Telnet is a simple yet powerful program that allows you to connect to a remote Cisco router or switch, and then configure it as though you were right at the console. Telnet is also one of those features that seems so very simple, until you get asked a half dozen questions about it on your CCNA exams. As with all topics, it's the details you know about Telnet that will help you pass the Intro and ICND exams.
Laptop Battery Let's take a look at a few of these details. We'll begin by debunking one common belief about Telnet:
Occasionally, during your CCNA and CCNP studies, you'll run into a term that just doesn't quite make sense to you. (Okay, more than occasionally!) One such term is "reverse telnet". As a Cisco certification candidate, you know that telnet is simply a protocol that allows you to remotely connect to a networking device such as a router or switch. But what is "reverse telnet", CCNP home lab setup
Thinkpad Telnet runs at layer 7 of the OSI model, not layer 3!
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Microsoft It's easy to think that Telnet runs at Layer 3 of the OSI model, the Network layer. After all, you're entering an IP address when you telnet in to a router or switch, and you may be on another router when you do it! None of that matters. Layer 3 is strictly the domain of routing. Like other features that require input from the end user, especially authentication, Telnet runs at the Application layer of the OSI model.
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Laptop Computers Speaking of authentication..
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Laptop Computer Cisco routers require a password to be set before anyone can telnet in.
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Desktop Computer Cisco routers can run quite a few passwords. We can set an enable password, an enable secret, an enable secret and enable password, a password for PPP connections, and even a console password.
Notebooks All of those are optional, but the telnet password is not. Makes sense - you wouldn't want just anyone telnetting into your router, would you?
Lenovo If you have no password set on the VTY lines of your router, no one can telnet in. If they try, they'll see this message:
Hard Drive R1#telnet 3.3.3.3
Travelstar Trying 3.3.3.3 ... Open
Gateway
Laptop Parts Password required, but none set
Software [Connection to 3.3.3.3 closed by foreign host]
Hard Drives
Electronics To allow telnet access into a Cisco router, configure the VTY lines with a password and the login command:
Canon R3#conf t
Desktop Pc Enter configuration commands, one per line. End with CNTL/Z.
Desktop Computers R3(config)#line vty 0 4
Think Pad R3(config-line)#login
Repair % Login disabled on line 2, until 'password' is set
Data Recovery % Login disabled on line 3, until 'password' is set
Cisco % Login disabled on line 4, until 'password' is set
Keyboard % Login disabled on line 5, until 'password' is set
Monitor % Login disabled on line 6, until 'password' is set
Desktop R3(config-line)#password cisco
Infosys Note the messages you get after enabling login. These messages simply indicate that the login won't work until a password is set. The order with which you use the login and password commands don't matter just make sure you use them both.
Refurbished Laptops We're not quite done, though. The remote user can now telnet in, but by default, that user will be placed into user exec mode. If the user is to be allowed to enter privileged exec mode during a telnet session, an enable password or enable secret must be set.
Wipro R1#telnet 3.3.3.3
Lap Top Trying 3.3.3.3 ... Open
Refurbished
Memory User Access Verification
Intel Password:
As400 R3>enable
Averatec % No password set
Hardware R3>
Dual Xeon The user is stuck in user exec until you set a local enable password. Doing so will allow the user to use that password to enter privileged exec mode.
Storage R3#conf t
Seagate R3(config)#enable password ccna
Computer Sales R3(config)#^Z
Computer Hardware R1#telnet 3.3.3.3
Printers Trying 3.3.3.3 ... Open
Technology User Access Verification
Mainframe Password: < user entered cisco here>
Samsung R3>enable
Computer Repair Password: < user entered ccna here >
Used Computers R3#
Network The user is now in privileged exec mode. There's also another method to use so the user is placed directly into privileged exec mode when telnetting in, avoiding the enable password prompt. Use the command privilege level 15 on the VTY lines to do so.
Digital Cameras R3#conf t
Desktops R3(config)#line vty 0 4
Cognos R3(config-line)#privilege level 15
Hosting R1#telnet 3.3.3.3
Netfinity Trying 3.3.3.3 ... Open
Internet User Access Verification
Cheap Computer Password: < user entered VTY line password here >
Digital Camera R3#
Printer Note that the user went straight to privileged exec mode.
Xseries Managing Telnet Connections
Maxtor
We already know how to use Telnet (a layer 7 application) to access a remote device there are also commands that help us manage telnet connections.
Data Storage "show sessions" is a common command to see what current telnet sessions are operating.
Hitachi Telnet sessions do not have to be exited they can be suspended as well. The command to suspend the Telnet session is followed by striking the "X" key.
Rational To resume this telnet session, enter the resume command followed by the session number ("resume 1") and press .
Websphere To end a suspended telnet session, enter the disconnect command followed by the session number ("disconnect 1") and press .
Battery
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