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Basic Router Troubleshooting

Hetal Presswala

August 09, 2008


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Objectives

One of the primary functions of a router is to determine thebest path to a given destination.

A router learns paths, also called routes, from anadministrator's configuration or from other routers by way ofrouting protocols

This module will describe methods for examining andinterpreting the contents of the routing table

Network testing and troubleshooting are perhaps the most timeconsuming components of every network administrators job.

Efficient testing and troubleshooting must be done in a logical,orderly, and well-documented fashion

This module will introduce several of the most important ofthese tools and provide practice in their use.


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The show ip route Command

The show ip route command displays the contentsof the IP routing table.


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Dublin# show ip route

Codes, howthe routes

were learnt

Networks beingadvertised

Administrative

Distance

Metric

NextHop

Timesince

lastupdate

Out

interface

Examining the Routing Table


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Static Vs Dynamic Routes


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Static Vs Dynamic Cont


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How to Configure Default Route

Alternatively use the default network command

Router(config)# ip default-network 172.16.1.2



Determining Route Source and Destination



Routing Role of Layer 2 and Layer 3 Addressing

Hop Source MAC Destination MAC Source IP Destination IP

1

2

3

192.168.1.5 192.168.3.10 00:ac:2d:34:56:11 00:01:46:26:23:a4

00:01:46:26:23:a4 00:04:24:98:55:44

00:04:24:98:55:4400:2a:3b:3c:a4:33

192.168.1.5 192.168.3.10

192.168.1.5 192.168.3.10



Determining Administrative Distance

AD measures the trustworthiness of the source of the route

information.

The lower the AD, the more trustworthy the source.

Different routing protocols have different default ADs.

When there are two paths available to a destination the path with the

lowest AD is usedin the routing table.



Determining the Route Metric

Routing algorithms generate a number, called the metric value, for each

path through the network .

The metric value is used to determine the best route to a destination.

The various protocols base their metrics on different factors:



Viewing the Routing Table

Use the following commands to find the lastrouting update:

show ip route

show ip route network show ip route connected show ip route static

show ip protocols show ip rip database



Show ip route Command



Introduction to Network Testing



Structured Approach to Troubleshooting



Broken cables Disconnected cables

Cables connected to the wrongports 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



Layer 1 Troubleshooting using show interfaces

BHM#show interface s0

Serial0 is up, line protocol is up

< output omitted >

Last clearing of "show interface" counters never< output omitted >

Received 73 broadcasts, 0 runts, 0 giants, 0 throttles

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0

abort

135 packets output, 7361 bytes, 0 underruns

0 output errors, 0 collisions, 37 interface resets0 output buffer failures, 0 output buffers swapped out

54 carrier transitions

Physical layer (linestatus)Up: has Layer 1connectivityDown: L1 problem

Administrativelydown: disabled

Faultyhardware,

cable ornoise

Bad line,badhardware

To reset: BHM# clearcounters

Lineinterruption

s, faultyhardware



Layer 1 Troubleshooting using show interfacesThe show interfaces serial command



Layer 1 Troubleshooting show controllers

show controllers serial 0

HD unit 0, idb = 0x1086D4, driver structure at 0x10E568

buffer size 1524 HD unit 0, V.35 DCE cable, clockrate 56000

Router#



Troubleshooting Using show controllers serialThe show controllers serial Command



Improperly configured serialinterfaces

Improperly configured Ethernetinterfaces

Improper encapsulation set Improper clock rate settings on

serial interfaces

Network interface card (NIC)problems

Typical Layer 2 Errors



Troubleshooting Using show cdp neighbors



Troubleshooting Using show cdp neighborstail


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Layer 2 Troubleshooting show interfaces

If the interface is up and the line protocol is down, a Layer 2 problem exists.

Among the possible causes are:

No keepalives ,No clock rate ,Mismatch in encapsulation type

BHM# show interface serial 0

Serial0 is up, line protocol is up

Hardware is HD64570

Internet address is 172.17.0.2/16

MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 1/25Encapsulation HDLC, loopback not set, keepalive set (10 sec)

Last input 00:00:01, output 00:00:00, output hang never

Last clearing of "show interface" counters never

< output omitted >

Data-Link layer (up ordown)Are keepalives beingreceived?

Encapsulation mustmatch at both ends of

connection


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Layer 2 Troubleshooting

If the interface is up and the line protocol is down, aLayer 2 problem exists.

Indicates whether the IOS processes consider the

interface usable.

Keepalives are defined as messages sent by one network

device to inform another network device that the virtual

circuit between the two is still active

If the interface misses three consecutive keepalives, line

protocol is down

When the line is down, the protocol is always down


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Show interfaces Command


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Routing protocol not enabled

Wrong routing protocol enabled

Incorrect IP addresses

Incorrect subnet masks

Typical Layer 3 Problem


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Layer 3 Troubleshooting Using Ping


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Ping

ping ScopeDelhiType escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 192.168.1.2, timeout is 2

seconds:

..!!!

Success rate is 60 percent (3/5),

round-trip min/avg/max = 32/33/36 ms

! indicate asuccessful echo

. indicate theapplication timed outwaiting for an echo

vista#

Extended ping

provides extra facilities for ping


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Troubleshooting Using traceroute Command

Test on a hop-by-hop basis Each time is the RTT


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Troubleshooting Routing IssuesThe show ip route Command


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Troubleshooting Routing IssuesThe show ip protocols Command


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Layer 7 Troubleshooting Using Telnet


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