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CCNA 2 v3.1 Module 9 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

This command does not work with IGRP

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Determining Route Source and Destination

The network layer provides best-effort, end-to-end, packet delivery across interconnected networks. The network layer uses the IP routing table to send packets from the source network to the destination network.

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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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IGRP Metric

• Metric=K1 *Bandwidth + (K2 * Bandwidth)/256-load) + K3 * Delay

• The default constant values are K1=K3=1 and K2=K4=K5=0 so:

• Metric=Bandwidth + Delay

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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

The show interfaces command is perhaps the single most important tool to discover Layer 1 and Layer 2 problems with the router. The first parameter (line) refers to the physical layer. The second parameter (protocol) indicates whether the IOS processes that control the line protocol consider the interface usable.

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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

Warning: The debug all command should be used sparingly as this can disrupt router operations.

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Summary