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Medium-Sized Routed Network Construction
Reviewing Routing Operations
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Static Route Uses a route that a
network administrator enters into the router manually
Dynamic Route Uses a route that a
network routing protocol adjusts automatically for topology or traffic changes
Static vs. Dynamic Routes
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What Is a Dynamic Routing Protocol?
Routing protocols are used between routers to determine paths to remote networks and maintain those networks in the routing tables.After the path is determined, a router can route a routedprotocol to the learned networks.
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nomous system is a collection of networks within on administrative domain.gateway protocols operate within an autonomous s
r gateway protocols connect different autonomous
Autonomous Systems: Interior and Exterior Routing Protocols
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Classes of Routing Protocols
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Selecting the Best Route Using Metrics
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Administrative Distance: Ranking Routing Sources
Routers choose the routing source with the best administrative distance: OSPF has an administrative distance of 110. EIGRP has an administrative distance of 90.
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Distance Vector Routing Protocols
Routers pass periodic copies of their routing table to neighboring routers and accumulate distance vectors.
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Sources of Information and Discovering Routes
Routers discover the best path to destinations from each neighbor.
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Maintaining Routing Information
Updates proceed step by step from router to router.
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Inconsistent Routing Entries:Counting to Infinity and Routing Loops
Each node maintains the distance from itself to each possible destination network.
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Counting to Infinity
Slow convergence produces inconsistent routing.
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Counting to Infinity (Cont.)
Router C concludes that the best path to network 10.4.0.0 is through router B.
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Counting to Infinity (Cont.)
Router A updates its table to reflect the new but erroneous hop count.
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Counting to Infinity (Cont.)
The hop count for network 10.4.0.0 counts to infinity.
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Solution to Counting to Infinity:Defining a Maximum
A limit is set on the number of hops to prevent infinite loops.
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Routing Loops
Packets for network 10.4.0.0 bounce (loop) between routers B and C.
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Solution to Routing Loops: Split Horizon
It is never useful to send information about a route back in the direction from which the original information came.
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Solution to Routing Loops:Route Poisoning and Poison Reverse
Routers advertise the distance of routes that have gone down to infinity.
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Solution to Routing Loops:Route Poisoning and Poison Reverse (Cont.)
Poison reverse overrides split horizon.
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Solution to Routing Loops: Hold-Down Timers
The router keeps an entry for the possibly down state in the network,allowing time for other routers to recompute for this topology change.
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Triggered Updates
The router sends updates when a change in its routing table occurs.
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Eliminating Routing Loops
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Eliminating Routing Loops (Cont.)
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Eliminating Routing Loops (Cont.)
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Link-State Routing Protocols
After an initial flood of LSAs, link-state routers pass small, event-triggered link-state updates to all other routers.
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OSPF Hierarchical Routing
Consists of areas and autonomous systems Minimizes routing update traffic
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Link-State Routing Protocol Algorithms
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Benefits of link-state routing: Fast convergence: Changes are reported immediately by the affected source
Robustness against routing loops: Routers know the topology Link-state packets are sequenced and acknowledged
Hierarchical network design enables optimization of resources.
Drawbacks of link-state routing: Significant demands for resources: Memory (three tables: adjacency, topology, forwarding) CPU (Dijkstras algorithm can be intensive, especially when there are many
instabilities) Requires very strict network design Configuration can be complex when tuning various parameters and
when design is complex
Benefits and Drawbacks of Link-State Routing
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Summary
Dynamic routing requires administrators to configure either a distance vector or link-state routing protocol. Distance vector routing protocols incorporate solutions such as
split horizon, route poisoning, and hold-down timers to prevent routing loops. Link-state routing protocols scale to large network infrastructures
better than distance vector routing protocols, but they require more planning to implement.
Medium-Sized Routed Network Construction Static vs. Dynamic RoutesWhat Is a Dynamic Routing Protocol?Autonomous Systems: Interior and Exterior Routing ProtocolsClasses of Routing ProtocolsSelecting the Best Route Using MetricsAdministrative Distance: Ranking Routing SourcesDistance Vector Routing ProtocolsSources of Information and Discovering RoutesMaintaining Routing InformationInconsistent Routing Entries:Counting to Infinity and Routing LoopsCounting to InfinityCounting to Infinity (Cont.)Counting to Infinity (Cont.)Counting to Infinity (Cont.)Solution to Counting to Infinity:Defining a MaximumRouting LoopsSolution to Routing Loops: Split HorizonSolution to Routing Loops:Route Poisoning and Poison ReverseSolution to Routing Loops:Route Poisoning and Poison Reverse (Cont.)Solution to Routing Loops: Hold-Down TimersTriggered UpdatesEliminating Routing LoopsEliminating Routing Loops (Cont.)Eliminating Routing Loops (Cont.)Link-State Routing ProtocolsOSPF Hierarchical RoutingLink-State Routing Protocol AlgorithmsBenefits and Drawbacks of Link-State Routing Summary