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CCNA Semester 2

Chapter 09Chap

ter 09

BASICS ROUTERBASICS ROUTERTROUBLESHOOTINGTROUBLESHOOTING

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ObjectivesObjectives

Use the show ip route command to gather detailed informationabout the routes installed on the router

Configure a default route or default network

Understand how a router uses both Layer 2 and Layer 3 addressingto move data through the network

Use the ping command to perform basic network connectivity tests

Use the telnet command to verify the application layer softwarebetween source and destination stations

Troubleshoot by sequential testing of OSI layers

Use the show interfaces command to confirm Layer 1 and Layer 2problems

Use the show ip route and show ip protocol commands toidentify routing issues

Use the show cdp command to verify Layer 2 connectivity

Use the traceroute command to identify the path packets takebetween networks

Use the show controllers serial command to ensure the propercable is attached

Use basic debug commands to monitor router activity

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Table of ContentTable of Content

1 Examining the Routing Table2 Network Testing

3 Troubleshooting Router Issues

Overview

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EXAMINING THE ROUTING TABLEEXAMINING THE ROUTING TABLE

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Initial IP routing tableInitial IP routing table

Initial routes are directly connected

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IP routing learning destinationIP routing learning destination

Static routes:Manually defined by the system

administrator as the next hop to a destination;

useful for security and traffic reduction. Default routes:Manually defined by the

administrator as the path to take when there is

no known route to the destination.

Dynamic routing:The router learns of paths to

destinations by receiving periodic updates from

other routers.

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The show ip route commandThe show ip route command

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Default route:Default route: Determining the gateway of last resortDetermining the gateway of last resort

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How a default route is usedHow a default route is used

Can use either of the following commands

Ip default-networknetworkor

ip route 0.0.0.0 0.0.0.0

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Using the IP DEFAULT-NETWORKcommand

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Using theUsing the IP ROUTE 0.0.0.0 0.0.0.0

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Determining the last routing updateDetermining the last routing update

show ip route 153.50.0.0

show ip route

show ip protocols

show ip ripdatabase

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Observing multiple paths to destinationObserving multiple paths to destination

Router# debug ip packet

Ping 183.8.0.130

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Observing multiple paths to destinationObserving multiple paths to destination

Router# debug ip packet

Ping 183.8.0.130

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Determining route source and destinationDetermining route source and destination

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Determining L2 and L3 addressesDetermining L2 and L3 addresses

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Determining the route administrative distanceDetermining the route administrative distance

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Determining the route metricDetermining the route metric

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Determining the route next hopDetermining the route next hop

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NETWORK TESTINGNETWORK TESTING

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Using a structured approach to troubleshootingUsing a structured approach to troubleshooting

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Five steps troubleshootingFive steps troubleshooting

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Typical layer 1 errorsTypical layer 1 errors

Broken cables Disconnected cables

Cables connected to the wrong ports

Intermittent cable connection

Wrong cables used

Transceiver problems DCE cable problems

DTE cable problems

Devices turned off

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Typical layer 2 errorsTypical layer 2 errors

Improperly configured serial interfaces

Improperly configured Ethernetinterfaces

Improper encapsulation set (HDLC isdefault for serial interfaces)

Improper clock-rate settings on serialinterfaces

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Typical layer 3 errorsTypical layer 3 errors

Routing protocol not enabled

Wrong routing protocol enabled

Incorrect IP addresses

Incorrect Subnet Masks Incorrect DNS to IP bindings

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Layer 3 troubleshooting using pingLayer 3 troubleshooting using ping

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Ping command outputPing command output

Sends ICMP echo packets.

These are the results that may be returnedby the ping test: ! successful receipt of an echo reply

. timed out waiting for datagram reply

U destination unreachable error

C congestion-experienced packet I ping interrupted (e.g. Ctrl-Shift-6 X)

? packet type unknown

& packet TTL exceeded

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ICMP MessagesICMP Messages

Carried in IP datagrams and are used tosend error and control messages.

Destination unreachable

Time exceeded Parameter problem

Source quench

Redirect Echo

Echo reply

..v..v..

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Layer 7 troubleshooting using TelnetLayer 7 troubleshooting using Telnet

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Telnet operationTelnet operation

LAB_A

LAB_B

Telnet LAB_B

Connect LAB_B

Ctrl-Shift-6 x

Enter

1

Exit

Disconnect

Show sessions

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TROUBLESHOOTING ROUTERTROUBLESHOOTING ROUTER

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Troubleshooting Layer 1 using show interfacesTroubleshooting Layer 1 using show interfaces

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Show interfaces serial S0/0Show interfaces serial S0/0

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Layer 1 problems could be a causeLayer 1 problems could be a cause

An increasing number ofcarrier transitionscounts: Line interruptions due to problems in the service

provider network.

Faulty switch, DSU, or router hardware. An increasing number ofinput errors:

Faulty telephone company equipment

Noisy serial line

Incorrect cable or cable length

Damaged cable or connection

Defective CSU or DSU

Defective router hardware

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Layer 1 problems could be a causeLayer 1 problems could be a cause

An increasing number of interface resets.These are the result of too many missedkeepalives:

Bad line causing carrier transitions Possible hardware problem at the CSU, DSU, or

switch

Ifcarrier transitions and interface resets

are increasing or ifinput errors are highwhile interface resets are increasing, theproblem is likely to be a bad link or defectiveCSU or DSU.

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Last clearing of the countersLast clearing of the counters

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Troubleshooting Layer 2 using show interfacesTroubleshooting Layer 2 using show interfaces

No keepalives

No clock rate

Mismatch in encapsulation

type

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Troubleshooting using show cdp neighborTroubleshooting using show cdp neighbor

LAB_A#Show cdp neighbor

Capability Codes: R - Router, T - Trans Bridge, B - Source Route

Bridge, S - Switch, H - Host, I - IGMP, r - Repeater

Device ID Local Intrfce Holdtme Capability PlatformPort ID

SCA0425051J Fas 0/24 158 T S WS-C6509

3/20Switch Fas 0/23 147 T S WS-C2924-XFas

0/24

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Troubleshooting using show cdp detailsTroubleshooting using show cdp details

Device ID: Switch

Entry address(es):IP address: 192.168.4.254

Platform: WS-C6509, Capabilities: Trans-Bridge Switch

Interface: FastEthernet0/24, Port ID (outgoing port): 3/20

Holdtime : 138 sec

Version :WS-C6509 Software, Version McpSW: 5.4(2) NmpSW: 5.4(2)

Copyright (c) 1995-2000 by Cisco Systems

advertisement version: 2

VTP Management Domain: 'SSP'

Native VLAN: 20Duplex: full

-------------------------

Device ID: Switch

Entry address(es):

Platform: cisco WS-C2924-XL, Capabilities: Trans-Bridge Switch

Interface: FastEthernet0/23, Port ID (outgoing port): FastEthernet0/24Holdtime : 127 sec

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Troubleshooting using tracerouteTroubleshooting using traceroute

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Traceroute operationTraceroute operation

LAB_A

LAB_B

TTL = 1

Time Exceed

LAB_C

TTL = 2

Time Exceed

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Traceroute command outputTraceroute command output

Use TTL messages.

Represent the time required for each of threeprobes to return.

These are the results: !H The probe was received by the router,

but not forwarded, usually due to anaccess list.

P The protocol was unreachable. N The network was unreachable.

U The port was unreachable.

* Time out.

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SHOW IP ROUTE command

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SHOW IP PROTOCOLS command

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Troubleshooting using show controllers serialTroubleshooting using show controllers serial

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Debug CommandsDebug Commands

debug ? undebug all

no debug all

terminal monitor

service timestamps debug uptime

show debug

Never:

debug all

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DEBUG IP RIP command

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SummarySummary

The show ip route command

Determining the gateway of last resort Determining the route source and destination address

Determining route administrative distance

Determining route metric

Determining route next hop

Determining last route update

Observing multiple paths to destination Using a structured approach to troubleshooting

Testing by OSI layers

Layer 1 troubleshooting using indicators

Layer 3 troubleshooting using ping

Layer 7 troubleshooting using Telnet

Troubleshooting Layer 1 using show interfaces Troubleshooting Layer 2 using show interfaces

Troubleshooting using show cdp

Troubleshooting using traceroute

Troubleshooting routing issues using show ip route and show ipprotocols

Troubleshooting using show controllers serial Troubleshooting using debug commands

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Q&AQ&A