Exploration_Routing_Chapter_8路由表.ppt

TheRoutingTable:ACloserLook,RoutingProtocolsandConceptsChapter8,Objectives,DescribethevariousroutetypesfoundintheroutingtablestructureDescribetheroutingtablelookupprocess.Describeroutingbehaviorinroutednetworks.,Introduction,ChapterFocus-Structureoftheroutingtable-Lookupprocessoftheroutingtable-Classlessandclassfulroutingbehaviors,RoutingTableStructure,LabTopology3routersetup-R1andR2shareacommon172.16.0.0/16networkwith172.16.0.0/24subnets.-R2andR3areconnectedbythe192.168.1.0/24network.-R3alsohasa172.16.4.0/24subnet,whichisdisconnected,ordiscontiguous,fromthe172.16.0.0networkthatR1andR2share.,RoutingTableStructure,Routingtableentriescomefromthefollowingsources-Directlyconnectednetworks-Staticroutes-Dynamicroutingprotocols,RoutingTableStructure,Level1RoutesAssoonasthenoshutdowncommandisissuedtherouteisaddedtoroutingtable,RoutingTableStructure,CiscoIProutingtableisahierarchicalstructure-Thereasonforthisistospeeduplookupprocess,RoutingTableStructure,Level1Routes-Haveasubnetmaskequaltoorlessthantheclassfulmaskofthenetworkaddress.Level1routecanfunctionas-Defaultroute-Supernetroute-Networkroute,RoutingTableStructure,Level1Routes-UltimateRouteIncludeseither:-Anext-hopaddressOR-Anexitinterface,ParentandChildRoutes-Aparentrouteisalevel1route-Aparentroutedoesnotcontainanynext-hopIPaddressorexitinterfaceinformation,RoutingTableStructure,RoutingTableStructure,Automaticcreationofparentroutes-OccursanytimeasubnetisaddedtotheroutingtableChildroutes-Childroutesarelevel2routes-Childroutesareasubnetofaclassfulnetworkaddress,RoutingTableStructure,Level2childroutescontainroutesource&thenetworkaddressoftherouteLevel2childroutesarealsoconsideredultimateroutesReason:theycontainthenexthopaddress&/orexitinterface,RoutingTableStructure,Bothchildrouteshavethesamesubnetmask-Thismeanstheparentroutemaintainsthe/24mask,RoutingTableStructure,Diagramillustrates2childnetworksbelongingtotheparentroute172.16.0.0/24,RoutingTableStructure,Inclasslessnetworks,childroutesdonothavetosharethesamesubnetmask,RoutingTableStructure,Parent&ChildRoutes:ClasslessNetworks,RoutingTableStructure,Parent&ChildRoutes:ClasslessNetworks,RoutingTableLookupProcess,TheRouteLookupProcessExaminelevel1routes-Ifbestmatchalevel1ultimaterouteandisnotaparentroutethisrouteisusedtoforwardpacketRouterexamineslevel2(child)routes-Ifthereisamatchwithlevel2childroutethenthatsubnetisusedtoforwardpacket-IfnomatchthendetermineroutingbehaviortypeRouterdeterminesclassfulorclasslessroutingbehavior-Ifclassfulthenpacketisdropped-Ifclasslessthenroutersearcheslevelonesupernetanddefaultroutes-Ifthereexistsalevel1supernetordefaultroutematchthenPacketisforwarded.Ifnotpacketisdropped,RoutingTableLookupProcess,LongestMatch:Level1NetworkRoutesBestmatchisalsoknownasthelongestmatchThebestmatchistheonethathasthemostnumberofleftmostbitsmatchingbetweenthedestinationIPaddressandtherouteintheroutingtable.,RoutingTableLookupProcess,FindingthesubnetmaskusedtodeterminethelongestmatchScenario:PC1pings192.168.1.2Routerexamineslevel1routeforbestmatchThereexistamatchbetween192.168.1.2&192.168.1.0/24Routerforwardspacketsouts0/0/0,RoutingTableLookupProcess,Theprocessofmatching-1sttheremustbeamatchmadebetweentheparentroute&destinationIP-IfamatchismadethenanattemptatfindingamatchbetweenthedestinationIPandthechildrouteismade.,RoutingTableLookupProcess,FindingamatchbetweenpacketsdestinationIPaddressandthenextrouteintheroutingtable-ThefigureshowsamatchbetweenthedestinationIPof192.168.1.0andtheleveloneIPof192.168.1.0/24thenpacketforwardedouts0/0/0,RoutingTableLookupProcess,Level1Parent&Level2ChildRoutesBeforelevel2childroutesareexamined-TheremustbeamatchbetweenclassfulleveloneparentrouteanddestinationIPaddress.,RoutingTableLookupProcess,AfterthematchwithparentroutehasbeenmadeLevel2childrouteswillbeexaminedforamatch-RoutelookupprocesssearchesforchildrouteswithamatchwithdestinationIP,RoutingTableLookupProcess,Howarouterfindsamatchwithoneofthelevel2childroutes-Firstrouterexaminesparentroutesforamatch-Ifamatchexiststhen:ChildroutesareexaminedChildroutechosenistheonewiththelongestmatch,RoutingTableLookupProcess,Example:RouteLookupProcesswithVLSM-TheuseofVLSMdoesnotchangethelookupprocess-IfthereisamatchbetweendestinationIPaddressandthelevel1parentroutethen-Level2childrouteswillbesearched,RoutingBehavior,Classful&classlessroutingprotocolsInfluencehowroutingtableispopulatedClassful&classlessroutingbehaviorsDetermineshowroutingtableissearchedafteritisfilled,RoutingBehavior,ClassfulRoutingBehavior:noipclasslessWhathappensifthereisnotamatchwithanylevel2childroutesoftheparent?-Routermustdetermineiftheroutingbehaviorisclasslessorclassful-Ifrouterisutilizingclassfulroutingbehaviorthen-Lookupprocessisterminatedandpacketisdropped,RoutingBehavior,ClassfulRoutingBehaviorSearchProcessAnexampleofwhenclassfulroutingbehaviorisineffectandwhytherouterdropsthePacket-Thedestinationssubnetmaskisa/24andnoneofthechildroutesleftmostbitsmatchthefirst24bits.Thismeanspacketisdropped,RoutingBehavior,ClassfulRoutingBehaviorSearchProcessThereasonwhytherouterwillnotsearchbeyondthechildroutesOriginallynetworkswereallclassfulThismeantanorganizationcouldsubnetamajornetworkaddressand“enlighten”alltheorganizationsroutersaboutthesubnettingTherefore,ifthesubnetwasnotintheroutingtable,thesubnetdidnotexistandpacketwasdropped,RoutingBehavior,ipClasslessBeginningwithIOS11.3,ipclasslesswasconfiguredbydefaultClasslessroutingbehaviorworksfor-DiscontiguousnetworksAnd-CIDRsupernets,RoutingBehavior,ClasslessRoutingBehavior:ipclasslessRoutelookupprocesswhenipclasslessisinuse-IfclasslessroutingbehaviorineffectthenSearchlevel1routesSupernetroutesCheckedfirst-IfamatchexiststhenforwardpacketDefaultroutesCheckedsecondIfthereisnomatchornodefaultroutethenthePacketisdropped,RoutingBehavior,ClasslessRoutingBehaviorSearchProcessRouterbeginssearchprocessbyfindingamatchbetweendestinationIPandparentrouteAfterfindingtheabovementionedmatch,thenthereisasearchofthechildroute,RoutingBehavior,ClasslessRoutingBehaviorSearchProcessIfnomatchisfoundinchildroutesofpreviousslidethenRoutercontinuestosearchtheroutingtableforamatchthatmayhavefewerbitsinthematch,RoutingBehavior,Classfulvs.ClasslessRoutingBehavior-ItisrecommendedtouseclasslessroutingbehaviorReason:sosupernetanddefaultroutescanbeusedwheneverneeded,Summary,Content/structureofaroutingtableRoutingtableentries-Directlyconnectednetworks-Staticroute-DynamicroutingprotocolsRoutingtablesarehierarchical-Level1routeHaveasubnetmaskthatislessthanorequaltoclassfulsubnetmaskforthenetworkaddress-Level2routeThesearesubnetsofanetworkaddress,Summary,RoutingtablelookupprocessBeginswithexamininglevel1routesforbestmatchwithpacketsdestinationIPIftheb