Reverse Telnet issue

Unanswered Question
Mar 14th, 2007
User Badges:

I was wondering if it were possible to reverse telnet from a Cisco 4000 router AUX port to a Cisco 2621XM router AUX port? I'm using the black RS232 DTE to RJ45 DTE console cable that usually comes with Cisco 6500's.

Here the config that I thought would work but doesn't:

Cisco 2621XM (AUX port 2065)

line aux 0

modem InOut

transport input all

transport output all

stopbits 1

Cisco 4000 (AUX port 2001)

line aux 0

modem InOut

transport input all

transport output all

stopbits 1

Note, I have loopbacks set on each router.

When I telnet from the 4000 to the 2621, I get a conection opened, but nothing happens. The Tx/Rx is set to 9600 on both ends.

CISCO4000#telnet 2001

Trying, 2001 ... Open

Any help or assistance would greatly be appreciated.


  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
peterledwidge Thu, 03/15/2007 - 08:25
User Badges:

Have you tried to set a password and login on the aux line?


bardak5328 Fri, 03/23/2007 - 07:53
User Badges:

Thanks, I tried that but it still didn't work. Do you think it's even possible to do an Aux to Aux connection without modems?

mohammedmahmoud Sat, 03/24/2007 - 09:46
User Badges:
  • Green, 3000 points or more

Hi there,

Cisco routers can be connected back-to-back (with no modems required) through the asynchronous AUX ports with the help of a null modem cable (rollover cable). The AUX ports of the two routers are directly connected with the help of a rollover cable with Point-to-Point Protocol (PPP) running on the link. The AUX ports are data terminal equipment (DTE) devices. You will require a null modem cable (rollover cable) to connect DTE to DTE devices.

You must add the following as illustrated in the document, its not like the reverse telnet (from Aux to Console port):

interface Async1

ip address

encapsulation ppp

async dynamic routing

async mode dedicated

Can i ask, why are you not using reverse telnet connection with Aux connected to Console ?


Mohammed Mahmoud.

mohammedmahmoud Wed, 04/11/2007 - 01:01
User Badges:
  • Green, 3000 points or more

Hi there,

You just have to put the following under the aux line on R1 (and connect the con and aux with a roll-over cable):

line aux 0

transport input telnet

stopbits 1

In order to connect from R1 aux port to R2 console port, you have to get 2 things:

1.An up/up interface IP on R1 (most commonly a loopback).

2.The tty line number of aux port on R1. (which is 1 in this case - its is most commonly 1 on most of the routers)

R1#sh line

Tty Typ Tx/Rx A Modem Roty AccO AccI Uses Noise Overruns Int

0 CTY - - - - - 1 155115 19/13410922 -

* 1 AUX 9600/9600 - - - - - 3 1 4/225411 -

* 2 VTY - - - - 55 1949 0 0/0 -

3 VTY - - - - 55 280 0 0/0 -

4 VTY - - - - 55 49 0 0/0 -

5 VTY - - - - 55 5 0 0/0 -

6 VTY - - - - 55 1 0 0/0 -

All that you have to do further is on R1:

"telnet 2001" is a loopback on R1.

2001 = 2000 + 1 (which is the line number of the aux line on R1)

HTH, Please rate if it does.

Best regrads,

Mohammed Mahmoud.

bardak5328 Wed, 04/11/2007 - 05:41
User Badges:

Hi Mohammed,

Thanks for the response. I actually don't need to route over the aux port, I just needed to get a console connection. I did get it to work with a CAB-500DTF - Rj45 ----------- Rolled Console ----------Rj45- Aux cable.

I know this is an unusual, Here's how I intend to use this setup.

Walgreens has over 5600 stores. Each store has a Cisco 2821 router and a Hughes satellite receiver for backup. The current VSAT solution in the stores is being phased out and a new Hughes 7700 receiver is taking it's place. The new receiver will have an RS-232 AUX port identical to the Cisco 4000 router. The only problem is the 7700 is still in development and won't be ready for a few months, so I'm forces to test with the Cisco 4000. The end goal is to have a working AUX to AUX or AUX to Console connection between the 2 devices. The business function is if there is a case of some outage where either the router or the vsat receiver is unreachable, that we can still manage each device via the aux/console port and troubleshoot the issue by either connecting to the store via the VSAT (Hughes receiver) or Terrestrial (Cisco router). It sounds complex but we actually got this to work last week.

We build a VSAT session on our hub equipment, pluged a laptop console session directly into the equipment. The console connections when from our data center hub went over the satillite link, through the Hughes receiver and into the console port of the router. I essentially now have the ability to troubleshoot a router in boot mode remotly via the satilite link which is pretty cool! The savings is not having to pay reocurring costs to run analog lines and modems to all stores with this new setup.

mohammedmahmoud Wed, 04/11/2007 - 11:18
User Badges:
  • Green, 3000 points or more

Hi Pete,

Nice topology there, i am glade that it worked out with you, we are using a similar topology for out of band management.

best regards,

Mohammed Mahmoud.


This Discussion