TCP/IP Suite Control Messages 779
However, this support is not always the case. If a router-solicitation message is sent to a
router that does not support the discovery process, the solicitation will go unanswered.
Figure 17-15 Router-Discovery Message
When a router that supports the discovery process receives the router-solicitation mes-
sage, a router advertisement is sent in return. Table 17-7 provides an explanation for
each field of the frame format.
Table 17-7 Router Frame Format
ICMP Field Description
Type 9.
Type 0.
Checksum The 16-bit 1s complement of the 1s complement sum of the
ICMP message starting with the ICMP type. For computing
the checksum, the checksum field should be 0.
Number of Addresses The number of router addresses advertised in this message.
Address Entry Size The number of 32-bit words of information per each router
address (2 in the version of protocol described here).
Lifetime The maximum number of seconds that the router addresses
are considered valid.
Router Addresses The sending router IP address(es).
The number of the address interface from which this mes-
sage is sent.
Preference Level The preference of each router address.
The number address as a default router address, relative to
other router addresses on the same subnet. A signed twos-
complement value; higher values mean more preferable.
08
16 31
Type (9)
Code (0)
Checksum

Lifetime
Router Address 1
Preference Level 1
Router Address 2
Preference Level 2
Number of
Addresses
Address Entry
Size
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780 Chapter 17: TCP/IP Error and Control Messages
Router-Solicitation Message
A host generates an ICMP router-solicitation message in response to a missing default
gateway. This message is sent using multicast. This message is the first step in the router-
discovery process. A local router responds with a router advertisement identifying the
default gateway for the local host.
Figure 17-16 identifies the frame format, and Table 17-8 gives an explanation of each
field.
Figure 17-16 Router-Solicitation Message Frame Format
Congestion and Flow-Control Messages
If multiple computers try to access the same receiver, the receiver can be overcome
with traffic. Congestion also can occur when traffic from a high-speed LAN reaches a
slower WAN connection. Congestion on the network or the receiver causes dropped
packets, which results in a loss of data. To reduce the amount of data lost, ICMP mes-
sages must be sent to the source of the congestion. This type of ICMP message is called
a source quench message. The source quench message notifies the sender of the conges-
tion and asks the sender to reduce the rate at which it is transmitting packets. In most
cases, congestion subsides after a short period of time. The source slowly increases the
transmission rate as long as no other source quench messages are received. Most Cisco
routers do not send source quench messages by default. This is because the source

quench message might, in itself, add to the network congestion.
Table 17-8 Router-Solicitation Message Frame Field Descriptions
ICMP Field Description
Type 10.
Code 0.
Checksum The 16-bit 1s complement of the 1s complement sum of the
ICMP message starting with the ICMP type. For computing
the checksum, the checksum field should be 0.
Reserved Sent as 0; ignored on reception.
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Summary 781
ICMP source quench message might be used effectively in a small office, home office
(SOHO). One such SOHO could consist of four computers networked together using
Category 5 cable and Internet connection sharing (ICS) over a 56-kbps modem. It is
easy to see that the 10-Mbps bandwidth of the SOHO LAN easily could overwhelm
the available 56-kbps bandwidth of the WAN link. This could result in data loss and
retransmissions. With ICMP messaging, the host acting as the gateway in the ICS can
request that the other hosts reduce their transmission rates to a manageable level.
Reducing the transmission rates prevents continued data loss.
Summary
These key points were discussed in this chapter:
■ IP is a best-effort delivery method that uses ICMP messages to alert the sender
that the data did not reach its destination.
■ ICMP echo request and echo reply messages enable the network administrator
to test IP connectivity to aid in the troubleshooting process.
■ ICMP messages are transmitted using the IP protocol, so their delivery is
unreliable.
■ ICMP packets have their own special header information, starting with a
Type field and a Code field.
■ Functions of ICMP control messages.

■ ICMP redirect/change request messages.
■ ICMP clock synchronization and transit time estimation messages.
■ ICMP information request and reply messages.
■ ICMP address mask request and reply messages.
■ ICMP router-discovery message.
■ ICMP router-solicitation message.
■ ICMP congestion and flow-control messages.
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.
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782 Chapter 17: TCP/IP Error and Control Messages
Key Terms
broadcast Data packet that are sent to all nodes on a network.
datagram In IP networks, packets often are called datagrams.
ICMP (Internet Control Message Protocol) Network layer Internet Protocol that
reports errors and provides other information relevant to IP packet processing. It is
documented in RFC 792.
multicast Single packets copied by the network and sent to a specific subset of
network addresses.
ping (Packet Internet Groper) ICMP echo message and its reply. Often used in
IP networks to test the reachability of a network device.
TCP/IP (Transmission Control Protocol/Internet Protocol) Common name for the
suite of protocols developed by the U.S. Department of Defense in the 1970s to sup-
port the construction of worldwide internetworks. TCP and IP are the two best-known
protocols in the suite.
unicast Message sent to a single network destination
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. ICMP is an error-reporting protocol for IP. True or false?
A. True
B. False
2. ICMP stands for what?
A. Internal Control Message Protocol
B. Internet Control Message Portal
C. Internal Control Message Protocol
D. Internet Control Message Protocol
3. ICMP messages are encapsulated as data in datagrams in the same way that any
other data is delivered using IP. True or false?
A. True
B. False
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Check Your Understanding 783
4. A default gateway must be configured if datagrams are to travel outside the local
network. True or false?
A. True
B. False
5. TTL stands for what?
A. Time-To-List
B. Time-To-Live
C. Terminal-To-Live
D. Terminal-To-List
6. All ICMP message formats start with which of these fields?
A. Type
B. Code
C. Checksum
D. All of the above
7. Default gateways send ICMP redirects/change requests only when which of the
following conditions are met?

A. The interface on which the packet comes into the router is the same interface
on which the packet gets routed out.
B. The subnet/network of the source IP address is the same subnet/network of
the next-hop IP address of the routed packet.
C. The datagram is not source-routed.
D. The route for the redirect is not another ICMP redirect or a default route.
E. All of the above.
8. The Type field on an ICMP timestamp message can be either a 13 (timestamp
request) or what?
A. 14 timestamp reply
B. 14 time
C. 14 reply
D. 26 timestamp reply
E. All of the above
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784 Chapter 17: TCP/IP Error and Control Messages
9. All ICMP timestamp reply messages do not contain the originate, receive, and
transit timestamps. True or false?
A. True
B. False
10. What is ICMP type number for a router-solicitation message?
A. 9
B. 10
C. 17
D. 18
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Objectives
Upon completion of this chapter, you will be able to
■ Perform basic network testing

■ Use the ping command to perform basic network connectivity tests
■ Use the telnet command to verify the application layer software between source
and destination hosts
■ Troubleshoot by testing OSI layers
■ Use the show interfaces command to confirm Layer 1 and Layer 2 problems
■ Use the show ip route and show ip protocol command to identify routing issues
■ Use the show cdp command to verify Layer 2 connectivity
■ Use the traceroute command to identify the path that a packet takes between
networks
■ Use the show controller serial command to ensure that the proper cable is
attached
■ Use basic debug commands to show router activity
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Chapter 18
Basic Router Troubleshooting
This chapter provides an overview of network testing and emphasizes the necessity of
using a structured approach to troubleshooting. This chapter also describes the funda-
mentals of troubleshooting routers.
Please be sure to look at this chapter’s associated e-Labs, movies, 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.
Introduction to Network Testing
Basic testing of a network should proceed in sequence from one OSI reference model
layer to the next, as shown in Figure 18-1.
Figure 18-1 Testing Utilities and the OSI Reference Model
It is best to begin with Layer 1 and work to Layer 7, if necessary. Beginning with Layer 1,
look for simple problems such as power cords unplugged at the wall.
Application
Presentation
Session

Transport
Network
Data Link
Physical
telnet
ping
trace
show ip route
show interfaces
7
6
5
4
3
2
1
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788 Chapter 18: Basic Router Troubleshooting
Layer 2 issues can include improperly configured serial or Ethernet interfaces, improper
clock rate settings on serial interfaces, or network interface card (NIC) problems.
The most common problems that occur on IP networks result from errors in the
addressing scheme, which are Layer 3 issues.
The various problems that can occur at all the layers of the OSI model are covered in
detail later in this chapter. It is important to test the address configuration before con-
tinuing with further configuration steps.
Each test presented in this section focuses on network operations at a specific layer of
the OSI model.
Structured Approach to Troubleshooting
Troubleshooting is a process that helps a user to find problems on a network. An orderly
process to troubleshooting should be used, based on the networking standards set in

place by an administration. Documentation is a very important part of the trouble-
shooting process. The flowchart in Figure 18-2 shows a recommended logical sequence
for troubleshooting network problems.
Figure 18-2 Troubleshooting Methodology

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