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Chapter 8

TCP/IP Suite Error and Control Messages/ Basic Router Troubleshooting

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Objectives

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Internet Control Message Protocol (ICMP)

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Error Reporting and Error Correction

• ICMP is an error-reporting protocol for IP.

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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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Unreachable Networks

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Ping

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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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ICMP Message Types

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Destination Unreachable Message

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Code Values for Destination Unreachable Messages

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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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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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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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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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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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Router Solicitation Message

• A host generates an ICMP router solicitation message in response to a missing default gateway.

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Congestion and Flow Control Messages

ICMP source-quench messages are used to reduce the amount of data lost from congestion.

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Summary

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Basic Router Troubleshooting

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Objectives

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The show ip route Command

The show ip route command displays the contents of the IP routing table.

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Static Routing

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Dynamic Routing

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Default Route

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Configuring a Default Route

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Determining Route Source and Destination

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Determining L2 and L3 Addresses

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Determining Administrative Distance

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Determining the Route Metric

Routing protocols use metrics to determine the best route to a destination.

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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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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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Observing Multiple Paths to a Destination

• Some routing protocols support multiple paths to the same destination.

• Unlike single path algorithms, these multi-path algorithms permit traffic over multiple lines, provide better throughput, and are more reliable.

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Introduction to Network Testing

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Structured Approach to Troubleshooting

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• 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

Typical Layer 1 Errors

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• Improperly configured serial interfaces

• Improperly configured Ethernet interfaces

• Improper encapsulation set • Improper clock rate settings

on serial interfaces • Network interface card (NIC)

problems

Typical Layer 2 Errors

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• Routing protocol not enabled • Wrong routing protocol

enabled • Incorrect IP addresses • Incorrect subnet masks

Typical Layer 3 Errors

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Layer 1 Problems in a Network

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Layer 3 Troubleshooting Using Ping

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Layer 7 Troubleshooting Using Telnet

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Troubleshooting Layer 1 Using show interfaces Command

The show interfaces serial command

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Troubleshooting Layer 2 Using show interfaces Command

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Troubleshooting Using show cdp neighbors Command

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Troubleshooting Using show cdp neighbors detail Command

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Troubleshooting Using traceroute Command

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Troubleshooting Routing Issues

The show ip route Command

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Troubleshooting Routing Issues

The show ip protocols Command

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Troubleshooting Using show controllers serial Command

The show controllers serial Command

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Introduction to debug

Debug syntax

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Summary

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Question/Answer