Estensione multicast OSPF

ThesynonymospfgenerallyreferstothemulticastextensionofOSPF.

Definition

OSPFroutingprotocolisatypicallink-stateroutingprotocol,generallyusedinthesameroutingdomain.Here,theroutingdomainreferstoanautonomoussystem(AutonomousSystem),orAS,whichreferstoagroupofnetworksthatexchangeroutinginformationwitheachotherthroughaunifiedroutingpolicyorroutingprotocol.InthisAS,allOSPFroutersmaintainthesamedatabasedescribingthestructureofthisAS.Thedatabasestoresthestatusinformationofthecorrespondinglinksintheroutingdomain.OSPFroutersusethisdatabasetocalculateitsOSPFroutingtable.of.

Asalinkstateroutingprotocol,OSPFtransmitslinkstatemulticastdataLSA(LinkStateAdvertisement)toallroutersinacertainarea,whichisdifferentfromthedistancevectorroutingprotocol.Therouterrunningthedistancevectorroutingprotocolpassespartoralloftheroutingtabletoitsneighboringrouters.

Intermsofthesecurityofinformationexchange,OSPFstipulatesthatanyinformationexchangebetweenrouterscanbeauthenticatedorauthenticatedwhennecessary,toensurethatonlytrustedrouterscantransmitoptions.Roadinformation.OSPFsupportsmultipleauthenticationmechanismsandallowsdifferentauthenticationmechanismstobeusedbetweenareas.OSPFoptimizestheapplicationoflinkstatealgorithmsinbroadcastnetworks(suchasEthernet)inordertousehardwarebroadcastcapabilitiesasmuchaspossibletotransmitlinkstatemessages.Usuallyanodeinthetopologydiagramofalinkstatealgorithmrepresentsarouter.IfKroutersareallconnectedtotheEthernet,whenthelinkstateisbroadcast,thenumberofpacketsabouttheseKrouterswillreachthesquareofK.Forthisreason,OSPFallowsanodetorepresentabroadcastnetworkinthetopologydiagram.Allroutersoneachbroadcastnetworksendlinkstatusmessagestoreportthelinkstatusoftheroutersinthenetwork.

Quomodo facitur

OSPFsimplymeansthattwoadjacentroutersbecomeneighborsintheformofsendingmessages,andtheneighborsthensendlinkstateinformationtoeachothertoformanadjacency,andthenEachcalculatestherouteaccordingtotheshortestpathalgorithm,putsitintheOSPFroutingtable,andcomparestheOSPFroutewithotherroutesandaddsittotheglobalroutingtable.Thewholeprocessusesfivetypesofmessages,threestages,andfourtables.

Fivetypesofmessages

Hellomessage:establishandmaintainneighborrelationships.

DBDmessage: Sendlinkstateheaderinformation.

LSRmessage:SendthelinkstateheaderinformationneededfromDBDtoneighbors,requestingcompleteinformation.

LSUmessage:SendthecompleteinformationcorrespondingtotheheaderinformationoftheLSRrequesttotheneighbor.

LSACK:AcknowledgethemessageafterreceivingtheLSUmessage.

Threestages

Neighbordiscovery:TheneighborrelationshipisformedbysendingHellomessages.

Routeannouncement:Neighborssendlinkstateinformationtoformanadjacencyrelationship.

Routingcalculation:Calculatetheroutingtablebasedontheshortestpathalgorithm.

Fourtables

Neighbortable:Themainrecordformstheneighborrelationshiprouter.

Linkstatedatabase:Recordlinkstateinformation.

OSPFroutingtable: derived fromthelinkstatedatabase.

Globalroutingtable:OSPFroutingiscomparedwithothers.

Workingprocess

(1) Knowitsownlink

Eachrouterknowsitsownlink,thatis,thenetworkdirectlyconnectedtoit.

(2) Lookingforneighbors

DifferentfromRIP,afterOSPFprotocolruns,itdoesnotbroadcastroutinginformationtothenetworkimmediately,butfirstsearchesforthelinkstatusthatcanbeexchangedwithitselfinthenetworkInformationperipheralrouter.Routersthatcanexchangelinkstateinformationareneighborstoeachother.

(III) Createalinkstatedatapacket

Oncetherouterhasestablishedaneighborrelationship,itcancreatealinkstatedatapacket.

(4)Linkstateinformationtransmission

TherouterfloodstheLSAdescribingthelinkstatetoneighbors,andfinallyformsalinkstatedatabasecontainingcompletenetworklinkstateinformation.

(5) Calculatingroutes

EachrouterintheroutingareacanusetheSPFalgorithmtoindependentlycalculateroutes.

Themainadvantages oftheOSPFprotocol

(1)OSPFissuitableforawiderangeofnetworks:ThereisnolimitonthenumberofhopsforroutingintheOSPFprotocol,sotheOSPFprotocolcanbeusedinInmanyoccasions,italsosupportsawidernetworkscale.Aslongasitisinamulticastnetwork,theOSPFprotocolcansupportdozensofrouterstoworktogether.

(2)Multicast-triggeredupdate:AftertheconvergenceoftheOSPFprotocoliscompleted,itwillsendtopologychangeinformationtootherroutersinatriggeredmanner,whichcanreducetheutilizationofnetworkbandwidth;atthesametime,itcanreduceSmallinterference,especiallywhenusingthemulticastnetworkstructure,whensendingoutinformation,itdoesnotcauseothereffectsonotherdevices

(3)Fastconvergencespeed:Ifthenetworkstructurechanges,theOSPFprotocolsystemwillSendoutnewpacketsasquicklyaspossible,sothatthenewtopologywillquicklyspreadtotheentirenetwork;moreover,OSPFusesshort-periodHELLOpacketstomaintainneighborstatus.

(4)Takethecostasthemetricvalue:WhentheOSPFprotocolisdesigned,theinfluenceofthelinkbandwidthontheroutemetricvalueisconsidered.TheOSPFprotocolusesthecostvalueasthestandard,andlinkcostandlinkbandwidthhaveaninverserelationship.Thehigherthebandwidth,thelowerthecost.Inthisway,OSPFroutingismainlybasedonbandwidthfactors.

(5)TheOSPFprotocolisdesignedtoavoidroutingloops:Undertheshortestpathalgorithm,thelinkstateintherouteisreceived,andthenthepathisgenerated,sothatnoloopwilloccur.

(6)Extensiveapplication:ItiswidelyusedontheInternet,andtherearealotofapplicationexamplesinothers.ProvethatthisisoneofthemostwidelyusedIGPs.

OSPFarea

ThepurposeofdividingtheareainOSPFistocontroltherangeoflinkstateinformationLSAflooding,reducethesizeofthelinkstatedatabaseLSDB,andimprovethescalabilityofthenetworkToachieverapidconvergence.

Whenthenetworkcontainsmultipleareas,theOSPFprotocolhasspecialregulations,thatis,theremustbeanArea0,whichisusuallycalledthebackbonearea(BackboneArea).WhendesigninganOSPFnetwork,oneisverygood.Themethodistostartfromthebackboneareaandthenexpandtootherareas.Thebackboneareaisatthecenterofallotherareas,thatis,allareasmustbephysicallyorlogicallyconnectedtothebackbonearea.ThereasonforthisdesignideaisthattheOSPFprotocolmustimporttheroutinginformationofallareasintothebackbonearea,andthentheroutinginformationfromThebackboneareaisdistributedtootherareas.

OSPFdividesareasintoseveraltypes.

Backbonearea:Asacentralentity,otherareasareconnectedtoit.Thenumberofthebackboneareais0.Inthisarea,alltypesofLSAsareallowedtobeissued.

Standardarea:Thedefaultareatypeexceptthebackbonearea.Inthistypeofarea,alltypesofLSAsareallowedtobepublished.

Stubarea:STUBarea.ThistypeofareadoesnotacceptroutinginformationaboutASoutside,thatis,itdoesnotaccepttype5ASexternalLSAs.Whenroutingtothenetworkoutsidetheautonomoussystem,therouterusesthelackForprovincialrouting(0.0.0.0),thestubareacannotcontaintheautonomoussystemborderrouterASBR.

Completelystubarea:ThistypeofareadoesnotacceptroutinginformationabouttheoutsideoftheAS,anddoesnotacceptsummaryroutesfromotherareasintheAS,thatis,type3,type4,andtype5LSAsarenotaccepted,ThecompletelystubareacannotcontaintheautonomoussystemborderrouterASBR.

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