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Appendix F. Answers to Review Questions

Appendix F. Answers to Review Questions

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Chapter1

1: Whatisthefirstthingyoushouldthinkaboutwhendesigninganewnetwork?



A1:



Answer:Thinkaboutthepurposeofthenetwork.Whatsortsoftrafficwillthe

networkcarry,andwhattypesofapplicationswillberunningonthenetwork?



2: Whataretheprimarygoalsinanynetworkdesign?



A2:



Answer:Theprimarygoalsarereliabilityandresilience,manageability,and

scalability.



3: Howdoyoudefineareliablenetwork?



A3:



4:



Answer:Areliablenetworkdeliversalmostallacceptedtraffictotheright

destinationwithinareasonableamountoftime.

WhatelementsinLayer3networkdesignimpactthedelayandjitterbudgets

throughthenetwork?



Answer:Thetopologyofthenetworkdeterminesthenumberofhopsanddelays

acrosseachhopthroughthenetwork.Routingprotocolsimpactthepaththat

traffictakesthroughthenetwork,impactingthedelayandjitter(traffic

engineering).ThereliabilityofthenetworkatLayer3impactsthedelayandjitter

A4: throughthenetwork;aconstantlyconvergingnetworkhasconstantlychanging

delayandjitter.

AlthoughthisbookfocusesonLayer3androuting,specifically,itisimportantto

rememberthatLayer2designhasamajorimpactonnetworkresiliencyand

reliability.

Whatarethetwoprimaryconsiderationswhendeterminingwhenanetworkhas



5: failed?



A5:



Answer:Arealltheapplicationsthatarerequiredforthebusinesstorunstill

available?Ifnot,whatisthecostofanapplicationbeingunusable?



Whatelementsareimportantindeterminingthetimerequiredtorestorea

6: networktoservice(oranapplicationrunningonanetworktoservice)afterithas

failed?

Answer:Thetwoelementsincludetheamountoftimethatittakesforthe

A6: networktodetectandrecoverfromafailure,andtheamountoftimerequiredto

troubleshootandtakeanyactionsnecessarytocorrectthefailure.

7: Whatareimportantday-to-daymanagementtasksinadministeringanetwork?



A7:



8:



A8:



9:



A9:



Answer:Thetwoimportantmanagementtasksaretakinganetworkbaselineand

documentingthenetwork.

Whattypesofdocumentationareimportantfornetworkmanagementand

troubleshooting?

Answer:TheimportantdocumentationincludesLayer2andLayer3network

diagrams,addressing,andsummarizationplans.

Whattwomethodscanyouapplytodeterminewhereredundancyisneededina

networktoincreaseresiliency?

Answer:Youcananalyzepointsoffailure(single,double,andsoon)and

performstatisticalanalysis.



10: WhataretheMTBFandMTTR?



Answer:Themeantimebetweenfailure(MTBF)istheamountoftimethata

systemcanbeexpectedtorunbeforeitfails.Themeantimetorepair(MTTR)is

A10:

theamountoftime,onaverage,requiredtorestoreasystemtooperationafterit

hasfailed.



11: Whattwotypesofinformationaretypicallyhiddeninnetworks?



A11:



Answer:Topologyinformationandreachabilityinformation(orreachability

detail)aretypicallyhiddeninnetworks.



Chapter2

1: Whatisoneofthekeyconceptsrequiredtobuildalarge-scalenetwork?

Answer:Oneofthekeyconceptsislayeringthroughabstraction.Itinvolves

breakingthenetworkintologicalverticallayersatthenetwork(IP)levelsothat

A1:

youcanbreakthenetworkintozones,givingyouedgesalongwhichtohide

information.

Whatfourtypesoftrafficareyouconcernedwithwhenconsideringhierarchical

2: networklayers?Whyisitimportanttoplaninterzoneandinterlayerconnectivity

carefullyinanetwork?

Answer:Thefourtypesoftrafficfoundinalayerednetworkdesignare

interlayer,intralayer,interzone,andintrazone.Youmustplaninterlayerand

interzoneconnectivitycarefullytokeepthenetworkcomplexitydown.More

A2:

interlayerconnectivitymustbeplannedcarefullytocontrolthecomplexityofthe

network.Asthesetypesofconnectivityincrease,thenetworkcomplexityalso

increases,makingitmoredifficulttoplantrafficflowandmanagethenetwork.



3:



Whatisthegeneralruleofthumbincommunicationsnetworksfortrafficthatis

destinedtolocalandremotedestinations?Isthisrulealwaystrue?



Answer:Generally,80percentofthetrafficisdirectedtolocaldestinations,

whereas20percentisdirectedtoremotedestinations.Thisruleisnotalways

A3:

true,however.Itdependsonthemostcommonlyusedresources,thespeedand

costofhigh-speedlinks,andotherfactors.

4: Whatisthepointoftrafficaggregationinanetworkdesign?

Answer:Trafficaggregationinanetworkdesigntakesadvantageofthe

A4: topologicallocalityoftrafficandprovidesedgesalongwhichtrafficcanleaveor

enterazone.



5:



Whatismeantbyroutingpolicy?Whattypesofpolicyareincludedinthis

concept?



Answer:Routingpolicyisthecontrolsthatareplacedontheroutinginformation

A5: asitispropagatedthroughthenetwork.Informationhidingbothtopologicaland

reachabilityandtrafficengineeringareincludedinroutingpolicy.

6: Whataretheprimaryfunctionsofeachlayerinatwo-layerhierarchy?

Answer:Thecorelayerforwardsinterlayerandinterzonetraffic,whereasthe

A6: aggregationlayeraggregatestraffic,providesuserattachment,andcontrolstraffic

admittance.Routingpolicyisimplementedalongthecore/aggregationedge.

7: Whataretheprimaryfunctionsofeachlayerinathree-layerhierarchy?

Answer:Thecoreanddistributionlayersaggregatetrafficandprovideinterzone

trafficforwarding,althoughthecoreistheprimaryhigh-speedinterconnection

betweentheroutingzones.Theaccesslayerprovidesuserattachment,andthe

A7:

accessanddistributionlayersprovidetrafficadmittance.Routepolicyis

primarilyimplementedinthedistributionlayer,althoughtrafficengineeringcan

occurinanylayer.



8:



Whataretheprimaryconsiderationswhendecidinghowmanylayersanetwork

shouldhave?



Answer:Theprimaryconsiderationsfordeterminingthenumberofnetwork

layersincludenetworkgeography(howlargeofanareathenetworkcovers),

A8: networktopologydepth(thenumberofhopsbetweentheedgeandthecore),

networktopologydesign(meshinessandcomplexityofthenetworkdesign),and

policyimplementation.

9: Explaintheconceptofhidinglayerswithinlayers.



Answer:Ratherthancreatingasinglesetoflayerswithinanetwork,youcan

divideanetworkintomultipleadministrativedomains,eachwithitsownlayered

A9: structure.Thus,asingleaggregationlayerzonewithinanetworkmightactually

containitsowncoreandaggregationlayers.



10: WhatisthebestmethodforassigningIPaddresseswithinahierarchicalnetwork?

Answer:AssigningIPaddressesbasedonthelogicaltopologyisthebest

A10: possiblemethod,althoughbitscanbe"stolen"fromtheIPaddressspaceto

indicateotherinformation,suchasthedivisionorphysicallocationofanaddress.



11:



Whataresometechniquesyoucanusetoworkaroundpooraddressingschemes

ormisplacedaddresses?



Answer:Toworkaroundpooraddressingschemesormisplacedaddresses,you

couldleakmorespecificsandsmallersummaryblocks.Essentially,youcanuse

A11:

"thelongestprefixmatchalwayswins"ruletosummarizeasmuchaspossible,

allowingmorespecificinformationtoleakthroughtoensureproperrouting.

12: Whydoesasummarizationblackholeoccur,andhowcanyousolveit?

Answer:Asummarizationblackholeoccurswhentworoutersthatare

advertisingthesamesummarycannotreachthesamedestinationswithinthose

A12: summaries.Thesimplestwaytoresolvethisproblemistomakecertainthatthe

tworoutesshareunsummarizedroutinginformationacrosssomelinkbetween

them.

13: WhatarethealternativestoIGP-to-IGPredistribution?

Answer:ThealternativestoIGP-to-IGPredistributionincludeusingstaticroutes

A13: toinjectroutinginformationattheedgeofthenetworkorcarryingtheexternal

routinginformationinanexternalprotocol,suchasBGP.



14: Whattwotechniquescanhelpyouovercomepossiblepermanentroutingloops

whenusingmultiplepointsofredistribution?

Answer:Youcanfilterroutesbasedonyourknowledgeofwhichrouteshave

originatedinwhichroutingdomain,oryoucanautomaticallytagroutesasthey

A14:

leaveoneroutingdomainandenteranotherandthenfilterthembasedonthese

tags.



Chapter3

1:



Whatarethetwobasictoolsyoucanusetosummarizeroutes(orhidedestination

details)inEIGRP?



A1: Answer:Thetworoutesaresummarizationanddistributionlists.

2: Howcanyoutellthatarouteisasummarywhenyoulookattheroutingtable?

A2: Answer:Itismarkedasasummaryandthenext-hopinterfaceisnull0.



3:



Whatisthedefaultadministrativedistanceforasummaryroute?Whatisthe

problemwiththis?



Answer:Alocalsummaryroutehasadefaultadministrativedistanceof5and

A3: candisplacevalidroutesthatarelearnedfromotherrouters,anditcancausea

routertothrowpacketsawayunintentionally.

4: Whatboundsaquery?



A4:



Answer:Distributionlistsandsummarizationboundaquery,becausetheylimit

knowledgeofspecificdestinations.



5: Howfarbeyondoneofthepossiblequeryboundsdoesaquerytravel?



A5:



Answer:Aquerygenerallytravelsonehop,oruntilarouterthatdoesnothave

informationaboutthatspecificdestinationreceivesthequery.



6:



Whatistheprimaryadvantagetosummarizingbetweencoreroutersratherthan

betweenthedistributionlayerandcore?



A6:



7:



Answer:Thecoreroutershasenoughinformationtomakeoptimalrouting

decisions.

Howisitpossibleto"blackhole"packetswhensummarizingdestinationsbehind

dual-homedremotesintothecore?



Answer:Evenifoneofthedistributionrouterslosesconnectivitywithoneofthe

A7: remotes,itstilladvertisedasummarycoveringthedestinationsthatareavailable

atthedisconnectedhost.



8:



Whyshouldsummarizationbeconfiguredoutboundfromthedistributionlayer

routerstowardaccesslayerroutersatremotesites?



Answer:Summarizationshouldbeconfiguredtoreducetheamountoftrafficon

A8: thedistributionlayertoremoterouterlinksandtoboundqueriesattheremote

router.



9:



Whatisthemostcommonproblemwithdual-homedremotes?Whatoptionsare

availabletoresolveit?



Answer:Themostcommonproblemisthatremoteroutersappeartobetransit

A9: pathstoEIGRP.Toresolvethatproblem,youcansummarizeroutesoutbound

fromthedistributionlayertowardtheaccesslayerrouters.

10: Whatmethodscanyouusetobreakaredistributionroutingloop?

Answer:Youcanusethefollowingmethodstobreakaredistributionrouting

loop:distributelists,routemaps,prefixlists,settingtheadministrativedistance

A10:

onroutesthatarelikelytoproduceloops,andusingadministrativetagsin

externalroutestomarktheroutesandblocktheirredistribution.



11:



UnderwhatconditionsistheadministrativedistanceignoredbetweenEIGRPand

IGRP?



Answer:TheadministrativedistanceisignoredwhenanIGRProuteandan

A11: EIGRProuteinthesameautonomoussystemcompeteforinclusionintherouting

table.

12: WhatoptionsdoyouhaveforgeneratingadefaultrouteinEIGRP?



A12:



Answer:Oneoptionistoredistributea0.0.0.0/0defaultroute.Anotheroptionis

toconfigureanipdefault-network,butthatisnotthepreferredmethod.



13:



Howcanyoupreventmultipleparallellinkswithinanetworkfrombeingusedas

transitpaths?



A13:



Answer:YoucanpreventthisbynotrunningEIGRPonsomepaths;you

accomplishthisbyusingthepassive-interfacecommand.



14: WhatdoesEIGRPusetopaceitspacketsonalink?



A14:



Answer:EIGRPusesthebandwidththatisconfiguredontheinterfacetopaceits

packetsonalink.



Chapter4

1: HowdoesOSPFbindthefloodingoftopologyinformation?

Answer:OSPFbindsthefloodingoftopologyinformationbybreakingthe

networkintoareas.Attheareaboundary,anABRsummarizestherouting

A1:

information,removingthetopologyinformation,andadvertisesjustreachability

andcostintotheadjoiningarea.



2:



Whatistheruleofthumbtoconsiderwhendesigningaroundsummarizationof

topologyinformationandsummarizationofreachabilityinformation?



Answer:Itisalwaysbettertodesignaroundthesummarizationoftopology

informationandthentweakthedesignasneededtoprovideoptimalreachability

A2:

summarization.Blockingtopologyinformationhasamuchlargerimpactonthe

convergencespeedofthenetwork.



3:



WhatisthegeneralruleofthumbondealingwithcomplexityinanOSPF

networkdesign?



A3: Answer:Alwaysseparatecomplexityfromcomplexitywherepossible.



4:



WhataresomeoftheadvantagesofplacingtheABRsinathree-layerhierarchy

attheedgeofthenetworkcore?



Answer:PlacingtheABRsinathree-layerhierarchyattheedgeofthenetwork

corecontainsthesizeandcomplexityofarea0,generallyprovidingforbetter

A4:

routeaggregation.Doingsoalsofacilitatespushingthecomplexityentailedfrom

addingredundancyinthedistributionlayeroutofarea0.



5:



WhatarethedisadvantagesofplacingtheABRsinathree-layerhierarchical

networkdesignattheedgeofthecore?



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Appendix F. Answers to Review Questions

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