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Reverse Telnet -how does this work

Posted on 2001-09-16
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Can someone give me a run down on how to set up a reverse telnet scenario between a Cisco router and 2 attached switches.
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Question by:Silas
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scraig84 earned 20 total points
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I don't know how much info you were looking for, but it is pretty simple.  First off, you need to have a router with async ports.  Attach the async ports to the switches' console ports and configure the lines in the IOS config with something similar to:

line 110 111
 no exec
 disconnect-character 81
 transport input telnet

Last you telnet to the Cisco router, using the line # plus 2000 as the port.  Following the example above, if I was trying to go to the switch on line 110 and an IP address on the Cisco router is 1.2.3.4, I would type the following to get to the switch:

telnet 1.2.3.4 2110

One thing to remember when doing reverse telnet is to exit the exec session before terminating the telnet session.  If you don't, you may wind up leaving the switch in enbable mode.

The following link may help as well:
http://www.cisco.com/warp/public/793/access_dial/comm_server.html

Hope that gets you started.
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by:Chriskohn
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Here is a link from Cisco that may help you
http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/12cgcr/dial_c/dcrtelnt.htm   Hope this helps, Worker
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by:Silas
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line vty 0 4 makes sense to me, but line 110?  What exactly does that refer to?
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by:scraig84
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It is a physical line, such as an asynch port.  If you add an async card to your router, you need to configure the physical aspects of these lines with the line command.  A vty line is a telnet line allowing inbound telnet connections.  When you reverse telnet, you are initiating a telnet session over a vty line to the router, and it is then initiating a physical connection over a physical line that was specified with the port used in the telnet command.

After you have added an async card, when you do a "show line", you will see the available TTY lines that it has associated with the async ports.  If you do a show line on any router, you will at least see the line numbers associated with the console port, and additionally you will see lines for an aux port and any vty lines you have created.

Hope that helps.
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by:irp
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If you do not have async ports (from NM-16A, NM-4A/S, WIC-2A/S) etc. you can use the AUX port (1700 series upwards), which appears as different line numbers on different platforms. This obviously only gives you one port though.
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