Module 9: Basic Router Troubleshooting

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Module 9 Basic Router Troubleshooting

• CCNA 2 Version 3.0

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The show ip route command

• Displays the contents of the IP routing table.
• This table contains entries for all known
  networks and subnetworks, as well as a code that
  indicates how that information was learned.
• Additional commands that can be used with the
  show ip route command
• show ip route connected
• show ip route network
• show ip route rip
• show ip route igrp
• show ip route static

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Routing Tables

• Lists available routes for forwarding data
• Selects the best path to a destination
• Network layer, layer 3, of OSI model

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Determining the gateway of last resort

• Routers keep a default route, or a gateway of
  last resort.
• Default routes are used when the router is unable
  to match a destination network with a more
  specific entry in the routing table.
• Default routes can be statically entered by an
  administrator or dynamically learned using a
  routing protocol.

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Determining route source and destination

• The network layer provides best-effort,
  end-to-end, packet delivery across interconnected
  networks.
• The network layer uses the IP routing table to
  send packets from the source network to the
  destination network.

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Determining L2 and L3 addresses

• For a packet to get from the source to the
  destination, both Layer 2 and Layer 3 addresses
  are used.
• The Layer 3 address is used to route the packet
  from the source network to the destination
  network.
• The source and destination IP addresses remain
  the same.
• The MAC address changes at each hop or router.
• A data-link layer address is necessary because
  delivery within the network is determined by the
  address in the Layer 2 frame header, not the
  Layer 3 packet header.

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Determining the route administrative distance

• The administrative distance is a number that
  measures the trustworthiness of the source of the
  route information.
• The lower the administrative distance, the more
  trustworthy the source.
• Connected 0 Static 1
• IGRP 100 RIP 120
• Static route advantages
• Secure operation, low processor overhead, precise
  control of path selection

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Determining the route metric

• Protocols base their metric on hop count,
  bandwidth, delay, load, reliability, ticks delay,
  maximum transmission unit (MTU), and cost.
• Factors such as bandwidth and delay are static
  because they remain the same for each interface
  until the router is reconfigured or the network
  is redesigned.
• Factors such as load and reliability are dynamic
  because they are calculated for each interface in
  real-time by the router.

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Determining the route next hop

• Destination/next hop associations tell a router
  that a particular destination can be reached
  optimally by sending the packet to a particular
  router.
• When a router receives an incoming packet, it
  checks the destination address and attempts to
  associate this address with a next hop.

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Determining the last routing update 

• Use the following commands to find the last
  routing update
• show ip route
• displays routing table info about all known
  networks and subnets
• show ip route network
• show ip protocols
• provides information on when the next update is
  due and which networks are advertised in the
  update
• displays sources of routing updates, networks
  being advertised, and update timer values
• show ip rip database

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Observing multiple paths to destination

• Some routing protocols support multiple paths to
  the same destination.
• Unlike single path algorithms, these multi-path
  algorithms permit traffic over multiple lines,
  provide better throughput, and are more reliable.

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Introduction to network testing

• Basic testing of a network should proceed in
  sequence from one OSI reference model layer to
  the next.
• It is best to begin with Layer 1 and work to
  Layer 7 if necessary.
• The most common problems that occur on IP
  networks result from errors in the addressing
  scheme.

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Using a structured approach to troubleshooting

• There should be an orderly process to
  troubleshooting based on the networking standards
  set in place by an administration.
• Documentation is a very important part of the
  troubleshooting process.

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• The steps in this model are
• Collect all available information and analyze the
  symptoms of failure
• Localize the problem to within a single network
  segment, to a single complete module or unit, or
  to a single user
• Isolate the trouble to specific hardware or
  software within the unit, module, or users
  network account.
• Locate and correct the specific problem.
• Verify that the problem has been solved.

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Testing by OSI layers

• Layer 1 errors can include
• Broken cables
• Disconnected cables
• Cables connected to the wrong ports
• Intermittent cable connection
• Wrong cables used for the task at hand (must use
  rollovers, crossover cables, and straight-through
  cables correctly)
• Transceiver problems
• DCE cable problems
• DTE cable problems
• Devices turned off

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• Layer 2 errors can include
• 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

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• Layer 3 errors can include
• Routing protocol not enabled
• Wrong routing protocol enabled
• Incorrect IP addresses
• Incorrect subnet masks

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Layer 1 troubleshooting using indicators

• Indicator lights are a useful tool for
  troubleshooting.
• Often this light is called the link light.
• The interface may also have lights to indicate
  whether traffic is being transmitted (TX) or
  received (RX).

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Layer 3 troubleshooting using ping

• The ping utility is used to test network
  connectivity.
• The ping command sends a packet to the
  destination host and then waits for a reply
  packet from that host.
• The ping output displays the minimum, average,
  and maximum times it takes for a ping packet to
  find a specified system and return.
• The ping command uses Internet Control Message
  Protocol (ICMP) to verify the hardware connection
  and the logical address of the network layer.
• The ICMP message type destination unreachable
  notifies source hosts that a receiving host or
  network is not available

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Layer 7 troubleshooting using Telnet

• Allows verification of the application layer
  software between source and destination stations.
  
• Most complete test mechanism available.
• The telnet utility is normally used to connect
  remote devices, to gather information, and to run
  programs.
• A successful Telnet connection indicates that the
  upper-layer application and the services of lower
  layers are functioning properly. (Verifies
  functionality of all seven layers of OSI model on
  a network host)

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Troubleshooting Layer 1 using show interfaces

• The show command used to check the status and
  statistics of the interfaces is the show
  interfaces command.
• When Serial 1 is down, line protocol is down is
  displayed, the show interfaces command will
  address if a layer 1 issue exists, an interface
  problem exists, or there is no usable media for
  the protocol
• show interface serial 1
• Could be an interruption in the line by service
  provider
• Faulty switch or CSU/DSU
• Problem with faulty telephone equipment
• Router hardware defective
• Cabling problem

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Troubleshooting Layer 2 using show interfaces

• If the interface is up and the line protocol is
  down, a Layer 2 problem exists. Among the
  possible causes are
• No keepalives
• No clock rate
• Mismatch in encapsulation type
• Serial 1 is down, line protocol is up will NEVER
  occur

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Troubleshooting using show cdp

• show cdp neighbors command displays information
  about directly connected neighbors.
• show cdp neighbors detail
• Displays specific device detail such as the
  active interfaces, the port ID, and the device.
  The version of Cisco IOS that is running on the
  remote devices is also shown.

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Troubleshooting using traceroute

• Used to discover the routes that packets take
  when traveling to their destination.
• Traceroute can also be used to help test the
  network layer (Layer 3) on a hop-by-hop basis and
  provide performance benchmarks.
• By obtaining the last good hop from the
  traceroute output and comparing it to a diagram
  of the internetwork, the problem area can be
  isolated.
• Traceroute sends out a sequence of User Datagram
  Protocol (UDP) datagrams

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Troubleshooting routing issues

• The show ip protocols and show ip route commands
  display information about routing protocols and
  the routing table.
• The show ip protocols command displays values
  about IP routing protocol information on the
  entire router.

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Troubleshooting using show controllers serial

• show controllers serial command is useful to
  determine the type of cable connected without
  inspecting the cables.
• The show controllers serial command queries the
  integrated circuit (chip) that controls the
  serial interfaces and displays information about
  the physical interface. This output varies from
  controller chip to controller chip.
• Without specific knowledge of the integrated
  circuit, this information is of little use.

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Introduction to debug

• The debug command is used to display dynamic data
  and events.
• The show commands only display static
  information, they provide a historical picture of
  the router operation.
• Using the debug command output gives more insight
  to the current events of the router.
• Warning The debug all command should be used
  sparingly as this can disrupt router operations.
  (because of high processor overhead)
• no debug all command or the undebug all command
  turn off all diagnostic output.

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• THE
• END