This littel snippet of IOS code provides a reverse telnet client from a Cisco IOS router to another device (usually the first switch after the router). It can be very handy to have console access to a remote branch core switch or similar if something goes ballistic and the reverse telnet provides as such a very affordable and easy way of setting this up on all branches. It does not scale in the same manner as a dedicated console server or an Async module for the ISR with multiple ports, but is a handy trick to known.

It is possible to utilize the AUX port of the ISR router as a console port termination. Just remember that it MUST be a roll-over cable between the two devices.

It is very simple to setup. Just configure a Loopback interface as the reverse telnet target as shown below
Note that the linenumber associated with the AUX port may vary between models and IOS versions.

router#config t
router(config)#int loopback 0
router(config-if)#ip address 172.18.0.1 255.255.255.255
router(config-if)#exit
router(config)#line aux 0
router(config-line)#modem InOut
router(config-line)#transport input all
router(config-line)#transport output all
router(config-line)#speed 9600
router(config-line)#end
router#
router#sh line
   Tty Typ     Tx/Rx    A Modem  Roty AccO AccI   Uses   Noise  Overruns   Int
     0 CTY              -    -      -    -    -      0       0     0/0       -
   225 AUX  19200/19200 - inout     -    -    -      0       0     0/0       -
*  226 VTY              -    -      -    -    -     10       0     0/0       -
   227 VTY              -    -      -    -    -      0       0     0/0       -
   228 VTY              -    -      -    -    -      0       0     0/0       -
   229 VTY              -    -      -    -    -      0       0     0/0       -
   230 VTY              -    -      -    -    -      0       0     0/0       -
router#
router#telnet 172.18.0.1 2225

When done you press CTRL+SHIFT+6, x and write “disconnect”. If you forget the line stays open and must later be cut off with the  “clear line x” command., where x is found by typing “who”.

(Edit) – Translated original post to English