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1、Medium-Sized Routed Network Construction,Reviewing Routing Operations,Static RouteUses a route that a network administrator enters into the router manually,Dynamic RouteUses a route that a network routing protocol adjusts automatically for topology or traffic changes,Static vs.Dynamic Routes,What Is
2、 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 routed protocol to the learned networks.,An autonomous system is a collection of networks
3、within 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 Protocols,Classes of Routing Protocols,Selecting the Best Route Using Metrics,Adminis
4、trative 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.,Distance Vector Routing Protocols,Routers pass periodic copies of their routing table to neighboring
5、 routers and accumulate distance vectors.,Sources of Information and Discovering Routes,Routers discover the best path to destinations from each neighbor.,Maintaining Routing Information,Updates proceed step by step from router to router.,Inconsistent Routing Entries:Counting to Infinity and Routing
6、 Loops,Each node maintains the distance from itself to each possible destination network.,Counting to Infinity,Slow convergence produces inconsistent routing.,Counting to Infinity(Cont.),Router C concludes that the best path to network 10.4.0.0 is through router B.,Counting to Infinity(Cont.),Router
7、 A updates its table to reflect the new but erroneous hop count.,Counting to Infinity(Cont.),The hop count for network 10.4.0.0 counts to infinity.,Solution to Counting to Infinity:Defining a Maximum,A limit is set on the number of hops to prevent infinite loops.,Routing Loops,Packets for network 10
8、.4.0.0 bounce(loop)between routers B and C.,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.,Solution to Routing Loops:Route Poisoning and Poison Reverse,Routers advertise the distance of rout
9、es that have gone down to infinity.,Solution to Routing Loops:Route Poisoning and Poison Reverse(Cont.),Poison reverse overrides split horizon.,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.,Triggered Updates,The router sends updates when a change in its routing table occurs.,
