Bài giảng Mạng máy tính nâng cao - Chapter 06: Distance vector routing protocols

DISTANCE VECTOR ROUTING PROTOCOLSMẠNG MÁY TÍNH NÂNG CAOChapter06Objectives Identify the characteristics of distance vector routing protocols.Describe the network discovery process of distance vector routing protocols using Routing Information Protocol (RIP).Describe the processes to maintain accurate routing tables used by distance vector routing protocols.Identify the conditions leading to a routing loop and explain the implications for router performance.Recognize that distance vector routing protocols are in use todayDistance Vector Routing Protocols Routing Information Protocol (RIP):RFC 1508Hop count is used as the metricMax hop is 15Routing updates are broadcast or multicast every 30 secondsInterior Gateway Routing Protocol (IGRP)Proprietary protocol developed by Cisco.Bandwidth, delay, load and reliability are used to create a composite metric.Routing updates are broadcast every 90 seconds,IGRP is the predecessor of EIGRP and is now obsoleteDistance Vector Routing ProtocolsEnhanced Interior Gateway Routing Protocol (EIGRP)It can perform unequal cost load balancing. It uses Diffusing Update Algorithm (DUAL) to calculate the shortest path. Routing updates are sent only when there is a change in the topology.Distance Vector Technology The Meaning of Distance Vector:A router using distance vector routing protocols knows 2 things:Distance to final destinationVector or directiontraffic should be directedDistance Vector Technology Characteristics of Distance Vector routing protocols:Periodic updatesNeighbors: Routing by rumorBroadcast updates 255.255.255.255Entire routing table is included with routing update Routing Protocol Algorithms At the core of the distance vector protocol is the algorithm. The algorithm is used to calculate the best path then send that information to the neighbors.Routing Protocol Algorithm: Defined as a procedure for accomplishing a certain taskAdvantages and Disadvantages NETWORK DISCOVERY After a cold start and before the exchange of routing information, the routers initially discover their own directly connected networks and subnet masks. Exchange of Routing Information Initial Exchange of Routing Information Convergence Convergence must be reached before a network is considered completely operable Speed of achieving convergence consists of 2 interdependent categories Speed of broadcasting routing information update Speed of calculating routesPeriodic Updates : RIPv1 & IGRP Periodic updates are time intervals in which a router sends out its entire routing table.Changes may occur for several reasons, including:Failure of a linkIntroduction of a new linkFailure of a routerChange of link parametersPeriodic Updates : RIPv1 & IGRP Distance vector routing loop Routing loop is a condition in which a packet is continuously transmitted within a series of routers without ever reaching its destinationDistance vector routing loop Routing loops may be caused by:Incorrectly configured static routesIncorrectly configured route redistributionSlow convergenceIncorrectly configured discard routesRouting loops can create the following issuesExcess use of bandwidthCPU resources may be strainedNetwork convergence is degradedRouting updates may be lost or not processed in a timely mannerProblem: Count to Infinity This is a routing loop whereby packets bounce infinitely around a networkSolution: Setting a maximum Distance Vector routing protocols set a specified metric value to indicate infinityOnce a router “counts to infinity” it marks the route as unreachablePreventing Routing Loops with Holddown Timers Network is unstable: a interface resets as up, then down, then upagain in rapid succession. Using triggered updates, the routers might react too quickly and unknowingly create a routing loop. A routing loop could also be created by a periodic update that is sent by the routers during the instability.Holddown timers prevent routing loops from being created by these conditions. Holddown timers also help prevent the count to infinity condition.Holddown timers allow a router to not accept any changes to a route for a specified period of timeHolddown timers Holddown timers work in the following way:A router receives an update from a neighbor indicating that a network that previously was accessible is now no longer accessible.Holddown timers 2. The router marks the network as possibly down and starts the holddown timerHolddown timers 3. If an update with a better metric for that network is received from any neighboring router during the holddown period, the network is reinstated and the holddown timer is removed.Holddown timers 4. If an update from any other neighbor is received during the holddown period with the same or worse metric for that network, that update is ignored. Thus, more time is allowed for the information about the change to be propagated.Holddown timers 5. Routers still forward packets to destination networks that are marked as possibly down. This allows the router to overcome any issues associated with intermittent connectivity. Split Horizon Rule Split Horizon rule: A router should not advertise a network through the interface from which the update cameFactors used to determine whether to use RIP or EIGRP include Network size Compatibility between models of routers Administrative knowledgeDVRP to day: RIP and EIGRP Routing Information Protocol :RIPIt is a distance-vector routing protocol. Hop count is used as the metric for path selection. If the hop count is greater than 15, the packet will be discarded. By default, routing updates are broadcast every 30 seconds. RIP is a classful routing protocol: Does not send subnet masks in routing updatesRIP implements split horizon and holddown mechanisms to prevent incorrect routing information from being propagated (Loop)Administrative Distance RIP’s default administrative distance is 120. When compared to other interior gateway protocols, RIP is the least-preferred routing protocol. IS-IS, OSPF, IGRP, and EIGRP all have lower default AD values.Configuring RIP Common RIP configuration issues To reduce routing loops and counting to infinity, RIP uses the following techniques:Count-to-infinity Split horizon Holddown counters Triggered updates Common RIP configuration issuesGAD(config-if)#no ip split-horizon turn off split-horzonRouter(config-router)#timers basic update invalid holddown flush change update, default 30s invalid, default 180s holddown, default 180s. flush, default 240s GAD(config-router)#update-timer seconds change the time to update. Default is 30sVerifying RIP configuration Verifying RIP configuration Verifying RIP configuration BHM(config)# show ip routeBHM(config)# show ip protocolsBHM(config)# show running-config BHM(config)# show interface s0/0 BHM(config)# show ip interface s0/0 Troubleshooting RIP update issues BHM(config)# debug ip rip Other commands to troubleshoot RIP:show ip rip database show ip protocols {summary} show ip route debug ip rip {events} show ip interface brief Preventing routing updates through an interface Integrating static routes with RIP GAD(config-router)# redistribute staticTo intergrateing static routes with RIPDefault-information originate command This command is used to specify that the router is to originate default information, by propagating the static default route in RIP update. Default route in RIP v1