I'm going to buy a Cisco 1750, but its lack the ADSL WIC Module. My question is, can I use the AUX Port to plug my RJ11 for DSL PPPoE Authentication? Like this:

          Cisco 1750
ISP --- AUX ------ F0/0 --- INTERNAL

If not, will I be able to do this with a consumer dlink modem?

ISP --- D-LINK MODEM ---- f0/0 CISCO 1750 f0/1 ---- INTERNAL
          BRIDGED              (PPPoE Auth)


2 Answers 2


The first diagram will not work, and in fact may result in damage to your router. The AUX port is a regular RS232 serial port, which is completely incompatible with ADSL. Don't do it.

The second diagram should work, provided your D-Link modem can operate in bridged mode.

  • That's what I was looking for. The modem does support bridge, so it's ok. Thx! Aug 29, 2014 at 18:00

Except the 1750 only has one fastethernet port. A WIC-1ENET is 10m, WIC-4ESW a 4 port switch... but they both live on the WIC bus that'll top out about 6-8m. Depending on your DSL speed, this may not be a problem. I use a WIC-1ADSL (and WIC-4ESW) in a 1720, but the DSL line is 6m anyway.

(and neither the 1720 nor 1750 support dot1q; that's a feature of the 17x1 line.)

  • Also consider the effects of Csma/CD if you negotiate at 10/half. This ultimately punishes you for bidirectional traffic Aug 29, 2014 at 19:12
  • 10base-whatever doesn't do autoneg. If you want full-duplex, you have to set both ends to full. (which is why my cable modem had to go through a switch first.)
    – Ricky
    Aug 29, 2014 at 19:44
  • Assuming the opposite side supports autoneg, it will see the lack of c1700 autoneg pulses and fall back to 10/half. I agree that negotiate is the wrong term here. Aug 29, 2014 at 22:01

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