OpenShortestPathFirst(OSP F)wasdesignedasan()gatewayprotocol,foruseinanautonomoussystemsuchasalocalareanetwork(LAN).ItimplementsDijkstra'salgorithm,alsoknownasthe(请作答此空)pathfirstalgorithm.Asalink-stateroutingprotocolitwasbasedonthelink-statealgorithmdevelopedfortheARPANETin1980andtheIS-ISroutingprotocol.OSPFwasfirststandardisedin1989asRFC1131,whichisnowknownasOSPFversion1.RoutingprotocolslikeOSPFcalculatetheshortestroutetoadestinationthroughthenetworkbasedonanalgorithm.Thefirstroutingprotocolthatwaswidelyimplemented,the(),calculatedtheshortestroutebasedonhops,thatisthenumberofroutersthatanIPpackethadtotraversetoreachthedestinationhost.RIPsuccessfullyimplementeddynamicrouting,whereroutingtableschangeifthenetworktopologychanges.ButRIPdidnotadaptitsroutingaccordingtochangingnetworkconditions,suchasdata-transferrate.Demandgrewforadynamicroutingprotocolthatcouldcalculatethefastestroutetoadestination.()wasdevelopedsothattheshortestpaththroughanetworkwascalculatedbasedonthecostoftheroute,takingintoaccountbandwidth,delayandload.ThereforeOSPFundertakesroutecostcalculationonthebasisoflink-costparameters,whichcanbeweightedbytheadministrator.OSPFwasquicklyadoptedbecauseitbecameknownforreliablycalculatingroutesthroughlargeandcomplexlocalareanetworks.Asalinkstateroutingprotocol,OSPFmaintainslinkstate(),whicharereallynetworktopologymaps,oneveryrouteronwhichitisimplemented.Thestateofagivenrouteinthenetworkisthecost,andOSPFalgorithmallowseveryroutertocalculatethecostoftheroutestoanygivenreachabledestination.Unlesstheadministratorhasmadeaconfiguration,thelinkcostofapathconnectedtoarouterisdeterminedbythebitrate(1Gbit/s,10Gbit/s,etc)oftheinterface.ArouterinterfacewithOSPFwillthenadvertiseitslinkcosttoneighbouringroutersthroughmulticast,knownasthehelloprocedure.AllrouterswithOSPFimplementationkeepsendinghellopackets,andthuschangesinthecostoftheirlinksbecomeknowntoneighbouringrouters.Theinformationaboutthecostofalink,thatisthespeedofapointtopointconnectionbetweentworouters,isthencascadedthroughthenetworkbecauseOSPFroutersadvertisetheinformationtheyreceivefromoneneighbouringroutertoallotherneighbouringrouters.Thisprocessoffloodinglinkstateinformationthroughthenetworkisknownassynchronisation.Basedonthisinformation,allrouterswithOSPFimplementationcontinuouslyupdatetheirlinkstatedatabaseswithinformationaboutthenetworktopologyandadjusttheirroutingtables.OSPFhasbecomeapopulardynamicroutingprotocol.OthercommonlyuseddynamicroutingprotocolsaretheRIPandtheBorderGatewayProtocol(BGP).Todayrouterssupportatleastoneinteriorgatewayprotocoltoadvertisetheirroutingtableswithinalocalareanetwork.FrequentlyimplementedinteriorgatewayprotocolsbesidesOSPFareRIP,IS-IS,andtheproprietaryInteriorGatewayRoutingProtocol(IGRP)byCisco.
A.shortest B.longest C.first D.begin正确答案A
