CS 6390 Advanced Computer NetworksPacket SwitchingBridges and LAN SwitchesTransparent BridgesSlide 5(1) Frame ForwardingSlide 7(2) Address Learning (Learning Bridges)Slide 9ExampleDanger of LoopsSpanning Trees / Transparent BridgesAlgorhymeConfiguration messages (ConfM)What do the ConfM do?ConceptsSlide 17Steps of Spanning Tree AlgorithmOrdering of MessagesInitializing the Spanning Tree ProtocolOperations of Spanning Tree ProtocolWhen to send your own ConfMWhen to forward a ConfMSelecting the Ports for the Spanning TreeBroadcast and MulticastLimitations of BridgesSlide 27Asynchronous Transfer ModeSlide 29ATM CellsATM LayeringATM LayersFunctions of ATM (Sub)LayersATM Network Layer (ATM Layer)ATM Layer HeadersATM Connection Setup/ReleaseATM Adaptation Layer: Segmentation and ReassemblyAAL5Slide 39Slide 40CS 6390Advanced Computer NetworksPacket SwitchingPacket Switching3.1 Switching and Forwarding3.2 Bridges and LAN Switches3.3 Cell Switching (ATM)Bridges and LAN Switches LANs have physical limitations (e.g., 2500m)Connect two or more LANs with a bridgeaccept and forward strategylevel 2 connection (does not add packet header)An Ethernet switch is a multi port bridgeABridgeB CX YZPort 1Port 2Transparent BridgesHow to efficiently forward frames Three principal approaches can be found:Fixed RoutingSource RoutingSpanning Tree Routing (IEEE 802.1d)The last one is in use in LANsBridges that execute the spanning tree algorithm are called transparent bridgesTransparent Bridges Overall design goal: Complete transparency•“Plug-and-play”: Self-configuring without hardware or software changes•Bridges should not impact operation of existing LANsThree parts to transparent bridges:(1) Forwarding of Frames(2) Learning of Addresses(3) Spanning Tree Algorithm(1) Frame ForwardingEach bridge maintains a forwarding database with entries< MAC address, port, age> MAC address: host or group addressport: port number of bridgeage: aging time of entrywith interpretation: a machine with MAC address lies in direction of the port number from the bridge. The entry is age time units old.Assume a MAC frame arrives on port x.(1) Frame ForwardingBridge 2Port A Port CPort xPort BIs MAC address of destination in forwardingdatabase for ports A, B, or C ?Forward the frame on theappropriate portFlood the frame, i.e., send the frame on all ports except port x.Found?Notfound ?Routing table’s entries are set automatically with a simple heuristic: The source field of a frame that arrives on a port tells which hosts are reachable from this port.(2) Address Learning (Learning Bridges)Port 1Port 2Port 3Port 4Port 5Port 6Src=x, Dest=ySrc=x, Dest=ySrc=x, Dest=ySrc=x, Dest=ySrc=x, Dest=ySrc=x, Dest=yx is at Port 3 Src=y, Dest=xSrc=y, Dest=xSrc=x, Dest=yy is at Port 4 Src=x, Dest=yAlgorithm: For each frame received, the bridge stores the source field in the forwarding database together with the port where the frame was received.All entries are deleted after some time (default is 15 seconds).(2) Address Learning (Learning Bridges)Port 1Port 2Port 3Port 4Port 5Port 6x is at Port 3 Src=y, Dest=xSrc=y, Dest=xy is at Port 4Example•Consider the following packets: (Src=A, Dest=F), (Src=C, Dest=A), (Src=E, Dest=C)•What have the bridges learned?•Bridge 1•Port1•LAN 1•A•LAN 2•C•B•D•LAN 3•E•F•Port2•Bridge 2•Port1•Port2Consider the two LANs that are connected by two bridges.Assume host n is transmitting a frame F with unknown destination.What is happening?Bridges A and B flood the frame to LAN 2.Bridge B sees F on LAN 2 (with unknown destination), and copies the frame back to LAN 1Bridge A does the same. The copying continuesWhere’s the problem? What’s the solution ?Danger of LoopsLAN 2LAN 1Bridge BBridge Ahost nFF FFFF FSpanning Trees / Transparent BridgesA solution is to prevent loops in the topology IEEE 802.1d has an algorithm that organizes the bridges as spanning tree in a dynamic environmentAlgorithm by Radia PerlmanNote: Trees don’t have loopsBridges that run 802.1d are called transparent bridgesBridges exchange messages to configure the bridge (Configuration Bridge Protocol Data Unit, Configuration BPDUs) to build the tree.LAN 2Bridge 2LAN 5LAN 3LAN 1LAN 4Bridge 5Bridge 4Bridge 3dBridge 1AlgorhymeI think that I shall never seeA graph more lovely than a tree.A tree whose crucial propertyIs loop-free connectivity.A tree that must be sure to spanSo packets can reach every LAN.First, the root must be selected.By ID, it is elected.Least-cost paths from root are traced.In the tree, these paths are placed.A mesh is made by folks like me,Then bridges find a spanning tree.- Radia PerlmanConfiguration messages (ConfM)What do the ConfM do? With the help of the ConfM, bridges can:Elect a single bridge as the root bridge.Calculate the distance of the shortest path to the root bridgeEach LAN can determine a designated bridge, which is the bridge closest to the root. The designated bridge will forward packets towards the root bridge.Each bridge can determine a root port, the port that gives the best path to the root.Select ports to be included in the spanning tree.ConceptsEach bridge has a unique identifier: Bridge ID Bridge ID = { Priority : 2 bytes; Bridge MAC address: 6 bytes }Priority is configuredBridge MAC address is lowest MAC addresses of all portsEach port within a bridge has a unique identifier (port ID).Root Bridge: The bridge with the lowest identifier is the root of the spanning tree.Root Port: Each bridge has a root port which identifies the next hop from a bridge to the root.ConceptsRoot Path Cost: For each bridge, the cost of the min-cost path to the root. Assume it is measured in #hops to the rootDesignated Bridge, Designated Port: Single bridge on a LAN that provides the minimal cost path to the root for this LAN: - if two bridges have the same cost, select the one with the lowest identifier - if the min-cost bridge has two or more ports on the LAN, select the port with the lowest identifierNote: We assume that “cost” of a path is the number of “hops”.Steps of Spanning Tree AlgorithmEach bridge is sending out ConfMs that contain the following information:The transmission of ConfMs results in the distributed computation of a spanning treeThe convergence of the algorithm is very quickroot bridge (what
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