Static versus Dynamic Routes Static Route Uses a protocol route that a network administrators enters into the router Static Route Uses a protocol route.

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  • Static versus Dynamic RoutesStatic RouteUses a protocol route that a network administrators enters into the router Dynamic RouteUses a route that a network routing protocol adjusts automatically for topology or traffic changes

  • Static Route ExampleABPoint-to-point or circuit-switched connection Only a single network connection with no need for routing updateStub NetworkFixed route to address reflects administrators knowledge

  • Default Route ExampleUse if next hop is not explicitly listed in the routing tableABC192. XInternetRouting TableNo entry for destination netTry router B deafult route

  • Adapting to Topology ChangeCBADXCan alternate route substitute for a failed route ?

  • Dynamic Routing OperationsRouting protocol maintains and distributes routing information


    Routing ProtocolRouting ProtocolRoutingtableRoutingtable

  • Representing Distance with MetricsInformation used to select the best path for routingBAE1E16464BandwidthDelayLoadReliabilityHop countTicksCost

  • Classes of Routing ProtocolsCBADCBADDistance VektorHybrid RoutingLink State

  • One Issue: Time to ConvergenceConvergence occurs when all routers use a consistent perspective of network topologyAfter a topology changes, routers must recompute routes, which disrupts routingThe process and time required for router reconvergence varies in routing protocols

  • Distance Vector ConceptPass periodic copies of routing table to neighbor routers and accumulate distance vectorsCBADDCBARoutingTableRoutingTableRoutingTableRoutingTable

  • Distance Vector Network DiscoveryBACWXYZRouting TableX 0 Y 0 Z 1 W 1

    Routing TableY 0 Z 0 X 1 W 2

    Routers discover the best path to destinations from each neighborRouting tblaW 0 X 0 Y 1 Z 2

  • Distance Vector Topology ChangesUpdates proceed step-by-step from router to routerBATopologyChangeCausesRoutingTableUpdateProcess to Update This Routing TableRouter ASends Out ThisUpdatedRoutingTableProcess to Update This Routing Table

  • Problem: Routing LoopsAEDCBX1Alternate Route:Network 1, Distance 3Network 1, UnreachableAlternate Route: Use ANetwork 1, Distance 4Alternate routes, slow convergence, inconsistent routingNetwork 1Down

  • Problem:Counting to InfinityAEDCBX1Network 1, Distance 5Network 1, Distance 7Network 1, Distance 4Network 1, Distance 6Routing loops increment the distance vectorNetwork 1Down

  • Routing TableMaximum metric is 16Network 1 is UnreachableSolution: Defining a MaximumAEDCBX1Network 1, Distance 12Network 1, Distance 14Network 1, Distance 15Network 1, Distance 13Specify a maximum distance vector metric as infinityNetwork 1Down

  • Network 1, unreachableSolution: Split HorizonIf you learn a protocols route on an interface, do not send information about that route back out that interfaceAEDCBX1Network 1DownB:Do not update router Aabout routes to network 1D: Do not update router Aabout routes to network 1

  • Network 1route to network 1has infinite CostSolution: Route PoisoningRouter keeps an entry for the network down state, allowing time for other routers to recompute for this topology changeAEDCBX1Network 1Down

  • Solution: Hold-Down TimersRouters ignore network update information for some period AEDCB?,X1Network 1 DownUpdate afterHold-Down TimeNetwork 1DownUpdate afterHold-Down TimeUpdate afterHold-Down TimeUpdate afterHold-Down Time

  • Link-State ConceptAfter initial flood, pass small event-triggered link-state updates to all other routersADCBLink-State PacketsTopologicalDatabaseShortest Path First TreeRoutingTableSPFAlgorithm

  • Link-State Network DiscoveryBACWXYZLink-State PacketW 0 X 0

    Link-State PacketX 0Y 0

    Link-State PacketY 0 Z 0

    TopologicalDatabaseTopologicalDatabaseTopologicalDatabaseA RoutingTableB RoutingTableC RoutingTableSPF TreeSPF TreeSPF TreeRouters calculate the shortest path to destinations in paralell

  • Link-State Topology ChangesTopology Change in Link- State UpdateProcess to Update This Routing TableUpdate processes proceed using the same link-state updateProcess to Update This Routing TableProcess to Update This Routing Table

  • Link-State ConcernsTopologicalDatabaseRoutingTableSPF TreeProcessing and memory required for link-state routingBandwidth consumed for initial link state flood

  • Problem: Link-State UpdatesADCBNetwork 1, Back Up NowNetwork 1, UnreachableNetwork 1, UnreachableNetwork 1, UnreachableX,okSlow path updateSlow path update arriveslastNetwork 1goes down then comes upFast path updates arrive firstWhich SPF tree to use for routing?Unsynchronized updates, inconsistent path decisions

  • Link-State Update Problems (cont.)Synchronizing large networks-which network topology updates are correct?Router startup-order of start alters the topology learnedPartitioned regions-slow updating part separated from fast updating part

  • Solution: Link-State MechanismsReduce the need for resourcesDampen update frequncyTarget link-state updates to multicastUse link-state area hierarchy for topologyExchange route summaries at area bordersCoordinate link-state updatesUse time stampsUpdate numbering and countersManage partitioning using an area hierarchy

  • Comparing Distance Vector Routing to Link-State Routing Distance Vector Link-State Views net topology from Gets common view ofneighbors perspective entire network topologyAdds distance vectors Calculates the shortestfrom router to router path to other routersFrequent, periodic updates: Event-triggered updates:slow convergence faster convergencePasses copies of routing Passes link-state routing updatestable to neighbor routers to other routers

  • Hybrid RoutingChoose a routing path basedon distance vectors

    Convergence rapidly usingchange-basedupdates

    Balanced Hybrid RoutingShare attributes of both distance-vector and link-state routingEIGRP


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