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 route
Trang 1However, 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
Type 9
Type 0
ICMP message starting with the ICMP type For computing the checksum, the checksum field should be 0
address (2 in the version of protocol described here)
are considered valid
The number of the address interface from which this mes-sage is sent
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
Type (9) Code (0) Checksum
Lifetime Router Address 1
Preference Level 1 Router Address 2 Preference Level 2
Number of Addresses
Address Entry Size
Trang 2780 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
Type 10
ICMP message starting with the ICMP type For computing the checksum, the checksum field should be 0
Type (10) Code (0) Checksum
Reserved
Trang 3ICMP 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:
that the data did not reach its destination
to test IP connectivity to aid in the troubleshooting process
unreliable
Type field and a Code field
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
Trang 4782 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.”
other data is delivered using IP True or false?
Trang 54. A default gateway must be configured if datagrams are to travel outside the local
network True or false?
following conditions are met?
on which the packet gets routed out
the next-hop IP address of the routed packet
request) or what?
Trang 6784 Chapter 17: TCP/IP Error and Control Messages
transit timestamps True or false?
A. 9
Trang 8Upon completion of this chapter, you will be able to
and destination hosts
networks
attached
Trang 9Chapter 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
Trang 10788 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
Start (Problem)
1 Collect all available information and analyze the symptoms of failure.
2 Localize the problem to within a single network segment,
to a single complete module or unit,or to a single user.
3 Isolate the trouble to specific hardware or software within the unit,module,or userÕs network account.
4 Locate and correct the specific problem.
5 Verify that the problem has been solved.
Other Problems?
Stop Yes
No