15-441 Computer Networking Lecture 8 – Switching and BridgingScaleSlide 3Problem: How to Connect LANs?OutlineBuilding Larger LANs: BridgesTransparent BridgesFrame ForwardingLearning BridgesSpanning Tree BridgesEthernet SwitchesEthernet EvolutionEthernet in a campus networkProblem: Bridging WeaknessesSlide 20What is an Internetwork?Logical Structure of InternetInternet Protocol (IP)Problem: Internetwork DesignGetting to a DestinationForwarding PacketsSimplified Virtual Circuits ExampleVirtual CircuitsSource Routing ExampleSlide 32Global Address ExampleGlobal AddressesComparison1Peter SteenkisteDepartments of Computer Science andElectrical and Computer Engineering15-441 Networking, Spring 2008http://www.cs.cmu.edu/~dga/15-441/S0815-441 Computer NetworkingLecture 8 – Switching and Bridging2ScaleWhat breaks when we keep adding people to the same wire?2yak yak…3ScaleWhat breaks when we keep adding people to the same wire?Only solution: split up the people onto multiple wires»But how can they talk to each other?3yak yak…4Problem: How to Connect LANs?When should these boxes forward packets between wires?How do you specify a destination?How does your packet find its way?4yak yak…5Outline BridgingInternetworks» Methods for packet forwarding56Building Larger LANs: BridgesExtend reach of a single shared mediumConnect two or more “segments” by copying data frames between them»Only copy data when needed key difference from repeaters/hubs»Reduce collision domain compared with single LAN»Separate segments can send at once much greater bandwidthChallenge: learning which packets to copy across links6LAN 1 LAN 27Transparent BridgesDesign goals:»Self-configuring without hardware or software changes»Bridge do not impact the operation of the individual LANsThree parts to making bridges transparent:1) Forwarding frames2) Learning addresses/host locations3) Spanning tree algorithm78Frame ForwardingA machine with MAC Address lies in the direction of number port of the bridgeFor every packet, the bridge “looks up” the entry for the packets destination MAC address and forwards the packet on that port.»Other packets are broadcast – why?Timer is used to flush old entries88711C98900AA2MAC AddressPortA21032C9A591199A323C908422301B2369011C2695519001190315Age36011611Bridge1329Learning BridgesManually filling in bridge tables?»Time consuming, error-proneKeep track of source address of packets arriving on every link, showing what segment hosts are on»Fill in the forwarding table based on this information9host host host host hosthost host host host hosthosthostBridge10Spanning Tree BridgesMore complex topologies can provide redundancy.»But can also create loops.What is the problem with loops?Solution: spanning tree10host host host host hosthost host host host hosthosthostBridge Bridge16Ethernet SwitchesBridges make it possible to increase LAN capacity.»Packets are no longer broadcasted - they are only forwarded on selected links»Adds a switching flavor to the broadcast LANEthernet switch is a special case of a bridge: each bridge port is connected to a single host.»Can make the link full duplex (really simple protocol!)»Simplifies the protocol and hardware used (only two stations on the link) – no longer full CSMA/CD»Can have different port speeds on the same switch–Unlike in a hub, packets can be stored –An alternative is to use cut through switching1617Ethernet Evolution17Current ImplementationsEthernetConc..RouterServerWANHUBSwitch•Switched solution•Little use for collision domains•80% of traffic leaves the LAN•Servers, routers 10 x station speed•10/100/1000 Mbps, 10gig coming: Copper, Fiber•95% of new LANs are EthernetCSMA - Carrier Sense Multiple AccessCD - Collision DetectionWANLANEthernet or 802.3•A Local Area Network•MAC addressing, non-routable•BUS or Logical Bus topology•Collision Domain, CSMA/CD•Bridges and Repeaters for distance/capacity extension•1-10Mbps: coax, twisted pair (10BaseT)B/REarly Implementations18Ethernet in a campus network18Server100 Mbps links10 Mbps linksServerServerServer100 Mbps links10 Mbps linksServer100 Mbps links10 Mbps linksServerGigabit Ethernet linksGigabit Ethernet linksServer farmDepartment ADepartment BDepartment CHub Hub Hub Ethernet switch Ethernet switch Ethernet switch Switch/router Switch/router Today: x 1019Problem: Bridging WeaknessesDoesn’t handle incompatible LAN technologies» Is interoperable within 802.* standardHow well does it scale?1920Outline BridgingInternetworks»Methods for packet forwarding2021What is an Internetwork?Multiple incompatible LANs can be physically connected by layer 3 switches called routersThe connected networks are called an internetwork»The “Internet” is one (very big & successful) example of an internetwork21hosthost hostLAN 1...hosthost hostLAN 2...router router routerWAN WANLAN 1 and LAN 2 might be completely different, totally incompatible LANs (e.g., Ethernet and ATM)22Logical Structure of Internet»Ad hoc interconnection of networks–No particular topology–Vastly different router & link capacities»Send packets from source to destination by hopping through networks–Router connect one network to another–Different paths to destination may exist22hosthostrouterrouterrouterrouterrouterrouter23Internet Protocol (IP)Hour Glass Model»Create abstraction layer that hides underlying technology from network application software»Make as minimal as possible»Allows range of current & future technologies»Can support many different types of applications23Network technologyNetwork applications email WWW phone...SMTP HTTP RTP...TCP UDP…IP ethernet PPP…CSMA async sonet... copper fiber radio...24Problem: Internetwork DesignHow do I designate a distant host?»Addressing / namingHow do I send information to a distant host?»What gets sent?»What route should it take?Must support:»Heterogeneity LAN technologies»Scalability ensure ability to grow to worldwide scale24hosthost hostLAN 1...hosthost hostLAN 2...router router routerWAN WAN25Getting to a DestinationHow do you get driving directions?IntersectionsroutersRoadslinks/networksRoads change slowly2526Forwarding PacketsTable of virtual circuits »Connection routed through network to set up state»Packets forwarded using connection state Source
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