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Routing and Routing Protocols Dynamic Routing Overview

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  • Routing and Routing ProtocolsDynamic Routing Overview

  • Dynamic RoutingDynamic routing allows the network to adjust to topology changes without administrative intervention.The routers exchange information to tell each other when a network link is no longer available.They also exchange information to notify when the link is working

  • Routed and Routing ProtocolsRouting Protocol:Allow one router to share information with other routers regarding the networks it knows about as well as its proximity to other routers.The information received using a routing protocol is used to build and maintain a routing table.Routed Protocol:Used to direct user traffic.Provide enough information in its network layer address to allow a packet to be forwarded from one host to another

  • Routing ConvergenceThe routing protocol learns all available routes, places the best routes into the routing table, and removes routes when they are no longer valid.Link failure, network modificationWhenever the topology of a network changes the routers need to communicate that change.When all routers in a network are operating with the same knowledge, the network is said to have converged.The sooner the routers converge, the quicker the routers will be able to make correct routing decisions.

  • Routing Protocol ClassificationRouting algorithms can be classified into one of two categories:Distance vector Determines the direction (vector) and distance to any link in the networkLink-state Also called shortest path first, recreates the exact topology of the entire network.

  • Distance Vector FeaturesPass periodic copies of the entire routing table from router to routerAlso known as Bellman-Ford algorithmsSimilar to the signs found at a highway intersection.A sign points towards a destination and indicates the distance to the destination.Further down the highway, another sign points toward the destination, but now the distance is shorter.

  • Distance Vector RoutingEach router begins by identifying its own neighbors.The interface that leads to each directly connected network has a distance of 0.Router W learns about other networks based on information received from Router XRouter W adds a distance vector number which increases the distance vector.Routers do not know the exact topology of an network as each router only sees its neighbor routers.

  • Link-State FeaturesAlso known as Dijkstras algorithm or as SPF (shortest path first) algorithmsMaintain a database of topology informationWhich routers are connected to each otherWhich routers connect to outside networksWhich routers are inside the network

  • More Link-State FeaturesLink-state advertisements (LSAs)A small packet of routing information that is sent between neighboring routers.Topological databaseA collection of information gathered from LSAsThe routers map of the entire networkSPF algorithmA calculation performed on the topological database resulting in the SPF tree, showing the best paths to destination networksRouting tablesA list of the known paths and interfaces

  • Link-State Network DiscoveryLSAs are exchanged between directly connected routers with information about directly connected networks.These LSAs are accumulated on each router and a topological database is constructed.The SPF algorithm uses this database to calculate shortest path.It then builds a tree, with itself as the root, consisting of all possible paths to each network.It sorts these paths Shortest Path First (SPF).The router lists the best paths and ports to these destination networks in the routing table.

  • Link-State RoutingThe router that first becomes aware of a topology change forwards the information to all neighboring routersEach router keeps track of its:Neighbor routersNeighbors nameLink (interface) statusCost of the link to the neighborConstruct an LSA packet that lists its neighbors detailsSend out this packetWhen it receives an LSA packet it records it in its databaseDraws a topology map with the accumulated LSAsEach time an LSA causes a change to the database, the SPF algorithm is recalculatedThe routing table is updated

  • Link-State ConcernsCPU Usage: more processing requirements than distance-vector protocolsRAM: holds info from various databases, holds the topology tree, and the routing tableBandwidth during the initial discovery process is heavy because LSA packets are sent to all neighboring routersAfter this initial flooding, bandwidth usage is minimal since LSAs are only sent when topology changes

  • Link-State vs. Distance Vector Routing Protocols

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