Friday, November 6, 2009

AUX back-to-back: poor man's connection

Hi all,

today I have focused my attention on the Aux port, the only free I have in my old 2600's lab....

So, first I have found some old docs on Cisco.com explaining clearly how to connect two routers back-to-back using the AUX port:

Connecting Routers Back-to-Back Through the AUX Ports (Document ID: 10365 )

and then was time to try it:

1) use an old rollover RJ-45 cable to connect AUX ports (pins 1-8 to 8-1... as learned in CCNA times)


2) Find the AUX tty number on both sides:

R3#sh line
Tty Typ Tx/Rx A Modem Roty AccO AccI Uses Noise Overruns Int
* 0 CTY - - - - - 0 0 0/0 -
65 AUX 9600/9600 - - - - - 0 0 0/0 -
66 VTY - - - - - 0 0 0/0 -
67 VTY - - - - - 0 0 0/0 -
68 VTY - - - - - 0 0 0/0 -
69 VTY - - - - - 0 0 0/0 -
70 VTY - - - - - 0 0 0/0 -

Line(s) not in async mode -or- with no hardware support:
1-64



R5#sh line
Tty Typ Tx/Rx A Modem Roty AccO AccI Uses Noise Overruns Int
* 0 CTY - - - - - 0 0 0/0 -
5 AUX 9600/9600 - - - - - 0 0 0/0 -
6 VTY - - - - - 0 0 0/0 -
7 VTY - - - - - 0 0 0/0 -
8 VTY - - - - - 0 0 0/0 -
9 VTY - - - - - 0 0 0/0 -
10 VTY - - - - - 0 0 0/0 -

Line(s) not in async mode -or- with no hardware support:
1-4

As you can see, different hardware/platform can use different tty numbers for AUX port, here we have tty 65 and tty 5

3) configure the AUX port on both sides:

R5(config)#line aux 0
R5(config-line)#transport input all
R5(config-line)#modem inOut
R5(config-line)#flowcontrol hardware
R5(config-line)#speed 115200 !-- better than 9600...
R5(config-line)#end

!-- same on R3


4) Create and configure the async interfaces (each interface uses the tty number of AUX port as point 2)


R5(config)#int async 5 !-- remember the tty number for AUX?
R5(config-if)#encapsulation ppp
R5(config-if)#async default routing
R5(config-if)#async mode dedicated
R5(config-if)#ip address 10.0.0.5 255.255.255.0
R5(config-if)#end
R5#


R3(config)#int async 65 !-- remember the tty number for AUX?
R3(config-if)#encapsulation ppp
R3(config-if)#async default routing
R3(config-if)#async mode dedicated
R3(config-if)#ip address 10.0.0.3 255.255.255.0
R3(config-if)#end



wait a little and you will see messages like

R5#
*Nov 6 16:58:20.237: %LINK-3-UPDOWN: Interface Async5, changed state to up
*Nov 6 16:58:23.394: %LINEPROTO-5-UPDOWN: Line protocol on Interface Async5, changed state to up
R5#


...Now you have a poor man's additional interface for your lab!
You can ping it and use it for dynamic routing (althrough u can't use it for mpls or other advanced features... ;-) ) (PS: Nov 14, another "informal meeting" aka a beer with Nicola Modena today (CCIE R&S #19119) and he said he used Aux back-to-back connections since a couple of years, and he runs mpls over it! ;-) so I corrected this post (and tryed it with mpls too ;-) )

R2#sh run int async 1 | beg int
interface Async1
ip address 192.168.0.2 255.255.255.0
encapsulation ppp
ip ospf 1 area 0.0.0.0
async dynamic routing
async mode dedicated
mpls ip
routing dynamic
end

R2#sh mpls interfaces detail
Interface Async1:
IP labeling enabled (ldp):
Interface config
LSP Tunnel labeling not enabled
BGP labeling not enabled
MPLS operational
MTU = 1500
R2#



Marco


NOTE: I have found an additional note on Document ID: 5465 (Configuring Dialout using a Modem on the AUX Port )
that say about the AUX speed:

speed 115200
!--- The AUX port on the 2600 supports a speed of 115200.
!--- Note: If you route through the AUX port, each character generates a
!--- processor interrupt. This is an abnormally high load on the CPU,
!--- which can be resolved if you use a lower AUX port speed.

I guess that in a lab environment the cpu usage will be low.

1 comment:

Luca Gervasi said...

Thanks Marco, you just gave me one p2p interface more for each router :)

(I wonder if there exists some kind of switch...)

Luca