Alternative connectivity tests

Alternative connectivity tests
4.2.5 This page will introduce some tools that are used to verify basic network connectivity.

Many network protocols support an echo protocol. Echo protocols are used to test if protocol packets are 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 functional. This is a basic test mechanism. This operation can be performed at either the user or privileged EXEC modes.

In Figure , the ping target 172.16.1.5 responded to all five datagrams that were sent. Each exclamation point (!) indicates a successful echo. Each period (.) on the display indicates that the application on the router timed out while it waited for a packet echo from a target. The ping user EXEC command can be used to diagnose basic network connectivity. The ping command uses Internet Control Message Protocol (ICMP).

The traceroute command, which is often referred to as the trace command in reference materials, can be used to find where data is sent in a network. The traceroute command is similar to the ping command. The main difference is that ping tests end-to-end connectivity and traceroute tests each step along the way. This operation can be performed at either the user or privileged EXEC levels.

In Figure , the path from York to Rome is traced. Along the way the path must go through London and Paris. If one of these routers is unreachable, three asterisks (*) will be returned instead of the name of the router. The traceroute command will attempt to reach the next step until the Ctrl-Shift-6 escape sequence is used.

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

The procedure to use the ping command is as follows:

• Enter the ping [IP address or name of destination ] command.
• Press the Enter key.

The procedure to use the traceroute command is as follows:

• Enter the traceroute [IP address or name of destination ] command.
• Press the Enter key.

The Lab Activities on this page will allow students to practice three network connectivity tests.

The next page will introduce some tools that are used to troubleshoot IP address-related problems.

Advanced Telnet operation

Advanced Telnet operation
4.2.4 This page will describe some features that can be used when several Telnet sessions are open at the same time. A user may switch back and forth between these sessions. The number of open sessions that are allowed at one time is defined by the session limit command.

Use the commands shown in Figure to escape from one session and resume a previously opened session.

A new connection can be made from the EXEC prompt.

Multiple Telnet sessions can be used and suspended with the Ctrl-Shift-6, then x sequence. The session can be resumed with the Enter key. When the Enter key is used, the Cisco IOS will resume the connection to the most recently suspended Telnet connection. If the resume command is used it requires a connection ID. Use the show sessions command to view the connection ID, as shown in Figure .

The Lab Activity on this page will show students how to use Telnet to remotely access routers.

The next page will describe some tests that can be used to verify connectivity.

Disconnecting and suspending Telnet sessions

Disconnecting and suspending Telnet sessions


4.2.3 This page will introduce two important features of the telnet command. These are the disconnect feature and the suspend feature.

A 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. The Enter key is used frequently. With a suspended Telnet session, it is possible to reconnect to another router. This is dangerous when changes are made to the configuration or EXEC commands are used. Always check which router is connected when the suspended Telnet feature is used.

The show sessions command will show which Telnet sessions are active.

The procedure that is used to disconnect a Telnet session is as follows:

• Enter the disconnect command.
• Follow the command with the name or IP address of the router or the session number. An example is as follows:

Denver>disconnect paris

The procedure that is used to suspend a Telnet session is as follows:

• Press Ctrl-Shift-6, then x.
• Enter the name of the router or IP address.

The Lab Activity will instruct students on how to suspend a Telnet session.

The next page will introduce some advanced features that can be used for multiple Telnet sessions.

Getting Information about Remote Devices

Getting Information about Remote Devices


4.2.1 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.


4.2.2 Establishing and verifying 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.

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.