Basic Router Troubleshooting

1
Basic Router Troubleshooting

• Module 9
• CCNA 2 Version 3.0

2
Determining Layer 2 and Layer 3 Addressing
3
Review of Routing

• One of the primary functions of a router is to
  determine the best path to a given destination
• A router learns paths (routes) from
• Static routing An administrator manually
  defines routes to one or more destination
  networks
• Dynamic routing Routers follow rules defined by
  a routing protocol to exchange routing
  information and independently select the best
  path
• Routers store this routing information in routing
  tables using on-board dynamic random access
  memory (DRAM)
• Routing tables contains a list of the best
  available routes used by routers to make packet
  forwarding decisions.

4
Static Routing Advantages/Disadvantages
5
Dynamic Routing Advantages/Disadvantages
6
Route Administrative Distance

• A router wants to install the best route to a
  destination in its routing table
• The administrative distance (AD) of the route is
  the key information that the router uses in
  deciding which is the best path
• The AD is a number that measures the
  trustworthiness of the source of the route
  information
• The lower the AD, the more trustworthy the source
• The path with the lowest administrative distance
  is installed in the routing table

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Administrative Distances

• Different routing protocols have different
  default administrative distances

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Default Routes

• What is a default route?
• A route that will be used when the router is
  unable to match a destination network with a more
  specific entry in the routing table
• A default route is also know as a gateway of last
  resort
• Whats the purpose of a default route?
• To keep routing tables as lean as possible
• How do you know if a default route has been set?
• Issue a show run check the running
  configuration file
• Issue a show ip route and look for the following
• Gateway of last resort is 172.16.1.2 to network
  0.0.0.0

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Defining a Default Route Method One

• The ip default-network command
• establishes a default route in networks using
  dynamic routing protocols

• Example) The global command ip default-network
  192.168.17.0
• defines the Class C network 192.168.17.0 as the
  destination path for packets that have no routing
  table entries

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Defining a Default Route Method Two

• To create a default route in a network that uses
  static routing the command is
• Router(config)ip route 0.0.0.0 0.0.0.0
  next-hop-ip-address exit-interface
• This command can either specify the exit
  interface (AD 0) or the next hop IP address(AD
  1)
• both will direct the packet to the same place

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Routing Metrics

• Routing protocols use metrics to determine the
  best route to a destination
• The metric is a value that measures the
  desirability of a route
• Some routing protocols use only one factor to
  calculate a metric
• Other routing protocols use several factors to
  determine a composite metric
• Routing protocols use an algorithm to generate a
  number, called the metric value, for each path
  through the network
• Typically, the smaller the metric number, the
  better the path

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Routing Metrics
Some static factors can be configured manually,
allowing precise control over what routes a
router chooses
Static Factors - remain the same for each
interface until the router is reconfigured or the
network is redesigned
Dynamic factors -calculated for each interface in
real-time by the router
Dynamic factors allow routers to make decisions
based on current conditions
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Determining the Last Routing Update

• Use the following commands to find the last
  routing update
• show ip route
• show ip route network
• show ip protocols
• show ip rip database

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Determining the Last Routing Update

• show ip route

Last update received 14 seconds ago
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Determining the Last Routing Update

• show ip route network

Last update received 11 seconds ago
16
Determining the Last Routing Update

• show ip protocols

Last update received 3 seconds ago
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Determining the Last Routing Update

• show ip rip database

Last update received 20 seconds ago
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Testing Process Overview

• It is best to begin with Layer 1 and work to
  Layer 7 if necessary

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5-Step Troubleshooting Model
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Layer 1 Errors

• 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 1 Troubleshooting

• Always check to make sure that the device is
  powered on!
• check the basics before running diagnostics or
  attempting complex troubleshooting
• Most interfaces or NICs will have indicator
  lights that show whether there is a valid
  connection
• Often this light is called the link light
• If the interface has indicator lights that do not
  show a valid connection
• Power off the device and reseat the interface
  card
• A faulty or incorrect cable could also cause a
  link light to indicate a bad connection or no link

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Layer 1 Troubleshooting Show Interface Command
Hardware Status - includes cables, connectors,
and interfaces showing the condition of the
physical connection between the devices
Software status - shows the state of messages
such as keepalives that are passed between
adjacent devices
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Is the Link Operational?

• Serial 1 is up, line protocol is up (says its
  operational)
• Keepalives - messages sent by one network device
  to inform another network device that the virtual
  circuit between the two is still active
• Note If the interface misses three consecutive
  keepalives, the line protocol is marked as down
• When the line is down, the protocol is always
  down, because there is no useable media for the
  Layer 2 protocol
• If the interface is up and the line protocol is
  down, a Layer 2 problem exists, could be
• No keepalives
• No clock rate
• Mismatch in encapsulation type

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Interpreting the Show Interface Output

• An increasing number of carrier transitions
  counts on a serial link may indicate one or more
  of the following problems
• Line interruptions due to problems in the service
  provider network.
• Faulty switch, DSU, or router hardware
• An increasing number of input errors may
  indicate
• Faulty telephone company equipment
• Noisy serial line
• Incorrect cable or cable length
• Damaged cable or connection
• Defective CSU or DSU
• Defective router hardware

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Errors Counters

• Interpret errors based on
• the amount of traffic that the router has
  processed
• the amount of time that the statistics have been
  captured
• The router tracks statistics that provide
  information about the interface
• The statistics reflect router operation since it
  was started or since the last time the counters
  were cleared
• show interfaces output shows the last clearing of
  the counters
• show version shows how long the router has been
  functional.
• Use the clear counters command to reset the
  counters to zero. These counters should always be
  cleared after an interface problem has been
  corrected.

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Layer 2 Errors

• 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

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

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Layer 3 Troubleshooting PING Utility

• The ping utility is used to test network
  connectivity
• tests whether protocol packets are being routed
• The ping command can be invoked from both user
  EXEC mode and privileged EXEC mode
• The use of an extended ping command directs the
  router to perform a more extensive range of test
  options
• To use extended ping, type ping at the command
  line
• press the Enter key without entering an IP
  address
• Prompts will appear each time the Enter key is
  pressed

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Layer 3 Troubleshooting Telnet Utility

• The telnet utility is
• a virtual terminal protocol
• part of the TCP/IP protocol suite
• the most complete testing mechanism available
• Telnet allows verification of the application
  layer software between source and destination
  stations
• Telnet is normally used to connect remote
  devices, to gather information, and to run
  programs.

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Troubleshooting Using CDP

• Cisco Discovery Protocol (CDP) advertises device
  information to its direct neighbors

• If the physical layer is properly functioning,
  then all other directly connected Cisco devices
  should be displayed
• If no known device appears, a Layer 1 problem
  likely exists
• Security can be an issue with CDP because of the
  amount of information CDP provides
• CDP should be configured only on links between
  Cisco devices and disabled on user ports or links
  that are not locally managed.

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Troubleshooting with Traceroute

• In order for the traceroute or ping data to
  successfully make the round trips between
  routers, there have to be known routes in both
  directions
• A failed response is not always an indication of
  a problem
• ICMP messages could be filtered at the host site
  (this is especially true across the Internet)

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Review Examination of a Routing Table
How Route was Learned
Next Hop to Destination Network
Interface through which to forward packet
Administrative Distance
Destination Networks
Metric (Hop Count)
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Troubleshooting with the Show IP Route Command

• If there is a problem reaching a host in a
  particular network, then the output of the show
  ip route command can be used to verify that the
  router has a route to that network

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Troubleshooting with the Show IP Protocols Command

• Displays values about IP routing protocol
  information on the entire router, can be used to
  confirm
• which protocols are configured
• which networks are being advertised
• which interfaces are sending updates
• the sources of routing updates

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Troubleshooting with the Show Controllers Serial
Command
Type of cable that the controller detects can be
determined useful for finding a serial interface
with no cable, the wrong type of cable, or a
defective cable.
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Troubleshooting with Debug

• The debug command is used to display dynamic data
  and events
• These events could be traffic on an interface or
  error messages generated by nodes on the network
• debug should be used conservatively
• produces high processor overhead that may disrupt
  normal router operation
• Warning The debug all command should be used
  sparingly as this can disrupt router operations
• By default, the router sends the debug output and
  system messages to the console
• the debug output and system messages can be
  redirected to the remote terminal by issuing the
  terminal monitor command
• no debug all command or the undebug all command
  turn off all diagnostic output

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Basic Router Troubleshooting

• Module 9
• CCNA 2 Version 3.0