Basic Router Trouble Shooting

7/25/2019 Basic Router Trouble Shooting

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1Version 3.1

Module 9Basic Router

Troubleshooting


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2Version 3.1

The Routing Table and show ip route

Command

Routers build routing tables to:

list available routes for forwarding data

select the best path to a destination

show ip routedisplays routing table information aboutall known networks and subnetworks


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3Version 3.1

Example Routing Table

When RT recei!es a pac"et destined #or 19$.1%&.'.'%(

it loo"s #or the pre#ix 19$.1%&.'.)*$' in its table.

RT then #or+ards the pac"et out an inter#ace,Ethernet)- based on the routing table entr.

/# RT recei!es a pac"et destined #or 1).3.$1.0( it sends

that pac"et out erial )


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4Version 3.1

Example Routing Table

' routes 2 directl connected

The router +ill drop an pac"et destined #or a net+or"

not listed in the routing table

o +hat do ou do +ith those pac"ets that this router

doesnt ha!e a route to4


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5Version 3.1

dding Routes

tatic routing

Manuall entered b the administrator5

more secure

lo+ processor o!erhead ha!e precise control o# path selection

6namic routing

router learns route #rom other routers( dnamicall


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6Version 3.1

7ate+a o# 8ast Resort

,6e#ault tatic Route-

T+o +as to staticall con#igure de#ault

routes5

rom global con#iguration mode

ip route 0.0.0.0 0.0.0.0 [next-hop

interface|exit-interface]

rom router con#iguration mode

ip default network [network number]


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ip default-network


8/238Version 3.1

ip route


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Version 3.1

Routing :rocesses

Remember that the /: source and destination do notchange in the pac"et( but the MC addresses do

change.

The pac"et is #or+arded based on the MC address o#

the next hop.

8aer 3 address is al+as used in routing( but the

MC address has to change because the data is

#ramed as it mo!es out on the data lin" laer.

ource to destination ;;; +e are al+as tal"ing about

net+or" addressing


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Route Metrics

Routing protocols use metrics to determine the best

route to a destination.

The metric is a !alue that measures the desirabilit o#

a route. The smaller the metric number( the better the path.


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M

E

T

R

I

C

S


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Version 3.1

Route Metrics

The more #actors that ma"e up a metric( the greater the#lexibilit to tailor net+or" operations to meet speci#icneeds.

B de#ault( /7R: uses the static #actors band+idth and

dela to calculate a metric !alue ,so band+idth and delacan actuall be entered b the administrator-.

/7R: ma also be con#igured to include the dnamic#actors( load and reliabilit( in the metric calculation.

Remember( /7R: uses the metric

Metric < band+idth = dela

Be sure to do the Interactive Media Activity in 9.1.6


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Version 3.1

Troubleshooting


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Version 3.1

Testing b >/ 8aers

8aer 1 2 chec" ob!ious #irst ,lights on a ?/C-

Bro"en cables

6isconnected cables

Cables connected to the +rong ports

/ntermittent cable connection

Wrong cables used #or the tas" at hand ,must userollo!ers( crosso!er cables( and straight;throughcables correctl-

Transcei!er problems 6CE cable problems

6TE cable problems

6e!ices turned o##


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15Version 3.1

Testing b >/ 8aers

8aer $ errors5

/mproperl con#igured serial inter#aces

/mproperl con#igured Ethernet inter#aces

/mproper encapsulation set ,@68C is de#ault #orserial inter#aces-

/mproper cloc"rate settings on serial inter#aces

?et+or" inter#ace card ,?/C- problems


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16Version 3.1

Testing b >/ 8aers

8aer 3 errors5

Routing protocol not enabled

Wrong routing protocol enabled

/ncorrect /: addresses /ncorrect subnet mas"s


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17Version 3.1

pingAtilit ,8aer 3 testing-

Mabe the host isnt a!ailable


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18Version 3.1

Telnet application ,8aer testing-

Actually verifies all seven layers of the OSI

If telnet is not working then that means that there are

upper!layer application layer problems"


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19Version 3.1

show interfaces serial

#sed to discover $ayer % & $ayer ' problems with the

router

(hen the line is down the protocol is always down

because there is no useable media for the $ayer ' protocol"If the interface is up and the line protocol is down a $ayer '

problem e)ists *no keepalives no clock rate encap different+

,ou would never see serial -.- is down line protocol is up


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20Version 3.1


Interpreting theShow Interfaces SerialCommand

One of the most important elements of theshow

interfaces serialcommand output is the display of the

line and data link protocol status.

The line status is triggered by a Carrier Detect signal, and

refers to the physical layer status. However, the line protocol,

triggered bykeepaliveframes, refers to the data link framing.


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21Version 3.1


no+ all

o# theseD


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22Version 3.1

show ip protocols

/he show ip protocolscommand shows the following:

Sources of routing updates

/he networks being advertised

#pdate timer values 0)" If a network is not being included in the routing

updates and should be an administrator could issue

this command to see which networks are being

advertised


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23

debug

1e 2areful333

4ebug will give you live changing information"

/herefore it will use processor time that may disrupt

normal router operation

0specially should not use 5debug all6 but be more

specific like

debug ip rip *only live info on rip routing ip+

/o turn off debug use undebug all or u all

Documents

Basic Router Trouble Shooting