Chapter 8
TCP/IP Suite Error and
Control Messages/ Basic
Router Troubleshooting
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1
Objectives
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2
Internet Control Message Protocol
(ICMP)
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3
Error Reporting and Error Correction
• ICMP is an error-reporting protocol for IP.
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4
ICMP Message Delivery
• ICMP messages are encapsulated as data
in datagrams in the same way any other
data is delivered using IP.
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5
Unreachable Networks
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6
Ping
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7
Detecting Excessively Long Routes
• Whether the actual path includes too many hops
or a circular routing path exists, the packet will
eventually reach the end of its life, known as its
time-to-live (TTL).
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8
ICMP Message Types
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9
Destination Unreachable Message
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10
Code Values for Destination
Unreachable Messages
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11
TCP/IP Suite Control Messages
• Control messages are used to inform
hosts of conditions such as network
congestion or the existence of a better
gateway to a remote network.
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12
ICMP Redirect/Change Requests
The default gateway
of the host may
need to use a
redirect/change
request to inform
the host of the best
path to a certain
network.
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13
Clock Synchronization and Transit Time
Estimation
• The ICMP timestamp request message
allows a host to ask for the current time
according to the remote host.
• The remote host uses an ICMP timestamp
reply message to respond to the request.
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14
Address Mask Requirements
• When a network administrator uses the
process of subnetting to divide a major IP
address into multiple subnets, a new
subnet mask is created.
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15
Router Discovery Message
• When a host on the network boots, and
the host has not been manually
configured with a default gateway, it can
learn of available routers through the
process of router discovery.
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16
Router Solicitation Message
• A host generates an ICMP router
solicitation message in response to a
missing default gateway.
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17
Congestion and Flow Control
Messages
ICMP source-quench messages are
used to reduce the amount of data lost
from congestion.
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18
Summary
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19
Basic Router Troubleshooting
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20
Objectives
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21
The show ip route Command
The show ip route command displays the
contents of the IP routing table.
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22
Static Routing
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23
Dynamic Routing
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24
Default Route
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25
Configuring a Default Route
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26
Determining Route Source and
Destination
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27
Determining L2 and L3 Addresses
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28
Determining Administrative Distance
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29
Determining the Route Metric
Routing protocols use metrics to
determine the best route to a destination.
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30
Determining the Route Next Hop
• Destination next hop associations
determine the best path and which router
to forward the packet to next. This router
represents the next hop on the way to the
final destination.
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31
Determining the Last Routing Update
• Use the following commands to find the
last routing update:
show ip route
show ip route network
show ip protocols
show ip rip database
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32
Observing Multiple Paths to a
Destination
• Some routing protocols support multiple
paths to the same destination.
• Unlike single path algorithms, these multipath algorithms permit traffic over multiple
lines, provide better throughput, and are
more reliable.
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33
Introduction to Network Testing
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34
Structured Approach to
Troubleshooting
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35
Typical Layer 1 Errors
• Broken cables
• Disconnected cables
• Cables connected to the
wrong ports
• Intermittent cable connection
• Wrong cables used for the
task at hand
• Transceiver problems
• DCE cable problems
• DTE cable problems
• Devices turned off
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36
Typical Layer 2 Errors
• Improperly configured serial
interfaces
• Improperly configured
Ethernet interfaces
• Improper encapsulation set
• Improper clock rate settings
on serial interfaces
• Network interface card (NIC)
problems
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37
Typical Layer 3 Errors
• Routing protocol not enabled
• Wrong routing protocol
enabled
• Incorrect IP addresses
• Incorrect subnet masks
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38
Layer 1 Problems in a Network
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39
Layer 3 Troubleshooting
Using Ping
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40
Layer 7 Troubleshooting
Using Telnet
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41
Troubleshooting Layer 1
Using show interfaces Command
The show interfaces serial command
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42
Troubleshooting Layer 2
Using show interfaces Command
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43
Troubleshooting Using show cdp
neighbors Command
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44
Troubleshooting Using show cdp
neighbors detail Command
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45
Troubleshooting Using traceroute
Command
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46
Troubleshooting Routing Issues
The show ip route Command
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47
Troubleshooting Routing Issues
The show ip protocols Command
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48
Troubleshooting Using show
controllers serial Command
The show controllers serial Command
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49
Introduction to debug
Debug syntax
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50
Summary
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51
Question/Answer
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52