Semester 2 Module 4 Learning about Other Devices

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Semester 2 Module 4 Learning about Other Devices

• Yuda college of business
• James Chen
• ydjames_at_ydu.edu.tw

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Outline

• Discovering and Connecting to Neighbors
• Getting Information about Remote Devices

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Introduction to CDP

• Cisco Discovery Protocol (CDP) is a Layer 2
  protocol that connects lower physical media and
  upper network layer protocols.
• CDP is used to obtain information about
  neighboring devices, such as the types of devices
  connected, the router interfaces they are
  connected to, the interfaces used to make the
  connections, and the model numbers of the
  devices.

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• CDP Version 2 (CDPv2) is the most recent release
  of the protocol.
• Cisco IOS (Release 12.0(3)T or later) supports
  CDPv2.
• CDP Version 1 (CDPv1) is enabled by default with
  Cisco IOS (Release 10.3 to 12.0(3)T).

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• When a Cisco device boots up, CDP starts up
  automatically and allows the device to detect
  neighboring devices that are also running CDP.
• It runs over the data link layer and allows two
  systems to learn about each other, even if they
  are using different network layer protocols.

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• Each device configured for CDP sends periodic
  messages, known as advertisements, to multiple
  routers.
• Each device advertises at least one address at
  which it can receive Simple Network Management
  Protocol (SNMP) messages.
• The advertisements also contain time-to-live or
  holdtime information, indicating the length of
  time that receiving devices should hold CDP
  information before discarding it.
• Additionally, each device listens to periodic CDP
  messages sent by others in order to learn about
  neighboring devices.

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Information obtained with CDP

• The primary use of CDP is to discover all Cisco
  devices that are directly connected to a local
  device.
• Use the show cdp neighbors command to display CDP
  updates on the local device.

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• CDP provides information about each CDP neighbor
  device by transmitting type length values (TLVs),
  which are blocks of information embedded in CDP
  advertisements.
• Device TLVs displayed by the show cdp neighbors
  command include the following
• Device ID
• Local Interface
• Holdtime
• Capability
• Platform
• Port ID
• VTP Management Domain Name (CDPv2 only)
• Native VLAN (CDPv2 only)
• Full/Half-Duplex (CDPv2 only)

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Implementation, monitoring, and maintenance of
CDP

• The following commands are used to implement,
  monitor, and maintain CDP information
• cdp run
• cdp enable
• clear cdp counters
• show cdp
• show cdp entry device-nameprotocol
  version
• show cdp interface type number
• show cdp neighbors type number detail

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Creating a network map of the environment

• Though a CDP frame can be small, it can retrieve
  a great deal of useful information about
  connected neighboring Cisco devices.
• This information can be used to create a network
  map of the connected devices.
• Devices connected to neighboring devices can be
  discovered by using Telnet to connect to the
  neighbors, and using the show cdp neighbors
  command to discover what devices are connected to
  those neighbors.

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Disabling CDP

• To disable CDP at the global level, use the no
  CDP run command in global configuration mode.
• If CDP is disabled globally, individual
  interfaces cannot be enabled for CDP.

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Troubleshooting CDP

• The following commands can be used to show the
  version, update information, tables, and traffic
  
• clear cdp table
• clear cdp counters
• show cdp traffic
• show debugging
• debug cdp adjacency
• debug cdp events
• debug cdp ip
• debug cdp packets
• cdp timer
• cdp holdtime
• show cdp

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Outline

• Discovering and Connecting to Neighbors
• Getting Information about Remote Devices

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Telnet

• Telnet is a virtual terminal protocol that is
  part of the TCP/IP protocol suite.
• It allows connections to be made to remote hosts.
  
• Telnet functions at the application layer of the
  OSI model.

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• A router can have multiple simultaneous incoming
  Telnet sessions.
• The range zero through four is used to specify
  five VTY or Telnet lines.
• These five incoming Telnet sessions could take
  place at one time.

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Establishing and verifying a Telnet connection

• The Telnet IOS EXEC command allows a user to
  Telnet from one Cisco device to another.
• With Cisco's implementation of TCP/IP, it is not
  necessary to enter the command connect or telnet
  to establish a Telnet connection.
• The hostname or the IP address of the remote
  router may be entered.
• To end a Telnet session, use the EXEC commands
  exit or logout.

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• To initiate a Telnet session any of the following
  alternatives can be used
• Denvergtconnect paris
• Denvergtparis
• Denvergt131.108.100.152
• Denvergttelnet paris
• The Telnet connection will terminate after ten
  minutes of inactivity by default or when the exit
  command at the EXEC prompt is entered.

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Disconnecting and suspending Telnet sessions

• One important feature of the Telnet command is
  the suspend feature.
• However, one potential problem exists when a
  Telnet session is suspended and the Enter key is
  pressed.
• Cisco IOS software resumes the connection to the
  most recently suspended Telnet connection.

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• The command show sessions will show what Telnet
  sessions are taking place.
• The procedure for disconnecting a Telnet session
  is as follows
• Enter the command disconnect
• Follow the command with the name or IP address of
  the router. Example Denvergtdisconnect paris
• The procedure for suspending a Telnet session is
  as follows
• Press Ctrl-Shift-6, then x
• Enter the name of the router or IP address

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Advanced Telnet operation

• A user may switch back and forth between
  concurrent Telnet sessions.
• The number of open sessions that are allowed at
  one time is defined by the session limit command.
  
• Multiple Telnet sessions can be used and
  suspended by using the Ctrl-Shift-6, then x
  sequence.
• The session can be resumed by using the Enter
  key.
• If the resume command is used it requires a
  connection ID.
• The connection ID is shown by using the show
  sessions command.

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Alternative connectivity tests

• Echo protocols are used to test whether protocol
  packets are being routed.
• The ping command sends a packet to the
  destination host and then waits for a reply
  packet from that host.
• Results from this echo protocol can help evaluate
  the path-to-host reliability, delays over the
  path, and whether the host can be reached or is
  functioning.
• This operation can be performed at either the
  user or privileged EXEC modes.

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• The ping target responded successfully to all
  five datagrams sent.
• The exclamation points ( ! ) indicate each
  successful echo.
• If one or more periods ( . ) are received instead
  of exclamations on the display, the application
  on the router timed out waiting for a given
  packet echo from the ping target.
• The command ping uses ICMP (Internet Control
  Message Protocol).

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• The traceroute command is the ideal tool for
  finding where data is being sent in a network.
• The traceroute command is similar to the ping
  command, except that instead of testing
  end-to-end connectivity, traceroute tests each
  step along the way.
• This operation can be performed at either the
  user or privileged EXEC levels.

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• If one of these routers is unreachable, three
  asterisks ( ) will be returned instead of the
  name of the router.
• The traceroute command will continue attempting
  to reach the next step until the Ctrl-Shift-6
  escape sequence is used.

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• A basic verification test also focuses on the
  network layer.
• Use the show ip route command to determine
  whether a routing table entry exists for the
  target network.
• This command will be discussed in more detail in
  a later module of this course.

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Troubleshooting IP addressing issues

• The following three commands are used to perform
  address-related troubleshooting
• ping uses the ICMP protocol to verify the
  hardware connection and the IP address of the
  network layer. This is a basic testing mechanism.
  
• telnet verifies the application layer software
  between source and destination. This is the most
  complete test mechanism available.
• traceroute allows the location of failures in the
  path from the source to the destination. Trace
  uses Time to Live values to generate messages
  from each router along the path.