Getting Information About Remote Devices 639
The statistics reflect router operation since the last time the counters were cleared. Use
the clear counters command to reset the counters to 0. By starting from 0, you get a
better picture of the current status of the network. The show interfaces command can
also be used to check the status of other interface types, such as Ethernet (E0), Fast
Ethernet (Fa0), and Integrated Services Digital Network (ISDN) (BRI0).
Checking Real-Time Traffic with the debug Command
The router includes hardware and software to aid it in tracking down problems on it
or on other hosts in the network. The debug privileged EXEC command starts the con-
sole display of the network events specified in the command parameter. Use the termi-
nal monitor command to forward debug output to your Telnet session terminal.
Figure 13-10 displays the data-link broadcasts received by the router using the debug
broadcasts command. Use the undebug all (or no debug all) command to turn off
debugging when you no longer need it. Debugging is really intended for solving
problems.
Figure 13-10 Checking Real-Time Traffic with the debug Command
By default, the router sends system error messages and output from the debug EXEC
command to the console terminal. However, you can redirect messages to a UNIX host
or to an internal buffer. The terminal monitor command enables you to redirect these
messages to a terminal, which can be useful in cases where you use a UNIX or Linux
host instead of the console terminal as a means of administering the devices.
22146 packets output, 2383680 bytes, 0 underruns
0 output errors, 0 collisions, 2 interface resets, 0 restarts
1 carrier transitions
Example 13-6 show interfaces Command Output (Continued)
CAUTION
Be careful with the
debug command on a
live network. Substan-
tial debugging on a
busy network slows

the network signifi-
cantly. Do not leave
debugging turned on;
use it to diagnose a
problem, and then
turn it off. Excessive
debugging on routers
with minimal RAM
can cause the router
to force a reload.
1102.book Page 639 Tuesday, May 20, 2003 2:53 PM
640 Chapter 13: Learning About Neighboring and Remote Devices
Summary
In this chapter, you learned the following the key points:
■ CDP is a media- and protocol-independent, Cisco-proprietary protocol used for
neighbor discovery.
■ CDP shows information only about directly connected neighbors.
■ You use CDP to show Layer 2 and 3 information about neighbors.
■ You should test network connectivity layer by layer.
■ Dealing with connectivity issues is the most common problem situation that
occurs on IP networks. The following three commands are used to perform
address-related troubleshooting:
— ping uses the ICMP protocol to verify the hardware connection and the
IP address of the network layer. This command is a very basic testing
mechanism.
— telnet verifies the application layer software between source and destina-
tion. This command is the most complete test mechanism available.
— traceroute allows the location of failures in the path from the source to des-
tination. traceroute, or trace, uses TTL values to generate messages from
each router along the path.

To supplement all that you’ve learned in this chapter, refer to the chapter-specific Videos,
PhotoZooms, and e-Lab Activities on the CD-ROM accompanying this book.
Lab Activity Troubleshooting IP Address Issues
In this activity, you configure IP addresses for workstations and the test end-to-
end network connectivity.
1102.book Page 640 Tuesday, May 20, 2003 2:53 PM
Check Your Understanding 641
Key Terms
CDP (Cisco Discovery Protocol) Used to obtain information about neighboring
devices, such as the types of devices connected, the router interfaces they are connected
to, the interfaces used to make the connections, and the model numbers of the devices.
debugging To find and remove errors (bugs) from a program or design.
ping Packet internet groper. ICMP echo message and its reply. Often used in IP
networks to test the reachability of a network device.
SNAP (Subnetwork Access Protocol) Internet protocol that operates between a
network entity in the subnetwork and a network entity in the end system. SNAP
specifies a standard method of encapsulating IP datagrams and ARP messages on IEEE
networks. The SNAP entity in the end system makes use of the services of the subnet-
work and performs three key functions: data transfer, connection management, and
QoS selection.
SNMP (Simple Network Management Protocol) Network management protocol
used almost exclusively in TCP/IP networks. SNMP provides a means to monitor and
control network devices, and to manage configurations, statistics collection, perfor-
mance, and security.
Telnet Standard terminal emulation protocol in the TCP/IP protocol stack. Telnet is
used for remote terminal connection, enabling users to log in to remote systems and
use resources as if they were connected to a local system.
TLVs (Type Length Values) Blocks of information embedded in CDP advertisements.
traceroute Program available on many systems that traces the path a packet takes to
a destination. It is used mostly to debug routing problems between hosts.

Check Your Understanding
Complete all the review questions to test your understanding of the topics and con-
cepts in this chapter. Answers are listed in Appendix C, “Check Your Understanding
Answer Key.”
1. What is a definition for telnet?
A. A command to determine whether a specific IP address is accessible. It works
by sending a packet to the specified address and waiting for a reply.
B. A command that uses Time-To-Live (TTL) values to generate messages from
each router used along the path.
1102.book Page 641 Tuesday, May 20, 2003 2:53 PM
642 Chapter 13: Learning About Neighboring and Remote Devices
C. A command used to verify the application layer software between source and
destination stations. This command is the most complete test mechanism
available.
2. What information does testing a network by using the show interfaces serial
command provide?
A. It displays line and data link protocol status.
B. It displays how the router directs traffic across the network.
C. It displays the path that packets follow across the network.
D. It displays the names of routers on the network.
3. What information does the show cdp neighbors command provide for each CDP
neighbor?
A. Device identifiers
B. Address list
C. Port identifier
D. All of the above
4. The show cdp interface command is used to display which of the following?
A. The values of the CDP timers and the interface status
B. The encapsulation used by CDP for its advertisement and discovery frame
transmission

C. The interface configuration of the neighboring routers
D. Both A and B
5. Which of these is true for the output of the command show cdp entry
[device name]?
A. It displays all the Layer 3 addresses present in the neighbor router.
B. It displays the value of neighbors that the router has.
C. It displays the list of the device numbers of all the neighboring routers.
D. It displays all the Layer 2 addresses on the interfaces of the neighbor router.
6. To display the information that is obtained both from the show cdp neighbors
and show cdp entry [device name] commands, which of the following commands
is used?
A. show cdp neighbors detail
B. show cdp interface entry
C. show cdp neighbors entry
D. show cdp details
1102.book Page 642 Tuesday, May 20, 2003 2:53 PM
Check Your Understanding 643
7. What information is displayed by the show cdp neighbors command?
A. Neighbor device ID
B. Local port type and number
C. Decremental holdtime value, in seconds
D. All of the above
8. What four important pieces of information do you receive after issuing a ping
command?
A. The size and quantity of ICMP packets; the timeout duration; the success
rate; and the minimum, average, and maximum round-trip times
B. The size and quantity of ICMP packets; the MAC address; the success rate;
and the minimum, average, and maximum round-trip times
C. All of the above
D. None of the above

9. What information does testing a network by using the traceroute command
provide?
A. It determines if the line protocol is operational.
B. It determines if a routing table entry exists for the target network.
C. It maps every router that a packet goes through to reach its destination.
D. It determines if upper-layer applications are functioning properly.
10. In reply to the ping command, exclamation points (!) indicate which of these?
A. The number of successful echos
B. The number of unsuccessful echos
C. The number of hops before reaching the destination
D. All of the above
11. What is true of the router debug commands?
A. The debug privileged EXEC command starts the console display of the net-
work events specified in the command parameter.
B. The undebug all command (or no debug all) turns off debugging.
C. The buffer debug command is used to buffer the login.
D. Both A and B.
1102.book Page 643 Tuesday, May 20, 2003 2:53 PM
Objectives
Upon completion of this chapter, you will be able to
■ Identify the stages of the router boot sequence
■ Determine how a Cisco device locates and loads Cisco IOS Software
■ Use the boot system command
■ Identify the configuration register values
■ Describe the process used to locate Cisco IOS Software
■ Describe the process and commands for creating and loading a software
image and configuration file backup
■ Describe Cisco IOS Software naming conventions
1102.book Page 644 Tuesday, May 20, 2003 2:53 PM
Chapter 14

Managing Cisco IOS Software
This chapter examines the stages of the router boot sequence, including how the Cisco
router locates and loads Cisco IOS Software. This chapter also introduces the boot system
command and explains how the boot system command tells the router to boot up. Also,
you learn how to use a variety of Cisco IOS Software source options and execute commands
to load Cisco IOS Software onto the router, maintain backup files, and upgrade Cisco
IOS Software. In addition, you learn about the functions of the configuration register and
how to determine which version of the Cisco IOS Software image file you have. Finally,
this chapter describes how to use a TFTP server as a software source for the Cisco IOS
Software image and configuration files.
Please be sure to look at this chapter’s associated e-Labs, Videos, and PhotoZooms that
you will find on the CD-ROM accompanying this book. These CD elements are designed
to supplement the material and reinforce the concepts introduced in this chapter.
Stages of the Router Power-On/Boot Sequence
A router initializes by loading the bootstrap, the operating system, and a configuration
file. If the router cannot find a configuration file, then it enters setup mode. The router
stores a backup copy of the new configuration from setup mode in nonvolatile random-
access memory (NVRAM). Figure 14-1 shows the events that occur as the router initializes.
1102.book Page 645 Tuesday, May 20, 2003 2:53 PM
646 Chapter 14: Managing Cisco IOS Software
Figure 14-1 Router Startup Sequence
The goal of the startup routines for Cisco IOS Software is to start the router opera-
tions. The router must deliver reliable performance in connecting to the user networks
it was configured to serve. To do this, the startup routines must do the following:
1. Make sure that the router loads ROM.
2. Find and load the Cisco IOS Software image that the router uses for its operating
system.
3. Find and apply configuration statements, including protocol functions and inter-
face addresses.
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.
Locating and Loading Cisco IOS Software
The default source for Cisco IOS Software startup depends on the hardware platform.
However, the router usually looks to the boot system commands saved in NVRAM.
Cisco IOS Software does, however, allow you to use several alternatives. You can spec-
ify that the router look at other sources for software. Also, the router can use its own
fallback sequence, as necessary, to load the software. Figure 14-2 illustrates how the
Cisco IOS Software image is located.
1102.book Page 646 Tuesday, May 20, 2003 2:53 PM
Locating and Loading Cisco IOS Software 647
Figure 14-2 Locating Cisco IOS Software
The settings in the configuration register enable the following alternatives:
■ You can specify global configuration mode boot system commands to enter fall-
back sources for the router to use in sequence. When the router restarts, it uses
these commands in sequence as needed.
■ If NVRAM does not have boot system commands that the router can use, the
system uses the Cisco IOS Software in Flash memory by default.
■ If Flash memory is empty, the router next attempts to use Trivial File Transfer
Protocol (TFTP) to load a Cisco IOS Software image from the network. The
router uses the configuration register value to form a filename from which to
boot a default system image stored on a network server.
■ If a TFTP server is unavailable, the router will load the limited version Cisco IOS
Software image stored in ROM.
Using the boot system Command
You can enter multiple boot system commands to specify the fallback sequence for
booting Cisco IOS Software. Examples 14-1, 14-2, and 14-3 show boot system entries
that specify that a Cisco IOS Software image will load first from Flash memory, then
from a network server, and finally from ROM.

Configuration Registers
Registers in NVRAM for Modifying Fundamental Cisco IOS Software;
Identifies Where to Boot Cisco IOS Image (For Example, Use config mode Commands)
Router# configure terminal
Router(config)# boot system flash IOS_filename
Router(config)# boot system tftp IOS_filename tftp_address
Router(config)# boot system rom
[Ctr–Z]
Router# copy running-config startup-config
Boot System Commands Not Found in NVRAM
Get Default Cisco IOS Software from Flash
Flash Memory Empty
TFTP Server Unavailable
Get Limited Cisco IOS Software from ROM
Get Default Cisco IOS Software from the TFTP Server
1102.book Page 647 Tuesday, May 20, 2003 2:53 PM
648 Chapter 14: Managing Cisco IOS Software
The command copy running-config startup-config saves the commands in NVRAM.
The router executes the boot system commands as needed and in the order in which
they were originally entered into configuration mode. To see if any boot system com-
mands have been entered, issue the show startup-config command.
Booting from Flash Memory
Booting from Flash memory involves loading a system image from electrically erasable
programmable read-only memory (EEPROM). The advantage here is that information
stored in Flash memory is not vulnerable to network failures that can occur when load-
ing system images from TFTP servers. Example 14-1 specifies that the gsnew-image
Cisco IOS Software file be loaded from Flash.
Booting from a Network Server
In case Flash memory becomes corrupted, a system image can provide a backup by
specifying that it should be loaded from a TFTP server. Example 14-2 specifies that the

test.ext image file be loaded from the TFTP server at IP address 172.16.13.111.
Booting from ROM
If Flash memory is corrupted and the network server fails to load the image, booting
from ROM is the final bootstrap option in software. However, the system image in
ROM will likely be a subset of Cisco IOS Software. This subset lacks the protocols,
features, and configurations of full Cisco IOS Software. Also, if the software has been
updated since the router was purchased, the router might have an older version of
Cisco IOS Software. Example 14-3 demonstrates this last bootstrap option.
Example 14-1 Booting from Flash Memory
Router# configure terminal
Router#(config)# boot system flash gsnew-image
[Ctrl-Z]
Router# copy running-config startup-config
Example 14-2 Booting from a Network Server
Router# configure terminal
Router#(config)# boot system tftp IOS_image 172.16.13.111
[Ctrl-Z]
Router# copy running-config startup-config
1102.book Page 648 Tuesday, May 20, 2003 2:53 PM