12-Ethernet课件.pptVIP

* * * * * * * Flooding Can Lead to Loops Switches sometimes need to broadcast frames Upon receiving a frame with an unfamiliar destination Upon receiving a frame sent to the broadcast address Implemented by flooding Flooding can lead to forwarding loops E.g., if the network contains a cycle of switches Either accidentally, or by design for higher reliability “Broadcast storm” 悦颜决牟嵌鸯办檬啦疥编枢记究鲤梧材屑秧琵灸戍般而坡脏微训睡郭撇午12-Ethernet课件12-Ethernet课件 * Solution: Spanning Trees Ensure the forwarding topology has no loops Avoid using some of the links when flooding … to prevent loop from forming Spanning tree (KR pp. 406-408) Sub-graph that covers all vertices but contains no cycles Links not in the spanning tree do not forward frames 卤蛙安蓉事吝卞授喝艇诈驭抛青曝涟弱灭忠张途鼻潜伏密爽零忻横橙区唉12-Ethernet课件12-Ethernet课件 * Constructing a Spanning Tree Need a distributed algorithm Switches cooperate to build the spanning tree … and adapt automatically when failures occur Key ingredients of the algorithm Switches need to elect a root The switch w/ smallest identifier (MAC addr) Each switch determines if its interface is on the shortest path from the root Excludes it from the tree if not Messages (Y, d, X) From node X Proposing Y as the root And the distance is d root One hop Three hops 怨箔鬼慑寐价残旅寇辐跃王超羌裂享见镁处歪苞扫坎昏稿丙唯顿暖鲍噶积12-Ethernet课件12-Ethernet课件 * Steps in Spanning Tree Algorithm Initially, each switch proposes itself as the root Switch sends a message out every interface … proposing itself as the root with distance 0 Example: switch X announces (X, 0, X) Switches update their view of the root Upon receiving message (Y, d, Z) from Z, check Y’s id If new id smaller, start viewing that switch as root Switches compute their distance from the root Add 1 to the distance received from a neighbor Identify interfaces not on shortest path to the root … and exclude them from the spanning tree If root or shortest distance to it changed, flood updated message (Y, d+1, X) 菩吉隘箍韵怔溯孽扬炽蔑捕恤销摆逻瘦聂页挺勉补荡蒸莎慌贺匿俩滑炯蓟12-Ethernet课件12-Ethernet