shown in Table 4.3.

TABLE 4.3 Router-command history

The following example demonstrates the show history command and how to change the history size as well as how to verify it with the show terminal command. First, use the show history command to see the last 10 commands that were entered on the router, as shown here:

Router#show history
 en
 sh history
 show terminal
 sh cdp neig
 sh ver
 sh flash
 sh int fa0
 sh history
 sh int s0/0
 sh int s0/1

Now use the show terminal command to verify the terminal history size.

Router#show terminal
Line 0, Location: “”, Type: “”
[output cut]
Modem type is unknown.
Session limit is not set.
Time since activation: 00:21:41
Editing is enabled.
History is enabled, history size is 20.
DNS resolution in show commands is enabled
Full user help is disabled
Allowed input transports are none.
Allowed output transports are pad telnet rlogin lapb-ta mop v120 ssh.
Preferred transport is telnet.
No output characters are padded
No special data dispatching characters

The terminal history size command, used from privileged mode, can change the size of the history buffer.

Router#terminal history size ?
 <0-256> Size of history buffer
Router#terminal history size 25

You verify the change with the show terminal command.

Router#show terminal
Line 0, Location: “”, Type: “”
[output cut]
Editing is enabled.
History is enabled, history size is 25.
Full user help is disabled
Allowed transports are lat pad v120 telnet mop rlogin
   nasi. Preferred is lat.
No output characters are padded
No special data dispatching characters
Group codes: 0

Gathering Basic Routing Information

The show version command will provide basic configuration for the system hardware as well as the software version and the boot images. Here's an example:

Router#show version
Cisco IOS Software, 2800 Software (C2800NM-ADVSECURITYK9-M), Version
   12.4(12), RELEASE SOFTWARE (fcl)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2006 by Cisco Systems, Inc.
Compiled Fri 17-Nov-06 12:02 by prod_rel_team

The preceding section of output describes the Cisco IOS running on the router. The following section describes the read-only memory (ROM) used, which is used to boot the router and holds the POST:

ROM: System Bootstrap, Version 12.4(13r)T, RELEASE SOFTWARE (fc1)

The next section shows how long the router has been running, how it was restarted (if you see a system restarted by bus error, that is a very bad thing), the location from which the Cisco IOS was loaded, and the IOS name. Flash is the default, and the “Flash:” before the IOS name shows where the IOS that is currently running was loaded from. The IOS could load from difference sources, such as from a TFTP server.

Router uptime is 2 hours, 30 minutes
System returned to ROM by power-on
System restarted at 09:04:07 UTC Sat Aug 25 2007
System image file is “flash:c2800nm-advsecurityk9-mz.124-12.bin”

This next section displays the processor, the amount of DRAM and flash memory, and the interfaces the POST found on the router:

[some output cut]
Cisco 2811 (revision 53.50) with 249856K/12288K bytes of memory.
Processor board ID FTX1049A1AB
2 FastEthernet interfaces
4 Serial(sync/async) interfaces
1 Virtual Private Network (VPN) Module
DRAM configuration is 64 bits wide with parity enabled.
239K bytes of non-volatile configuration memory.
62720K bytes of ATA CompactFlash (Read/Write)
Configuration register is 0x2102

The configuration register value is listed last—it's something I'll cover in Chapter 5.

In addition, the show interfaces and show ip interface brief commands are very useful in verifying and troubleshooting a router as well as network issues. These commands are covered later in this chapter. Don't miss it!