Telnet / Establishing and verifying a Telnet connection

Telnet
4.2.1 This page will introduce 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 provides a network terminal or remote login capability. 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.

A router can have simultaneous incoming Telnet sessions. The numbers zero through four are used to specify five vty or Telnet lines.

The verification of application layer connectivity is a by-product of Telnet. Telnet is mainly used to establish remote connections to network devices. Telnet is a simple and universal application program.

The next page will explain how a Telnet connection is initiated and verified.

Establishing and verifying a Telnet connection

4.2.2

This page will show students how to establish and test a Telnet connection.

The Telnet IOS EXEC command allows a user to Telnet from one Cisco device to another. In the Cisco implementation of TCP/IP, it is not necessary to enter the connect or telnet commands 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.

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

Denver>connect paris

Denver>paris

Denver>131.108.100.152

Denver>telnet paris

A hostname table or access to DNS for Telnet must be present for a name to work. Otherwise, the IP address of the remote router must be entered.

Telnet can be used to determine if a remote router can be accessed. As shown in Figure , if Telnet is used successfully to connect the York router to the Paris router, then a basic test of the network connection is successful. This operation can be performed at either the user or privileged EXEC levels.

If remote access can be obtained through another router, then at least one TCP/IP application can reach the remote router. A successful Telnet connection indicates that the upper-layer application functions properly.

If Telnet to one router is successful, failure to another router is likely caused by addressing, naming, or access permission problems. The problem may exist on the original router or on the router that failed as a Telnet target. The next step is to use the ping command, which is covered later in this lesson. The ping command can be used to test end-to-end connections at the network layer.

Once the Telnet is completed, log off the host. The Telnet connection will terminate after ten minutes of inactivity by default or when the exit command is entered at the EXEC prompt.

Students can use the Lab Activity on this page to establish and verify a Telnet connection.

The next page will explain how to disconnect or suspend a Telnet connection.

Disabling CDP / Troubleshooting CDP

Disabling CDP

4.1.5  This page will show students how to disable 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.

On Cisco IOS Release 10.3 or higher, CDP is enabled by default on all supported interfaces to send and receive CDP information. However, on some interfaces, such as asynchronous interfaces, CDP is disabled by default. If CDP is disabled use the cdp enable command in interface configuration mode. To disable CDP on a specific interface after it has been enabled, use the no cdp enable command in interface configuration mode.

The next page will introduce some commands that can be used to troubleshoot CDP.

Troubleshooting CDP 

4.1.6

This page will introduce some commands that can be used to show the version and update CDP 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

The Lab Activities on this page will require students to use CDP commands to learn about other devices.

This page concludes this lesson. The next lesson will explain how to test and obtain information from remote devices.  

Creating a network map of the environment

 Creating a network map of the environment

4.1.4 This page will explain how the information that is obtained by CDP can be used to create a network map.

CDP was designed and implemented as a simple, low-overhead protocol. Though a CDP frame can be small, it can retrieve a lot of useful information about directly connected Cisco devices.

This information can be used to create a network map of the connected devices. To discover devices that are connected to neighbor devices, use Telnet to connect to the neighbors. Then use the show cdp neighbors command.

The Lab Activity will show students how to use CDP commands to learn about neighboring network devices.

The next page will describe how CDP can be disabled.

Creating a network map of the environment

Creating a network map of the environment

4.1.4  

Implementation, monitoring, and maintenance of CDP / Creating a network map of the environment

Implementation, monitoring, and maintenance of CDP

4.1.3 This page will introduce the commands that are used to implement, monitor, and maintain CDP information: 

• cdp run
• cdp enable
• show cdp traffic
• clear cdp counters
• show cdp
• show cdp entry {*|device-name [*][protocol | version]}
• show cdp interface [type number]
• show cdp neighbors [type number] [detail]

The cdp run command is used to enable CDP globally on a router. By default, CDP is globally enabled. The cdp enable command is used to enable CDP on a particular interface. On Cisco IOS Release 10.3 or higher, CDP is enabled by default on all supported interfaces to send and receive CDP information. CDP can be enabled on all device interfaces with the cdp enable command.
The Lab Activity on this page will teach students about some basic CDP commands.
The next page will explain how CDP can be used to create a network map. 

Creating a network map of the environment

4.1.4

This page will explain how the information that is obtained by CDP can be used to create a network map.

CDP was designed and implemented as a simple, low-overhead protocol. Though a CDP frame can be small, it can retrieve a lot of useful information about directly connected Cisco devices.

This information can be used to create a network map of the connected devices. To discover devices that are connected to neighbor devices, use Telnet to connect to the neighbors. Then use the show cdp neighbors command.

The Lab Activity will show students how to use CDP commands to learn about neighboring network devices.

The next page will describe how CDP can be disabled. 

Module 4: Overview / Learning about Other Devices / Introduction to CDP / Information obtained with CDP

Overview

Sometimes network documentation is incomplete or inaccurate. Cisco Discovery Protocol (CDP) is a useful tool in these situations because it can build a basic picture of a network. CDP is a media and protocol independent, Cisco proprietary protocol used for neighbor discovery. CDP will only show information about directly connected neighbors but it is still a powerful tool.

After a router is initially configured it is often difficult to connect directly to the router for configuration changes or other activities. Telnet is a TCP/IP-based application that allows remote connection to the router command-line interface (CLI) for configuration, monitoring, and troubleshooting purposes. Telnet is an essential tool for network professionals.

This module covers some of the objectives for the CCNA 640-801, INTRO 640-821, and ICND 640-811 exams. -

Students who complete this module should be able to perform the following tasks:

• Enable and disable CDP
• Use the show cdp neighbors command
• Determine which neighbor devices are connected to each local interface
• Gather network address information about neighbor devices that use CDP
• 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 

 
4.1 Discovering and Connecting to Neighbors

Introduction to CDP 

4.1.1 This page will introduce Cisco Discovery Protocol (CDP). CDP is a Layer 2 protocol that connects lower physical media and upper network layer protocols, as shown in Figure  . CDP is used to obtain information about neighboring Cisco 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. CDP is media and protocol independent, and runs on all Cisco equipment over the Subnetwork Access Protocol (SNAP).

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.

When a Cisco device boots up, CDP starts up automatically and allows the device to detect neighbor devices that use CDP. CDP operates at the data link layer and allows two systems to learn about each other, even if they use different network layer protocols.

Each device that is configured for CDP sends periodic messages, which are known as advertisements, to directly connected Cisco devices. 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, which indicates the length of time that receiving devices should hold CDP information before they discard it. Each device also listens to periodic CDP messages that are sent by others to learn about neighbor devices.

The next page will explain how CDP collects and delivers information. 
Information obtained with CDP 

4.1.2 This page will explain how CDP is used to obtain information about network devices.

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.

Figure displays an example of how CDP delivers its collection of information to a network administrator. Each router that uses CDP exchanges protocol information with its neighbors. The network administrator can display the results of this CDP information exchange on a console that is connected to a local router.

An administrator can use the show cdp neighbors command to display information about the networks that are directly connected to a router. CDP transmits type length values (TLVs) to provide information about each CDP neighbor device. TLVs 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

The following TLVs are only included in CDPv2:

• VTP management domain name
• Native VLAN
• Full or half-duplex

Notice that the router at the bottom of Figure is not directly connected to the console router that is used by the administrator. To obtain CDP information about this device, the administrator would need to Telnet to a router that is directly connected to this device.

The next page will introduce the commands that are used to monitor CDP information.  
 

Summary

Summary

This page summarizes the topics discussed in this module.

A router has several modes that are used to accomplish specific tasks. The user EXEC mode is used primarily to check the status of a router. The privileged EXEC mode allows administrators to set usernames and passwords for access to router commands. Global configuration mode is used to apply configuration statements that affect a whole system.

One of the first configuration tasks is to give a unique name to a router. For security purposes, passwords and user IDs for authorized users should be set. The show command is used to examine the contents of files and for troubleshooting.

Serial interfaces require a clock signal to control the timing of the communications. An interface must have an IP address and subnet mask to route IP packets. By default, interfaces are turned off or disabled. Use the no shutdown command to turn on an interface. Use the show running-config command to display the current running configuration to verify any modifications.

Configuration standards are developed for consistency, to reduce network complexity, to reduce downtime, and to maximize network performance. Some standards for configuration files include the number of files to maintain, how they are stored, and where they are stored. Interface descriptions, login banners, and MOTDs can be standardized to inform users about events such as downtime and to warn unauthorized users.

Host name resolution translates names to IP addresses. The Cisco IOS software maintains a cache of host name-to-address mappings for use by EXEC commands. The cache speeds up the conversion process. Unlike DNS, host names are only significant to the router on which they are configured. Host names are entered in global configuration mode.

Configuration backup can be stored on a TFTP server, on a network server, or on a disk. A specific backup plan will ensure that the files are available if a problem occurs.