CCNA思科认证网络工程师【CCNA 7-3】实施生成树.ppt

PVRSTImplementationCommandsConfiguringspanning-treemoderapid-pvstVerifyingshowspanning-treevlan101Debuggingdebugspanning-treeHowtoImplementRapidPVRST(RPVST+)VerifyingPVRSTDisplayspanningtreemodeissettoPVRST.Switch#showspanning-treevlan30VLAN0030SpanningtreeenabledprotocolrstpRootIDPriority24606Address00d0.047b.2800ThisbridgeistherootHelloTime2secMaxAge20secForwardDelay15secBridgeIDPriority24606(priority24576sys-id-ext30)Address00d0.047b.2800HelloTime2secMaxAge20secForwardDelay15secAgingTime300InterfaceRoleStsCostPrio.NbrType-------------------------------Gi1/1DesgFWD4128.1P2pGi1/2DesgFWD4128.2P2pGi5/1DesgFWD4128.257P2p**Emphasize:Layer2hasnomechanism(likeaTTL)tostoploops.**Layer3of3Emphasize:Layer2hasnoTTLmechanismtostoploopingframes.*Layer2of2*Layer2of2*Emphasize:Aloopedtopologyisoftendesiredtoprovideredundancy,butloopedtrafficisundesirable.TheSpanning-Treeprotocolwasoriginallydesignedforbridges.Today,itisalsoappliedtoLANswitchesandroutersoperatingasabridge.Spanning-Treeprotocolensuresthatallbridgedsegmentsarereachablebutanypointswhereloopsoccurwillbeblocked.*Emphasize:ThethreegeneralruleswhendealingwithSTPareasfollows:1.Onerootbridgepernetwork.TherootisthebridgewiththelowestbridgeID.Alltheportsontherootbridgearedesignatedports(forwarding).2.Foreverynon-rootbridge,thereisarootport(forwarding).Therootportistheportwiththelowestaccumulatedpathcosttotherootbridge.3.Foreverysegment,thereisonlyonedesignatedport.Thedesignatedportforwardstrafficforthesegment.Thedesignatedporthasthelowestaccumulatedpathcosttotherootbridge.*Emphasize:Bydefault,theswitchwiththelowestMACaddresswillbetherootbridg