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Basic Router Troubleshooting: IP Route and Gateway of Last Resort

Learn how to troubleshoot basic router issues using the "show ip route" command, determine the default gateway, and use ping to test network connectivity. Understand the advantages and disadvantages of static and dynamic routing.

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Basic Router Troubleshooting: IP Route and Gateway of Last Resort

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  1. CCNA 2 v3.1 Module 9

  2. Basic Router Troubleshooting CCNA 2, Module 9

  3. Router#show ip route • Displays the content of the IP routing table • Entries for all known networks and subnets • Code that indicates how network was learned

  4. New Routes can be Added • Statically • Administrator manually defines routes • Routes do not change until administrator manually programs the changes • Dynamically • Routing protocol exchange routing information • Router independently selects the best path • Routes change automatically as neighboring routers update each other with new information

  5. Static Routing Advantages • Lower processor overheads • Lower memory overheads • No bandwidth utilization • Secure operation • Predictability Static Routing Disadvantages • High maintenance configuration • No adaptability to change

  6. Dynamic Routing Advantages • High degree of adaptability • Low maintenance configuration Dynamic Routing Disadvantages • Increased Processor overheads • Increased memory utilization • High bandwidth utilization

  7. Gateway of Last Resort (Default Gateway) • Used when the router is unable to match a destination network • keep routing tables as lean as possible • Router can forward packets destined to any Internet host without having to maintain entry for every Internet network • Configured by administrator • Router(config)#ip default network <network no> Or Router(config)#ip route 0.0.0.0 0.0.0.0 <next-hop or outgoing port> • To find out the default route use show ip route

  8. Route Source and Destination • Path determination occurs at network layer • The network layer provides best-effort, end-to-end, packet delivery across interconnected networks • Path determination enables a router to • Evaluate the available paths to a destination • Establish the preferred handling of a packet • Uses the IP routing table to do this • After the router determines which path to use • it takes the packet from one interface and forwards it to another interface (switching)

  9. Determine L2 and L3 Address • L3 address • Used to route the packet from the source network to the destination network • L2 address • Delivery within a network • To get packets from one router to the next • The IP source and destination headers do not change

  10. Do interactive media lab 9.1.4

  11. Determining Route Administrative Distance • Administrative distance of the route • Used by router to decide the best path to the destination • A number that measures the trustworthiness • Lower administrative distance is more trustworthy the source • Default administrative distance • RIP - 120 • IGRP -100 • OSPF – 110 • A route is not installed in the routing table if the administrative distance from another source is lower

  12. Determining Route Metrics • metrics • Determine the best route to a destination • Measures the desirability of a route • The smaller the metric value the better the path • RIP – hop count • IGRP – bandwidth, delay, (reliability, load) • Static values • bandwidth and delay don’t change • Dynamic values • Reliability and load change between interfaces

  13. Determining the Route Next Hop When a router receives an incoming packet Router checks the destination address Router attempts to associate this address with a next hop

  14. Determine the last Routing Update • Show ip route • Show ip route <network address> • Show ip protocols • Show ip rip database

  15. Observing Multiple Paths to Destinations • Some routing protocols support multiple paths to the same destination • Multi-path algorithms • permit traffic over multiple lines • provide better throughput • more reliable • IGRP uses load balancing • Show ip route

  16. Network Testing Work tests up to layer 7 Start testing network at Layer 1

  17. Layer 1 Errors • Broken cables • Disconnected cables • Cables connected to the wrong ports • Intermittent cable connection • Wrong cables used for the task at hand • rollovers, crossover and straight-through cables • Transceiver problems • DCE cable problems • DTE cable problems • Devices turned off Indicator lights are a useful tool for troubleshooting

  18. Layer 2 Errors • Improperly configured serial interfaces • Improperly configured Ethernet interfaces • Improper encapsulation set (HDLC is default for serial interfaces) • Improper clockrate settings on serial interfaces • Network interface card (NIC) problems

  19. Layer 3 Errors • Routing protocol not enabled • Wrong routing protocol enabled • Incorrect IP addresses • Incorrect subnet masks

  20. Layer 3 troubleshooting using Ping • Used to test network connectivity (Layer 3) • ping sends a packet to the destination host and then waits for a reply packet from that host (echo protocol) • Results help evaluate • path-to-host reliability • delays over the path • whether the host can be reached or is functioning • ping uses Internet Control Message Protocol (ICMP) • Verifies hardware connection • Verifies logical address of the network layer • 5 datagrams sent • Success = ! • Timout = . • Extended Ping also

  21. Layer 7 troubleshooting using Telnet • telnet normally used to • connect remote devices • gather information • run programs • Telnet application provides • A virtual terminal for connection to routers running TCP/IP • Useful to verify that a connection can be made using Telnet • A successful Telnet connection indicates that the upper-layer application and the services of lower layers are functioning properly

  22. Troubleshooting L1& L1 using show interface <specific interface> • Show interfaces displays the status of • physical hardware (layer 1) • logical software (layer 2) Part 2 is hardware Line status L1 problems Part 1 is configuration L2 problems

  23. A high number of interface resets means too many keepalives have been missed. Causedby: • Bad line causing carrier transitions • Possible hardware problem at the CSU, DSU, or switch • Use the clear counters command to reset the counters to zeroafter an interface problem has been corrected • Starting from zero gives a better picture of the current status of the network

  24. Keepalives • Messages sent by one network device to inform another network device that the virtual circuit between the two is still active • If the interface misses three consecutive keepalives, the line protocol is marked as down • If the line is down • Layer 1 problem • Cabling, connectors, equipment powered off or malfunctioning • Administratively down - manually disabled in the configuration • Interface is up and line protocol is down • Layer 2 problem exists • No keepalives, No clock rate, Mismatch in encapsulation type • When the line is down, the protocol is always down • no useable media for the Layer 2 protocol

  25. Troubleshooting using Cisco Discovery Protocol • CDP advertises device information to its direct neighbors • MAC address, IP addresses, outgoing interfaces • show cdp neighbors • Displays information about directly connected neighbors • show cdp neighbors detail • displays active interfaces, port ID, device, IOS version • For security reasons CDP should be • Configured only on links between Cisco devices • disabled on user ports or links not locally managed

  26. Troubleshooting using traceroute • traceroute command is used to • Discover the routes that packet take on way to destination • Test network layer (Layer 3) on a hop-by-hop basis • provide performance benchmarks • Traceroute output • A list of hops that were successfully reached • Every router that the datagram passes through. • indicate the specific hop at which the failure is occurring • * indicates the packet failed • round trip time (RTT) - relative performance of links

  27. Troubleshooting using show ip protocols • displays values about IP routing protocol information on the entire router • which protocols are configured • which networks are being advertised • which interfaces are sending updates • timers, filters, route summarization, route redistribution

  28. Troubleshooting using show ip route • Displays the contents of the IP routing table • entries for all known networks and subnetworks • how that information was learned • used to verify that the router has a route to that network

  29. Troubleshooting usingshow controllers serial • determine the type of cable connected without inspecting the cables • Useful for finding a serial interface with • No cable • The wrong type of cable • Defective cable

  30. Debug command • Produces high processor overhead • Should be used isolate problems • not monitor normal network operation. • By default, the router sends the debug output and system messages to the console. • terminal monitor command cam redirect this to a remote terminal • Turn off diagnostics • no debug all or undebug all • The time when debug events occurred • timestamps • GAD(config)#service timestamps debug uptime

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