网络方面知识ccna等中英文icnd210s03l

1、Medium-Sized Routed Network Construction Reviewing Routing Operations Static RouteUses a route that a network administrator enters into the router manuallyDynamic RouteUses a route that a network routing protocol adjusts automatically for topology or traffic changesStatic vs. Dynamic RoutesWhat Is a

2、 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 routed protocol to the learned networks.An autonomous system is a collection of networks wit

3、hin a common administrative domain.Interior gateway protocols operate within an autonomous system.Exterior gateway protocols connect different autonomous systems.Autonomous Systems: Interior and Exterior Routing ProtocolsClasses of Routing ProtocolsSelecting the Best Route Using MetricsAdministrativ

4、e Distance: Ranking Routing SourcesRouters choose the routing source with the best administrative distance:OSPF has an administrative distance of 110.EIGRP has an administrative distance of 90.Distance Vector Routing ProtocolsRouters pass periodic copies of their routing table to neighboring routers

5、 and accumulate distance vectors.Sources of Information and Discovering RoutesRouters discover the best path to destinations from each neighbor.Maintaining Routing InformationUpdates proceed step by step from router to router.Inconsistent Routing Entries:Counting to Infinity and Routing LoopsEach no

6、de maintains the distance from itself to each possible destination network.Counting to InfinitySlow convergence produces inconsistent routing.Counting to Infinity (Cont.)Router C concludes that the best path to network is through router B.Counting to Infinity (Cont.)Router A updates its tab

7、le to reflect the new but erroneous hop count.Counting to Infinity (Cont.)The hop count for network counts to infinity.Solution to Counting to Infinity:Defining a MaximumA limit is set on the number of hops to prevent infinite loops.Routing LoopsPackets for network bounce (loop) be

8、tween routers B and C.Solution to Routing Loops: Split HorizonIt is never useful to send information about a route back in the direction from which the original information came.Solution to Routing Loops:Route Poisoning and Poison ReverseRouters advertise the distance of routes that have gone down t

9、o infinity.Solution to Routing Loops:Route Poisoning and Poison Reverse (Cont.)Poison reverse overrides split horizon.Solution to Routing Loops: Hold-Down TimersThe router keeps an entry for the “possibly down” state in the network, allowing time for other routers to pute for this topology change.Tr

10、iggered UpdatesThe router sends updates when a change in its routing table occurs.Eliminating Routing LoopsEliminating Routing Loops (Cont.)Eliminating Routing Loops (Cont.)Link-State Routing ProtocolsAfter an initial flood of LSAs, link-state routers pass small, event-triggered link-state updates t

11、o all other routers.OSPF Hierarchical RoutingConsists of areas and autonomous systemsMinimizes routing update trafficLink-State Routing Protocol AlgorithmsBenefits of link-state routing:Fast convergence: Changes are reported immediately by the affected sourceRobustness against routing loops:Routers

12、know the topologyLink-state packets are sequenced and acknowledgedHierarchical 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, especiall

13、y when there are many instabilities)Requires very strict network design Configuration can be complex when tuning various parameters and when design is complexBenefits and Drawbacks of Link-State Routing SummaryDynamic 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 poison