[geeks] ADSL Modem

[Sridhar Ayengar]

Phil Brutsche 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 ;)

I'm reasonably certain even my provider does that, but I'm a special 
case.  Most of my ISP's customers probably don't have a block.

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

There's no way that I can get the Zyxel to bridge ethernet over PPPoA? 
I'd like to be able to use PPPoA just so I don't have to shrink my 
outbound MTU.

> 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.

This is not an option because it would drastically cut performance. 
Otherwise, I wouldn't be running my Cisco anyway.  I'd just use a PC.  I 
use *switched* FDDI on my network, and there wouldn't be much reason to 
do that if I didn't want good performance.

>> 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.

Interesting.  I really wish there were a way to get PPPoA + bridging. 
According to the manual I've been reading for the Zyxel 645, the two 
setup screens appear not to have much to do with each other.  Will it 
not allow me to configure PPPoA+CHAP if I have bridging configured?

>>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

I know for a fact that the Zyxel in question can definitely do 
PPPoE+routing in addition to PPPoA+routing.

> 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.

I'll see if I can get the VPI and VCI from the ISP tomorrow.  (Or today, 
it's after midnight here.)

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

They treat me quite well, because I have five separate DSL services. 
They want to keep me.

Peace...  Sridhar