第3章 03路由器应用之OSPF 2英文版

Chapter-11OSPFIntroductionOSPFOSPFDevelopmentTimelineEncapsulatedOSPFmessageEncapsulatedOSPFmessageEncapsulatedOSPFmessageOSPFPacketTypesHelloPackets OSPFpacketType1istheOSPFHellopacket.Hellopacketsareusedto:DiscoverOSPFneighborsandestablishneighboradjacencies.Advertiseparametersonwhichtworoutersmustagreetobecomeneighbors.ElecttheDesignatedRouter(DR)andBackupDesignatedRouter(BDR)onmulti-accessnetworkslikeEthernetandFrameRelay.OSPFmessageformatHelloProtocolLink-stateupdates(LSUs) Link-stateupdates(LSUs)arethepacketsusedforOSPFroutingupdates.AnLSUpacketcancontaintendifferenttypesofLink-StateAdvertisements(LSAs),asshowninthefigure. ThedifferencebetweenthetermsLink-StateUpdate(LSU)andLink-StateAdvertisement(LSA)cansometimesbeconfusing.Attimes,thesetermsareusedinterchangeably. AnLSUcontainsoneormoreLSAsandeithertermcanbeusedtorefertolink-stateinformationpropagatedbyOSPFrouters.Link-stateupdates(LSUs)Dijkstra'sshortestpathfirst(SPF)algorithm OSPFroutermaintainsalink-statedatabasecontainingtheLSAsreceivedfromallotherrouters.OncearouterhasreceivedallofLSAsandbuiltitslocallink-statedatabase,OSPFusesDijkstra'sshortestpathfirst(SPF)algorithmtocreateanSPFtree.TheSPFtreeisthenusedtopopulatetheIProutingtablewiththebestpathstoeachnetwork.Dijkstra'sshortestpathfirst(SPF)algorithmDefaultAdministrativeDistanceAuthentication RIPv2,EIGRP,OSPF,IS-IS,andBGPcanallbeconfiguredtoencryptandauthenticatetheirroutinginformation.Thispracticeensuresthatrouterswillonlyacceptroutinginformationfromotherroutersthathavebeenconfiguredwiththesamepasswordorauthenticationinformation.OSPFCommand OSPFisenabledwiththerouterospfprocess-idglobalconfigurationcommand.Theprocess-idisanumberbetween1and65535andischosenbythenetworkadministrator.Theprocess-idislocallysignificant,whichmeansthatitdoesnothavetomatchotherOSPFroutersinordertoestablishadjacencieswiththoseneighbors. Inourtopology,wewillenableOSPFonallthreeroutersusingthesameprocessIDof1.WeareusingthesameprocessIDsimplyforconsistency.R1(config)#routerospf1R1(config-router)#EnablingOSPFRoutingNetworkCommand ThenetworkcommandusedwithOSPFhasthesamefunctionaswhenusedwithotherIGProutingprotocols:AnyinterfacesonarouterthatmatchthenetworkaddressinthenetworkcommandwillbeenabledtosendandreceiveOSPFpackets.Thisnetwork(orsubnet)willbeincludedinOSPFroutingupdates. Thenetworkcommandisusedinrouterconfigurationmode.Router(config-router)#network

network-addresswildcard-mask

area

area-id TheOSPFnetworkcommandusesacombinationofnetwork-addressandwildcard-masksimilartothatwhichcanbeusedbyEIGRP.ConfiguringOSPFSubnetworksDeterminingtheRouterIDVerifyingtheRouterIDLoopbackAddressNewRouterIDVerifyingneighborAdjacency TheshowipospfneighborcommandcanbeusedtoverifyandtroubleshootOSPFneighborrelationships.Foreachneighbor,thiscommanddisplaysthefollowingoutput:NeighborID-TherouterIDoftheneighboringrouter.Pri-TheOSPFpriorityoftheinterface.Thisisdiscussedinalatersection.State-TheOSPFstateoftheinterface.FULLstatemeansthattherouteranditsneighborhaveidenticalOSPFlink-statedatabases.OSPFstatesarediscussedinCCNP.DeadTime-TheamountoftimeremainingthattherouterwillwaittoreceiveanOSPFHellopacketfromtheneighborbeforedeclaringtheneighbordown.ThisvalueisresetwhentheinterfacereceivesaHellopacket.Address-TheIPaddressoftheneighbor'sinterfacetowhichthisrouterisdirectlyconnected.Interface-Theinterfaceonwhichthisrouterhasformedadjacencywiththeneighbor.VerifyingneighborAdjacencyOSPFCommand PowerfulOSPFtroubleshootingcommandsinclude:showipprotocolsshowipospfshowipospfinterfaceCiscoOSPFCostValues TheCiscoIOSusesthecumulativebandwidthsoftheoutgoinginterfacesfromtheroutertothedestinationnetworkasthecostvalue.OSPFAccumulatesCosts ThecostofanOSPFrouteistheaccumulatedvaluefromoneroutertothedestinationnetwork.TheipospfcostCommand Analternativemethodtousingthebandwidthcommandistousetheipospfcostcommand,whichallowsyoutodirectlyspecifythecostofaninterface.Forexample,onR1wecouldconfigureSerial0/0/0withthefollowingcommand:R1(config)#interfaceserial0/0/0R1(config-if)#ipospfcost1562EquivalentCommandMultiaccessvspoint-to-pointnetwork Amulti-accessnetworkisanetworkwithmorethantwodevicesonthesamesharedmedia. Apoint-to-pointnetworkthereareonlytwodevicesonthenetwork,oneateachend.Multiaccessvspoint-to-pointnetworkOSPFNetworkTypesOSPFdefinesfivenetworktypes:Point-to-pointBroadcastMultiaccessNonbroadcastMultiaccess(NBMA)Point-to-multipointVirtuallinks NBMAandpoint-to-multi-pointnetworksincludeFrameRelay,ATM,andX.25networks.MultiaccessNetworks MultiaccessnetworkscancreatetwochallengesforOSPFregardingthefloodingofLSAs:1.Creationofmultipleadjacencies,oneadjacencyforeverypairofrouters.2.ExtensivefloodingofLSAs(Link-StateAdvertisements).MultipleAdjacenciesDR/BDRElectionHowdotheDRandBDRgetelected?