[geeks] ADSL Modem

[Phil Brutsche]

Sridhar Ayengar wrote:
>   I just have to figure out how to make sure that the Cisco gets the 
> external IP address, and not the Zyxel.  There's probably a "bridge 
> mode" or something similar in the Zyxel.

If the Zyxel is in bridge mode you're using PPPoE. Some ISPs (ie mine,
but I live in a rural county with less than 100k people and the
telephone company *is* the only real ISP) configure the bridged DSL
modems so that you use DHCP and it all Just Works, like on a cable
modem. But the majority (ie yours) aren't like that ;)

If the Zyxel is in PPPoA mode the Zyxel gets the IP address.

In the latter case you would end up making the Zyxel the default gateway
for ethernet-attached devices and using the Cisco router for the
non-ethernet stuff.

You'd be splitting up your IP block though.

> Unfortunately, there's nothing they can do about getting rid of PPPoE. 
> That seems to be dictated by Verizon.  I'm going to ask them tomorrow if 
> they can do PPPoA.  We'll see what happens.

My experience is that it's all router/modem configuration and the ISP
has little to do with it.

>From a configuration POV the difference between PPPoA and PPPoE is:
 * in PPPoA the DSL modem handles PPP and is your default gateway
 * in PPPoE the DSL modem is configured to bridge the ATM and ethernet
interfaces, while your PC/firewall/whatever does the PPP heavy lifting

Aside from the bridging and the PPP, the configurations are nearly
identical. I have save configs for a Cisco 1720 w/WIC-1ADSL to do both.

If you know:
 * your PPP username
 * your PPP password
 * the VPI and VCI numbers for the ATM PVC
then you can setup your DSL modem to be a router without bothering your ISP.

If they say they don't support PPPoA they may be hesitant to tell you
the VPI and VCI information.

(I think some of that might answer the question you emailed me about
earlier today)

-- 

Phil Brutsche
phil at tux.obix.com