The OSI Model...

One of the first things that you begin to learn about in the CCNA course is the OSI model. Up until recently I’d never heard of the layers of the OSI model and so it’s taken a some time to become familiar with what each layer of the model does.

The OSI (Open Systems Interconnection) was created by the International Organisation for Standardisation (ISO) as a framework used to describe how data is transferred over a medium from one networked device to another. It is made up of 7 layers:

• 7. Application
• 6. Presentation
• 5. Session
• 4. Transport
• 3. Network
• 2. Data Link
• 1. Physical

The 4 main advantages of the OSI are:

1. It divides the communication process into simpler components, aiding component development, design, and troubleshooting.
2. It encourages industry standardization by defining what functions occur at each layer of the model.
3. It prevents changes in one layer from affecting other layers, so that it does not hamper development.
4. It allows various types of network hardware and software to communicate.

Below is my summary chart of the OSI model...

Loopback & Telnet commands.

I’ve configured 4 routers each with the loopback addresses shown below.

A loopback interface is a virtual interface that doesn’t connect to any other device but is an easy way to identify a particular router on the network. The loopback interface never goes down unless the router is turned off.

Lookpack can be configured with :

                  Router(config)interface loopback <ip>

I’ve already created a telnet password (‘cisco’) on each router and also an enable password (also ‘cisco’) so that I can telnet into and manage each router.

I am on Router1 and would like to telnet from Router1 into all 3 of the other routers.

I start off by telneting into Router 2:

                                Router1#telnet 2.2.2.2

        Trying 2.2.2.2 ...Open

        User Access Verification

        Password:  cisco

        Router2>en

        Password:  cisco

        Router2#

To get back to Router1 I need to press Ctrl Shift 6 all together, and then the letter x (Ctrl+Shift+6, & then 'x'). Now I am back on Router1 but still have a telnet session connecting to Router2.

I then want to telnet from Router1 into Router3:

                                Router1#telnet 3.3.3.3

        Trying 3.3.3.3 ...Open

        User Access Verification

        Password: cisco

        Router3>enable

        Password: cisco

        Router3#

I then go back to Router1 (Ctrl+Shift+6, & then 'x') and telnet into Router 4 using the same telnet command shown above.

Once I’m back on Router1 I can use the command show sessions to see all of the telnet sessions that I have open from Router1.

                                Router1#show sessions

Conn Host                Address             Byte  Idle Conn Name

  1       2.2.2.2             2.2.2.2                0     1      2.2.2.2

  2       3.3.3.3             3.3.3.3                0     1      3.3.3.3

*3       4.4.4.4             4.4.4.4                0     0      4.4.4.4

Now if I want to go back into any of my telnet sessions I just need to type the connection number – Eg. ‘3’ will take me to Router 4 (4.4.4.4).

          Router1#3

          [Resuming connection 3 to 4.4.4.4 ... ]

          Router4#

From Router1 if I want to disconnect any of my telnet sessions I need to use the command disconnect <connection number>:

                               

                          Conn Host                Address             Byte  Idle Conn Name

     1     2.2.2.2             2.2.2.2                0     4      2.2.2.2

     2     3.3.3.3             3.3.3.3                0     4       3.3.3.3

*   3     4.4.4.4             4.4.4.4                0     0       4.4.4.4

Router1#disconnect 2

Closing connection to 3.3.3.3 [confirm]

The final command that I can use is show users which will show me how many people have telnetted into a particular router.

Eg. I want to know how many people have telnetted into Router4. The below output shows me that one person has telnetted into Router4 on vty 0 (the first telnet line).

                                    Router4#show users

                             Line       User       Host(s)          Idle           Location

*    67          vty 0        idle             00:00:00   192.168.1.1

Configuring Telnet

Telnet is used as a way of connecting to a device remotely and in the case of cisco devices it is used to remotely view and make changes to device configuration.

To setup telnet access into a cisco device there are a number of commands involved:

1.      

Router# configure terminal

Router(config)# line vty 0 4

Router(config-line)# password cisco

Router(config-line)# exec-timeout 20

Router(config-line)# login

Router(config-line)# exit

The line vty 0 4 command is used to configure telnet and define the number of users that connect to the device at once. 0 4 means that 5 users can connect at any one time – 0,1,2,3,4. If you used the command line vty 0 9 it would mean that 10 users would be able to connect simultaneously.

The exec-timeout command specifies the number of minutes until the telnet connection is dropped if it’s not being used.

The login command specifies that the password must be used for the user to telnet into the device.

If you typed no login instead of login then no password would be required.

** You also need to make sure that the device that you are telneting into has an enable password so that you can access the privileged exec mode via telnet.

Router# configure terminal

Router(config)# enable password cisco