《路由原理基础》PPT课件.ppt

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1、 2007 Cisco Systems,Inc.All rights reserved.ICND2 v1.03-1Medium-Sized Routed Network Construction Reviewing Routing Operations 2007 Cisco Systems,Inc.All rights reserved.ICND2 v1.03-2Static RouteUses a route that a network administrator enters into the router manuallyDynamic RouteUses a route that a

2、 network routing protocol adjusts automatically for topology or traffic changesStatic vs.Dynamic Routes 2007 Cisco Systems,Inc.All rights reserved.ICND2 v1.03-3What Is a Dynamic Routing Protocol?Routing protocols are used between routers to determine paths to remote networks and maintain those netwo

3、rks in the routing tables.After the path is determined,a router can route a routed protocol to the learned networks.2007 Cisco Systems,Inc.All rights reserved.ICND2 v1.03-4An autonomous system is a collection of networks within a common administrative domain.Interior gateway protocols operate within

4、 an autonomous system.Exterior gateway protocols connect different autonomous systems.Autonomous Systems:Interior and Exterior Routing Protocols 2007 Cisco Systems,Inc.All rights reserved.ICND2 v1.03-5Classes of Routing Protocols 2007 Cisco Systems,Inc.All rights reserved.ICND2 v1.03-6Selecting the

5、Best Route Using Metrics 2007 Cisco Systems,Inc.All rights reserved.ICND2 v1.03-7Administrative 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 90.2007 Cisco Sys

6、tems,Inc.All rights reserved.ICND2 v1.03-8Distance Vector Routing ProtocolsRouters pass periodic copies of their routing table to neighboring routers and accumulate distance vectors.2007 Cisco Systems,Inc.All rights reserved.ICND2 v1.03-9Sources of Information and Discovering RoutesRouters discover

7、the best path to destinations from each neighbor.2007 Cisco Systems,Inc.All rights reserved.ICND2 v1.03-10Maintaining Routing InformationUpdates proceed step by step from router to router.2007 Cisco Systems,Inc.All rights reserved.ICND2 v1.03-11Inconsistent Routing Entries:Counting to Infinity and R

8、outing LoopsEach node maintains the distance from itself to each possible destination network.2007 Cisco Systems,Inc.All rights reserved.ICND2 v1.03-12Counting to InfinitySlow convergence produces inconsistent routing.2007 Cisco Systems,Inc.All rights reserved.ICND2 v1.03-13Counting to Infinity(Cont

9、.)Router C concludes that the best path to network 10.4.0.0 is through router B.2007 Cisco Systems,Inc.All rights reserved.ICND2 v1.03-14Counting to Infinity(Cont.)Router A updates its table to reflect the new but erroneous hop count.2007 Cisco Systems,Inc.All rights reserved.ICND2 v1.03-15Counting

10、to Infinity(Cont.)The hop count for network 10.4.0.0 counts to infinity.2007 Cisco Systems,Inc.All rights reserved.ICND2 v1.03-16Solution to Counting to Infinity:Defining a MaximumA limit is set on the number of hops to prevent infinite loops.2007 Cisco Systems,Inc.All rights reserved.ICND2 v1.03-17

11、Routing LoopsPackets for network 10.4.0.0 bounce(loop)between routers B and C.2007 Cisco Systems,Inc.All rights reserved.ICND2 v1.03-18Solution to Routing Loops:Split HorizonIt is never useful to send information about a route back in the direction from which the original information came.2007 Cisco

12、 Systems,Inc.All rights reserved.ICND2 v1.03-19Solution to Routing Loops:Route Poisoning and Poison ReverseRouters advertise the distance of routes that have gone down to infinity.2007 Cisco Systems,Inc.All rights reserved.ICND2 v1.03-20Solution to Routing Loops:Route Poisoning and Poison Reverse(Co

13、nt.)Poison reverse overrides split horizon.2007 Cisco Systems,Inc.All rights reserved.ICND2 v1.03-21Solution to Routing Loops:Hold-Down TimersThe router keeps an entry for the“possibly down”state in the network,allowing time for other routers to recompute for this topology change.2007 Cisco Systems,Inc.All rights reserved.ICND2 v1.03-22Triggered UpdatesThe router sends updates when a change in its routing table occurs.2007 Cisco Systems,Inc.All rights reserved.ICND2 v1.03-23