From: Baety Wayne A1C 18 CS/SCBX (Wayne.Baety@xxxxxxxxxxxxx)
Date: Thu Feb 21 2002 - 12:30:18 GMT-3
You'll need 2 external modems configured back to back and ignoring CD
(usually ATX0 or ATX1 configuration command) to get a simulation like this
working. If you use 56K modems you will only be able to connect at 33.6K
because of the double digital-to-analog conversions and quantization errors.
R1 (aux) ----- (aux) R3
Figure 1. Back to Back AUX port connections with rolled cable.
The reason that this setup doesn't work for calling out in both directions
is because the DTR and DSR leads are shared between the two devices, both
believe each other to be the DCE. The DTR signal is raised when a router
wants to communicate with a DCE, but in order to do so it waits until DSR is
raised, signaling that the distant DCE is online and ready to receive
traffic. In this simulation, R1 is configured to lower DTR when bringing
down the dialer interface (modem host or modem callout), holding the signal
low and only raising it when it wants to pass interesting traffic along.
This makes R3 believe that the DCE is offline and there isn't a callin
connection in progress. In order for this communication flow to proceed
correctly R3 must always keep its DTR (r1's DSR) signal high. R3 is
configured to watch DSR (r1's DTR signal) and treat a raised signal as a
callin connection. If R3 were to lower its DTR signal, like R1 does, then
when R1 wants to initiate a connection it will see DSR low and think the DCE
device is offline. Eventually it will timeout the attempt unless R3
magically needs to send interesting traffic at the same time. If R1 were
configured to keep DTR constantly raised, like R3 does in this scenario
(modem inout or modem dialin), then the connection would never drop, since
R3 will interpret the raised DSR signal (r1's DTR) as a callin connection.
In short, only one side can be configured as a DTR activator. The other
must always keep DTR high so that both signals are high when the activator
initiates communication.
The workaround is to use true DCE devices, 2 modems configured back to back.
WAYNE BAETY, MCSE, A1C, USAF
Network Systems Trainer
> -----Original Message-----
> From: Wu, Sean [mailto:sean.wu@capitalone.com]
> Sent: Thursday, February 21, 2002 11:39 AM
> To: CCIELab (E-mail)
> Subject: aux port back-to-back
>
> Hi, Guys,
>
> Question about Aux port back-to-back connection to simulate ISDN DDR
>
> Below is a list of urls about aux b2b info.
>
> http://www.cisco.com/warp/public/793/access_dial/auxback.html
> I used the sample config, but it didn't work
>
> http://www.cisco.com/warp/public/471/mod-aux-dialout.html
>
> http://www.groupstudy.com/archives/ccielab/200201/msg00998.html
>
> http://www.groupstudy.com/archives/ccielab/200201/msg00997.html
>
> This config works great expect that only allows one way call, i.e, you can
> only ping
> from r1 to r3, not r3 to r1 in order to bring up the circuit.
>
> the modem type under "line aux 0" is a magic. I wonder if we have to use
> "host" in one end and the other end "dialin". Apparently, when I use
> "modem
> inout" on both ends, it won't work.
>
> Any comments or working config examples?
>
> thanks.
>
>
> Sean Wu
>
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