CCNA2_M7_Distance_vector_Routing_Protoco l (1130735)

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CCNA – Semester2Module 7Routing and Routing ProtocolsObjectives• Distance vector routing• RIP• IGRPDistance Vector RoutingDistance vector routing updates• Routing table updatesoccur periodically orwhen the topology in adistance vectorprotocol networkchanges.• Distance vectoralgorithms call for eachrouter to send its entirerouting table to each ofits adjacent neighbors.Distance vector routing loop issuesRouting decisions based on incorrect information results in packets takingpaths that return them to already visited routers••••••Before network 1 goes down B & D goes via A to this network, C may go via Bto this networkWhen net 1 is down, A informs B & D about it, C does not yet knowC sends update to D informing that it canget to net 1 via BD updates this incorrect route overwritingthe previously correct update from AD sends update to A followed by A sendsupdate to B causing both A & B to alsoupdate the incorrect routeConsequence: Packet originating from Cdestined to net 1 will loop:CÆBÆAÆDÆCProblem: Count to infinity•Large number of routing loops results in forever-increasing value of routingmetric, e.g.

hop count, or count to infinityLoop prevention: Maximum metricLoop prevention: Split horizon• The routing protocol advertises routes out aninterface only if they were not learned from updatesentering that interface.Route poisoning• To overcome large routing loops• Set the hop count to one more than the maximum.Avoiding routing loops with triggered updates• Triggered update is sent immediately in response tosome change in the routing table.• Triggered updates, used in conjunction with routepoisoning, ensure that all routers know of failed routesbefore any holddown timers can expire.Loop prevention: Hold-down timer•If at any time during hold-down timer an update is received from– The same neighbor indicating that the network is again accessible, the router marksthe network as accessible and removes the hold-down timer.– A different neighboring router with a better metric than originally recorded for thenetwork, the router marks the network as accessible and removes the hold-downtimer.– A different neighboring router with a poorer metric, the update is ignored.Routing Information Protocol- RIP -Classful Routing vs.

Classless Routing• Classful routing must be used with classfulnetwork or subnets with the same subnetmask.It does not include subnetmask in its routingtable (FLSM)• Classless routing allows updating routes withdifferent subnetmasks therefore it includesubnetmask in routing update (VLSM)RIP• Distance vector protocol• Hop count is used as the metric for pathselection• Maximum hop count is 15• Routing update interval is 30sRIPv1 and RIPv2• RIP Version 1 is a Classful Routing Protocol, RIP Version 2is a Classless Routing Protocol. RIP v2 enhancementsinclude:– Ability to carry additional packet routing information.– Authentication mechanism to secure table updates.– Supports variable length subnet masking (VLSM).Configuring RIPip classless command•If a packet is destined for an unknown subnet of a network that hasdirectly connected subnets on router, it will be discarded.

The ipclassless command allows it to be forward over default route.Common RIP configuration issuesCommandDescription(config-router)#timers basicupdate invalid holddown flush[sleeptime]Change routing update timerparameters(conf-router)#passive-interfacetype numberStop sending routing update outthis interface(config-router)#redistributestaticpropagate static routes over routingupdates#clear ip route *Clear routing tableConfigure RIPv1 and RIPv2Show ip protocolsSHOW IP ROUTE commandShow ip routeR183.8.0.128 [120/1] via 183.8.128.130 , 00:00:17 , Serial0via 183.8.64.130 , 00:00:17 , Serial1• 183.8.0.128 : destination network number• [120/1] : [administrative distance/hop count]• 183.8.128.130: next hop IP address• 00:00:17 : latest update was 17 seconds ago• Serial0 : outgoing interface is Serial0• Second line : load sharingTroubleshooting RIP update issues•Other commands to troubleshoot RIP:–––––show ip rip databaseshow ip protocolsshow ip routedebug ip rip {events}show ip interface briefLoad Balancing with RIPRouter route selection• Among multiple routes to a specific network:– Use the route with the lowest AD.– If AD are equal, use the route with the lowest metric– If AD and metric are equal, proceed load sharingDefault Route with RIP• A router running RIP can receive a default route via anupdate from another router running RIP.• Another option is for the router to generate the defaultroute itself.Interior Gateway Routing Protocol- IGRP -IGRP• IGRP is a distance vector routing protocoldeveloped by Cisco.• IGRP sends routing updates at 90 secondintervals, advertising networks for a particularautonomous system.IGRP features• Key design characteristics of IGRP are a follows:– The versatility to automatically handle indefinite, complex topologies– The flexibility needed to segment with different bandwidth and delaycharacteristics– Scalability for functioning in very large networksIGRP Metrics• The metrics that IGRP uses are:– Bandwidth – The lowest bandwidth value in the path– Delay – The cumulative interface delay along the path– Reliability – The reliability on the link towards the destination as determinedby the exchange of keepalives– Load – The load on a link towards the destination based on bits per second– MTU – The Maximum Transmission Unit value of the path.• Default metric use only bandwidth and delay• Hop-count is used as a secondary metric to prevent loopSHOW IP PROTOCOLS commandIGRP routes• IGRP advertises three types of routes:– Interior– System– ExteriorIGRP RoutesAS = 100Configuring IGRPAS = 200Verifying IGRP configuration• Additional commands for checking IGRPconfiguration are as follows:– show interface interface [brief]– show running-config– show ip protocolsTroubleshooting IGRP• The following commands are useful whentroubleshooting IGRP:––––––show ip protocolsshow ip routedebug ip igrp eventsdebug ip igrp transactionspingtracerouteDefault static ip route• Create a default route on the router which isbeing configured:ip route 0.0.0.0 0.0.0.0 {ip_next_hop| interface}• Static routes can be advertised to other routervia dynamic routing update using:redistribute static• IGRP has no concept of route 0.0.0.0/0Default Network• Tell other router to choose the network as thedefault route:ip default-network {network_number}• Network_number must be a known classfullnetwork (taken from routing table)• IGRP may use this with default static ip route• For a network, default route configuration isonly configured on gateway routerDefault RouteSummary•••••••How routing loops occur in distance vectorPreventing routing loop solutionConfiguring RIPIGRP featuresIGRP metricsIGRP routesConfiguring IGRPLab Topology 1 - RIPF0/0F0/0Site1Site2S0/0S0/1Lab_ELab_BS0/1S0/0S0/0S0/1S0/1Lab_CLab_AS0/0S0/0F0/0F0/0S0/1Lab_FLab_DF0/0F0/0PortLab_ALab_BLab_CLab_DLab_ELab_FS0/0201.10.11.1/24199.6.13.1/24204.204.7.1/24192.168.3.1/2434.10.120.1/16N/AS0/1N/A201.10.11.2/24199.6.13.2/24204.204.7.2/24192.168.3.2/2434.10.210.5/16F0/0172.17.11.1/16172.16.11.1/16172.17.11.2/16172.18.11.1/16172.16.11.2/16172.18.11.2/16Lab Topology 2 – 2 IGRP AS with default routesInternet172.16.2.1/16IGRP 100Lab_B172.16.2.2/16Lab_E172.20.0.0/16Lab_AIGRP 200192.168.1.0/24Lab_CLab_DLab_FCCNA2 – Module7.

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