Cisco CCNP Certification Exam Training:
Reverse Telnet And IP Host Tables
By Chris Bryant, CCIE #12933
Knowing reverse telnet and how to use IP Host tables is helpful for your CCNA certification exam prep, and those of you with your own Cisco home labs may well have used reverse telnet without even knowing it! But if you're like me, the first time I heard the term, I thought "how do you reverse the telnet process?"
Think of it this way - when you're using regular Telnet, you're accepting a connection into a line, a forward connection.
When Reverse Telnet is in use, a Telnet session is being initiated out an asynchronous line.
Assuming the router has an ethernet0 IP address of 150.1.1.1 and the modem is connected to line 2, the syntax would be as follows:
AccessServer# telnet 150.1.1.1 2002
Trying 150.1.1.1, 2002 ... Open
Where does the "2002" come from? The TCP base port for the router's individual lines is 2000. Just add the line number to 2000 and you've got the rest of the command.
Testing the connection is just as easy. Simply send an AT command to the router; if it responds with OK, everything really is OK! If not, run show line 1 - if no CTS appears there, there's a problem with the physical connection. I don't care what you're troubleshooting, we always start at Layer 1!
Finally, the Reverse Telnet session must be suspended and then terminated. If you don't disconnect properly, the line cannot accept incoming calls because it will still be considered busy. Or worse, another user could connect that still-active session....without being prompted for a password!
To suspend the Reverse Telnet session: < ctrl-shift-6> < x >
To terminate the session: AccessServer# disconnect
The IP Host table makes Reverse Telnet sessions a little easier. The ip host command is used to build such a table, and here's what a typical IP Host table on an access server in a home lab looks like:
ip host R3 2003 100.1.1.1
ip host R1 2001 100.1.1.1
ip host R2 2002 100.1.1.1
ip host R4 2004 100.1.1.1
ip host SW1 2005 100.1.1.1
ip host SW2 2006 100.1.1.1
The IP address in use here is a loopback interface, but it could be the ethernet address as well. From here, all you need to do is type "R1" to connect to the device R1 via line 1 (2001).
166#r1
Trying R1 (100.1.1.1, 2001)... Open
R1#
Knowing how to use IP Host tables and reverse telnet helps you on your way to CCNP certification and makes working with your Cisco lab much easier. Have fun!
To your success,
Chris Bryant
CCIE #12933
chris@thebryantadvantage.com
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