CCNA2

Module 4

Discovering and Connecting to Neighbors

• Enable and disable CDP • Use the show cdp neighbors command • Determine which neighboring devices are

connected to which local interfaces • Gather network address information about

neighboring devices using CDP

Discovering and Connecting to Neighbors

• Establish a Telnet connection • Verify a Telnet connection • Disconnect from a Telnet session • Suspend a Telnet session • Perform alternative connectivity tests • Troubleshoot remote terminal connections

Discovering and Connecting to Neighbors

• 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

• CDP is media and protocol independent, and runs on all Cisco equipment

Discovering and Connecting to Neighbors

• 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

• Each device configured for CDP sends periodic messages, known as advertisements, to multiple routers.

Discovering and Connecting to Neighbors

• 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

• Additionally, each device listens to periodic CDP messages sent by others in order to learn about neighboring devices

Discovering and Connecting to Neighbors

• The primary use of CDP is to discover all Cisco devices that are directly connected to a local device

• Each router that is running CDP exchanges protocol information with its neighbors

• The administrator uses the show cdp neighbors command to display information about the networks directly connected to the router

• CDP provides information about each CDP neighbor device by transmitting type length values (TLVs), which are blocks of information embedded in CDP advertisements

Discovering and Connecting to Neighbors

Discovering and Connecting to Neighbors

• Notice that the lowest router in the figure is not directly connected to the administrator’s console router. To obtain CDP information about this device, the administrator would need to Telnet to a router that is directly connected to this device.

Discovering and Connecting to Neighbors

• 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-name[*][protocol |

version]} • show cdp interface [type number] • show cdp neighbors [type number] [detail]

Discovering and Connecting to Neighbors

• The cdp run command is used to enable CDP globally on the router. By default, CDP is globally enabled. The cdp enable command is used to enable CDP on a particular interface

• CDP could be enabled on each of the devices interfaces by using the cdp enable command.

Discovering and Connecting to Neighbors

• 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.

Discovering and Connecting to Neighbors

• 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

Discovering and Connecting to Neighbors

Discovering and Connecting to Neighbors

• 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 is an IOS EXEC command used to verify the application layer software between source and destination. This is the most complete test mechanism available.

• Telnet functions at the application layer of the OSI model. Telnet depends on TCP to guarantee the correct and orderly delivery of data between the client and server

Discovering and Connecting to Neighbors

• 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.

Discovering and Connecting to Neighbors

Discovering and Connecting to Neighbors

• To initiate a Telnet session any of the following alternatives can be used:

• Denver>connect parisDenver>parisDenver>131.108.100.152Denver>telnet paris

Discovering and Connecting to Neighbors

Discovering and Connecting to Neighbors

Discovering and Connecting to Neighbors

• 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.

Discovering and Connecting to Neighbors

• The ping user EXEC command can be used to diagnose basic network connectivity. The command ping uses ICMP (Internet Control Message Protocol).

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Discovering and Connecting to Neighbors

• 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.

Discovering and Connecting to Neighbors

Discovering and Connecting to Neighbors

• 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.