CCNA2_M9_Basic_Router_Troubleshooting (1130741)

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CCNA – Semester2Module 9Basic Router TroubleshootingObjectives• Examining the routing table• Network testing• Troubleshooting router issuesExamining the Router TableStatic RouteDynamic RouteDefault 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}• This route can be advertised to other router viadynamic 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 routeDefault RouteDetermining the route administrative distance• If a path has the lowest administrative distance it isinstalled in the routing tableDetermining the route metric• The metric is a value that measures the desirability of aroute.

The smaller the metric number, the better thepath.Determining the last routing update• Use the following commands to find the lastrouting update:– show ip route– show ip protocols– show ip rip databaseObserving multiple paths to destination• Some routing protocols support multiple pathsto the same destination.• Unlike single path algorithms, these multi-pathalgorithms permit traffic over multiple lines,provide better throughput, and are morereliable.Network TestingTroubleshooting stepsStartProblem1. Collect all available information and analyze thesymptoms of failure2.

Localize the problem to within a single networksegment, single module/unit or user3. Isolate the trouble to specific hardware orsoftware within unit/module/user4. Locate and correct the specific problem5. Verify that the problem has been solvedEndNAnotherProblemYLayer 1 errors•Cables– Disconnected cables or Intermittentcable connection– Broken– Cables connected to the wrong ports– wrong cables used (must userollovers, cross-connects, andstraight-through cables correctly)••••Transceiver problemsDCE cable problemsDTE cable problemsDevices turned offLayer 2 errors• Improperly configured serialinterfaces• Improperly configuredEthernet interfaces• Improper encapsulation set(HDLC is default for serialinterfaces)• Improper clock-rate settingson serial interfacesLayer 3 errors• Routing protocol notenabled• Wrong routing protocolenabled• Incorrect IP addresses• Incorrect Subnet Masks• Incorrect DNS to IPbindingsTroubleshooting Router IssuesOverviewTroubleshooting Layer 1 using show interfacesShow interfaces serial: Interface status• Refers to the hardware layer and essentially reflectswhether CD signal is received from the other end.• If the line is down, a problem may exist with the cabling,equipment somewhere in the circuit may be powered offor malfunctioning, or one end may be administrativelydown.• If the interface is administratively down it has beenmanually disabled in the configuration.Show interfaces serial: Carrier transitions• An increasing number of carrier transitionscounts on a serial link may indicate one or moreof the following problems:– Line interruptions due to problems in the service providernetwork.– Faulty switch, DSU, or router hardware.Show interfaces serial: Error• If number of input errors is increasing, there are severalpossible sources of those errors.

Some of these related toLayer 1 problems are:––––––Faulty telephone company equipmentNoisy serial lineIncorrect cable or cable lengthDamaged cable or connectionDefective CSU or DSUDefective router hardwareShow interfaces serial:interface reset• Another area to examine is number of interface resets.These are the result of too many missed keepalives :– Bad line causing carrier transitions– Possible hardware problem at the CSU, DSU, or switch• If carrier transitions and interface resets are increasingor if input errors are high while interface resets areincreasing, the problem is likely to be a bad link ordefective CSU or DSU.Clear Counter• Use the clear counters command to reset thecounters to zero.• These counters should always be cleared afteran interface problem has been corrected.• Starting from zero gives a better picture of thecurrent status of the network and will help verifythat the issue has indeed been corrected.Show interface serialTroubleshooting Layer 2 using show interfaces• If the interface misses three consecutive keepalives, theline protocol is marked as down.• When the line is down, the protocol is always down.• If the interface is up and the line protocol is down, aLayer 2 problem exists.

Among the possible causes are:– No keepalives– No clock rate– Mismatch in encapsulation typeTroubleshooting using show cdp• The output from the show cdp neighbors commanddisplays information about directly connectedneighbors.Troubleshooting using traceroute• Traceroute also provides information indicating therelative performance of links. The round trip time(RTT) is the time required to send an echo packetand get a response. This is useful for anapproximate idea of the delay on the link.Troubleshooting routing issues•Show ip route displays the contents of the IP routing table:–––––•Routing methodAdministrative distanceRoute metricNext-hop, ougoing interfaceTime past since the last updateThe show ip protocols command displays values about IProuting protocol information on the entire router:––––––which protocols are configuredwhich networks are being advertisedwhich interfaces are sending updatessources of routing updatestimers, filters, route summarization, route redistributionother parameters that are specific to each routing protocol.Troubleshooting using show controllers serial• Useful to determine the type of cable connectedwithout inspecting the cables and clock on thelink.• Other than the cable type, most of this output isinternal technical detail regarding the controllerchip status.Introduction to debug• debug commands assist in the isolation ofprotocol and configuration problems and isused to display dynamic data and events.• show commands only display static information.• Debug reduce performance cost, produce highprocessor overhead that may disrupt normalrouter operation.

For this reason, debug shouldbe used conservatively.Debugging• By default, the router sends the debug outputand system messages to the console.• The debug output and system messages can beredirected to the remote terminal by issuing theterminal monitor command.• “Never debug any activity on the port where thesession is established”.Debugging• timestamps command put a timestamp on adebug message.(config)#service timestamps debug uptime• no debug all or undebug all: turn off alldebugging.• The “no” form of the command disable aspecific debugging• show debugging to view all current debuggingLogging• (config)# no logging consolestop sending information to console• (config-line)# logging synchronoussending information to console withoutinterrupting current command lineSummary••••Determine route parametersTroubleshooting OSI layersTroubleshooting with show interfaceTroubleshooting with show ip route and show ipprotocol• Troubleshooting with traceroute• Troubleshooting with debugLab TopologyRequirement•••••••Assign IP address to all nodes in the networkSerial connection between Office2 and Branch1 use clock rate 128k andPPP encapsulation with network 203.162.120.0/24, others use clock rate64k and HDLC encapsulationConfig routing using proper routing methodBe able to telnet to and config all routers using nameAll access to router must be authenticated (vty, console, aux)Interfaces should have meaningful descriptions, bannerBe able to ping to domains on the Internet using DNS server203.162.10.98CCNA2 – Module9.

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