网络路由的原理和常见协议的工作机制.docx

网络路由的原理和常见协议的工作机制.docx

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网络路由的原理和常见协议的工作机制

网络路由的原理和常见协议的工作机制

RoutingPage1of9WhatistheRoleoftheRouter?

Routersinterconnectworksinordertoallowmunicationandresourcesharingamongstthoseworks.Connectionsaremadebetweenroutersusingcablesorawirelessmedium.Variousroutingprotocolsenabletherouterstoexchangedata,andcanbeusedtocontroltheflowofthatdata.Someroutershaveswitchesbuiltintothem,andsomerequireaseparateswitch.Apictureofasmallroutercanbefoundbelow:

BEFOREMOVINGFORWARD,ITISIMPORATANTTOKNOWTHATAROUTERISNOTAFIREWALLANDAFIREWALLISNOTAROUTER.THATSAID,SOMEROUTERSHAVEBUILTINFIREWALLCAPABILITIESRoutingPage2of9HowdoesaRouteruseRoutingTables?

Todetermihedestinationofapacket,therouterwillexamihepacketandsendthepackettotheproperdestination.Toexamihepacket,therouterstripsoffthelayer-2informationandputstheresultingpacketintoaqueue–thisprocessourswhenthepacketarrivesattherouter.TherouterwilltheninspectthedestinationIPaddressofthepacketandsendittothecorrectport.Toperformthis,therouterwillusearoutingtable.Theroutingtabletellstherouterexactlywheretosendthepacketsforthegivendestination.Anexampleofaroutingtablecanbefoundbelow:

DestinationNetworkSubMaskGatewayInterface10.12.14.0255.255.255.00.0.0.0LAN76.30.4.0255.255.254.00.0.0.0WAN0.0.0.00.0.0.076.30.4.1WANTheponentsoftheroutingtablearefurtherdescribedbelow:

1）DestinationLANIP–ThedefinedworkID.EveryworkIDdirectlyconnectedtooneoftherouter’sportsisalwayslistedhere.2）SubMask–IdentifiestheworkIDfromanIPaddress.3）Gateway–TheIPaddressforthehoprouter（i.e.wherethepacketshouldgo）.IfthepacketisdestinedforaworkIDthatisnotdirectlyconnectedtotherouter,thegatewaycolumntellsroutertheIPaddressofaroutertosendthepacketto;thenextrouterhandlesthepacketandtheoriginalrouterisrelievedofitsresponsibilityforthatpacket.IftheworkIDisdirectlyconnected,thenagatewayisn’tneeded.4）Interface–TheInterfacefieldidentifieswhichofitsphysicalportstouse.Theprovidedexampleshows“LAN”and“WAN”butsomeroutersmayshowaport’sIPaddressoranothertypeofabbreviation（likef0/0,f0/1asfoundonCiscorouters）.***Whenlookingattheroutingtable,a0represents“anything.”AnexampleofthiscanbefoundintheprovidedroutingtablewiththeDestinationLANIPof10.12.14.0andthesubmaskof255.255.255.0.Thismeansthatanyvaluebetween1and254isaeptable.Exercise:

Usetheroutingtableabovetoanswerthefollowingquestions.1）WhatinterfacewouldapacketreceivedwithaDestinationLANIPof76.30.4.27besentto?

________2）IfapacketisreceivedforaDestinationLANIPof69.36.252.61,whatGatewayshouldthepacketbesentto?

____________________________________________________________________________RoutingPage3of9HowtoIdentifytheDefaultRouteintheRoutingTableMostroutershaveadefaultroute.Adefaultroutetellstherouterwheretoforwardpacketswhennootherroutecanbedeterminedbyusingtheroutingtable.Intheroutingtablebelow,thedefaultrouteisrepresentedinthethirdline:

DestinationLANIPSubMaskGatewayInterface10.12.14.0255.255.255.00.0.0.0LAN76.30.4.0255.255.254.00.0.0.0WAN0.0.0.00.0.0.076.30.4.1WANIsaRoutertheOnlyThingthathasaRoutingTable?

No,everynodeonaworkshouldhavearoutingtable.Takethetimetoopenamandpromptorterminalwindowandtypethemandstat–r.YoumayalsousethemandrouteprintintheWindowsmandpromptasanalternativeoptionorrouteinlinux.Thedefaultroute.Thisroutingtableshowsroutesfor10.12.14.0and76.30.4.0;thethirdlineintheroutingtablehas0.0.0.0forthedestinationandsubmask,representinganyotherdestination.Essentially,thisruleintheroutingtablesays（anydestinationaddress）（withanysubmask）（forwarditto76.30.4.1）（usingtheWANport）.RoutingPage4of9WhatisNetworkAddressTranslation（NAT）?

NetworkAddressTranslation（NAT）istheprocessofmodifyingIPaddressinformationinIPv4headerswileittransitsthrougharoutingdevice.Thismodificationisdoneforsecuritypurposes,andtohidetheIPaddressesofputersontheLANwhileallowingtheputertoaesstotheInter.NATessentiallyextendsthelifeofIPv4addressesontheInter.ItmustbenotedthatNATisnotrouting,itisaseparatetechnologythatmanyroutersofferasafeature.WhatdoesthebasicsetupofNATlooklike?

Inthisscenario,youareaworkadministrator.YouhaveaLANwitheightputersonitthatneedaesstotheInter.YoucalltheInterServiceProviderandpurchaseaworkID（example:

1.2.3.136/29）.YouthenassignanIPaddresstoeachputerandtotheLANconnectionontherouter,andthenassigntheIPaddressfortheISP’sroutertotheWANconnectiononthelocalrouter（example1.2.3.4）.Theputersontheworkthenusetherouterastheirdefaultgateway（aesspointtotheInter）.AnexampleofNATcanbefoundbelow:

InNATsmostbasicstructure,whentherouterreceivestherequestfromtheputerontheLANitwillreplacethesourceIPaddresswiththeIPaddressoftheWANonoutgoingpackets.Therouterwillthentrackthebasicdataabouttheactiveconnection（destinationaddressandport）.Whenthereplyreturnstotherouter,therouterwillusetheconnectiontrackinginformationthatitstoredduringfromtheoutboundrequesttodetermiheprivateaddressontheLANandtherouterwillforwardthereplytotheappropriateputer.RoutingPage5of9WhatisPAT?

TherearemanymethodsusedtoNAT.ThemostmonmethodisPortAddressTranslation（PAT）.PATusesportnumberstomaptrafficfromspecificputersontheLAN.WhenaputerontheLANmakesarequest,likeusingawebbrowsertoaessawebsite,therouterwillrecordthesourceIP,destinationIP,andtheportnumber.TheprivateIPaddressisthenswappedoutforthepublicIPaddressandauniqueportnumberoneachpacket–thisinformationisalsorecordedbytherouter.Anexampleofthisinteractioncanbeseenbelow:

Whenthetrafficisreturned,therouterwillparetheiningdestinationportandsourceIPaddresstotheentryintheNATtranslationtabletodetermineinternalIPaddressandmodifythepacketwiththeinternalsourceIPaddress.ThepacketwillthenbesenttotheoriginalputerwiththeoriginatingsourceIPaddress.Thisprocesswillevenworkwhenmorethan1puterontheworkareaessingthesameserveroutsideoftheworkbecausethoughtsofportsareavailableforPAT.Onethingtobeawareofisthatthisprocessonlyworksfortrafficleavingthework,notinboundtraffic.Noticehowthesource（Src:

）IPandporthavebeenchangedinthepacket.OriginalIPis10.0.0.2AfterPATIPis157.55.1.10Originalport3123AfterPATportis4135TheNATtranslationtable.Therouterhasstoredtheinformationsothatthereturnedpacketscanbesenttocorrecthost.RoutingPage6of9Then,WhatisPortForwarding?

Portforwardingtakescareofthelimitationsassociatedwithinboundrequests,likeaputeringfromoutsideoftheworkattemptingtoaessawebserverinsidethework.Portforwardingallowsaworkadministratortodesignateaspecificlocaladdressfordifferentworkservices.AputeroutsideoftheworkcanmakearequestforservicesusingthepublicIPaddressoftheworkandtheportnumberofthedesiredservice.Therouterwillexamihepacket,reviewthelistofservicesmappedtolocaladdresses,thesendthepackettotheproperhost.Anexampleofportforwardingcanbefoundbelow:

WhatistheDifferenceBetweenDynamicandStaticNATs?

BothstaticanddynamicNATrequirestheworktohaveenoughpublicaddressestosatisfythetotalnumberofsimultaneoususersessions.Fundamentally,staticNATsareusedforinboundtrafficanddynamicNATsareusedforoutboundtrafficbutbotherarefurtherexplainedbelow:

StaticNAT:

AstaticNAT（AKAinboundmapping）istheprocessofmappinganunregisteredIPaddresstoaregisteredIPaddress.Thismappingisdoneonaone-to-onebasis.Theunregistered（ormapped）IPaddressisassignedwiththesameregisteredIPaddresseachtimetherequestismade.Amonapplicationofthisisforwebserversorhoststhatmusthaveaconsistentaddressthatcanbeaessedfromoutsideofthework.AnexampleofstaticNATingcanbefoundbelow:

RoutingPage7of9DynamicNAT:

AdynamicNATprovidesapermanentmappingbetweenspecificinternalIPaddressesandaspecificpublicIPaddress.Toachievethis,dynamicNATusesapoolofpublicIPaddressesandassignstheIPaddressesonafirstefirstservebasiswhenthehostmakesarequestforaserviceoutsideofthework.AnexampleofdynamicNATingcanbefoundbelow:

WhatisDynamicRouting?

Thisjobsheethasshownmanyexamplesoftablesthatlookingliketheyarestatic.Don’tletthatfoolyouthough;mostroutershavetheabilitytoupdatetheirroutesdynamically,providedtheyareabletousedynamicroutingprotocols.Whilemanyroutingprotocolshavebeendefinedovertheyears,routingprotocolsaretypicallydividedintothreedistinctgroups:

1）distancevector;2）linkstate,and3）hybrid.Tounderstandthesegroups,youmustunderstandmetricsbeforeunderstandingthedifferentroutingprotocols.RoutingMetricsAmetricissimplyarelativevaluethatrouterswillusewhentheyhavemorethanoneroutetogettoanotherwork.Thesemetricsidentifythe“cost”ofusingagivenroute.Alowermetricnumberidentifiesthefirstpaththattheroutershouldtake;iftheroutercan’ttakethatpathitwilluseahighermetriumberifitisavailable.Anexampleofaroutingtablewithmetricscanbeseenbelow.Theroutingmetricandinterfacehasbeenoutlinedandidentifiestheinterfacetosendpacketstoforworkdestinationsof224.0.0.0withasubmaskof240.0.0.0.Inthisinstance,5pathsareavailableforthatdestinationbut4oftheinterfacescarrythesamemetric–meaningthatall4interfacescanbeused;ifthesamemetriumberisused,thenthewidestsubmaskwilltakeprecedence.Ifthereisaproblemwiththoseinterfaces,theroutershouldusetheinterfacewiththenexthighestmetric.RoutingPage8of9Dynamicroutingprotocolsallowaroutertofindalternativeroutesifaconnectionislostorunavailable.Therolethatthemetricplaysinthisprocessallowstheroutertofindthenextroute–therouterwillalwayschoseroutewiththelowestmetric.Itisimportanttoknowthatthereisnomonstandardusedtodetermihemetricvaluebutvarioustypesofdynamicprotocolsusedifferentcriteria.Youwillfindsomeofthemostmoncriteriafordete