Cisco Router Reverse Telnet

28. September 2015 Cisco Router 5,757

If you connect the router’s aux port to another device’s console port, you can telnet to the router using reverse telnet to connect to the other device’s console via your aux port. Here’s how to set it up.

  1. Connect your router’s aux port to another device’s console port using a Cisco flat crossover cable, or basically a rollover cable.  So pin 1 is on pin 8 on the other end of the cable.
  2. TELNET to your router, enable, and set the aux port up as follows:
    # conf t
    # line aux 0
    (config-line)# modem InOut
    (config-line)# transport preferred all
    (config-line)# transport input all
    (config-line)# transport output all
    (config-line)# exit
  3. Make sure you have an address on your loopback interface, and the interface is up.
  4. Back in enable mode, find out the line number of your AUX port with#show line. It will look something like this:
       Tty Typ     Tx/Rx    A Modem
         0 CTY              -    - 
    *    1 AUX   9600/9600  - inout
    *   98 VTY              -    - 
        99 VTY              -    - 
       100 VTY              -    - 
       101 VTY              -    - 
       102 VTY              -    - 

    On this router it is line 1. It’s different from router to router but I’ve never seen it change.

  5. Now all you need to do is add 2000 to the line number, and telnet to your loopback0 address, port 2001.  (i.e. telnet 2001)

At this point, you may get a login prompt.  Login to your router as normal, press enter a few times, and the console of the other device should come up.


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