PPPoE over L2TP over GigE questions

Wed Jun 11 16:13:23 UTC 2008

Dear Mezei,

> Would the L2TP payload be an ethernet packet which contains a PPPoE
> packet, or would the L2TP payload be the PPPoE packet only ?

ppp frame in l2tp (udp packet).
http://www.faqs.org/rfcs/rfc2661.html
5.0 Protocol Operation

l2tp is designed for minimal overhead.

> Also, while I am at it:
>
> Architecturally, is a BAS considered a router, or a bridge/switch ?
> (since the PPPoE packet has no routing information (source,
> destination), it is the BAS which maintains the table of
> source/destination for each PPPoE session ID. Yet, the BAS machines are
> supposedly Juniper ERX routers in Bell territory...

the BAS is a LAC (L2TP Access Concentrator), which preauth the pppoe 
session, create, if needed a L2TP tunnel to a LNS (L2TP Network Server), 
handle the authentication between client (pppoe) and LNS.
L2TP use one tunnel for 1 LAC - LNS link, meaning more than one pppoe 
tunnel use a L2TP tunnel link.

> And while I am at it:
>
>> From the end user point of view, the ADSL modem sends all ATM frames to
> a predetermined ATM destination (VPI/VCI). I assume that VPI/VCI points
> to the BAS.

Depends on network design.
As adsl use ATM as line protocol, you need VPI/VCI.
protocol stack:
pppoe
ethernet
ATM

at the provider side you have various options.
it is very common that the dslam, that terminates the adsl line has an 
ethernet upstream port.

> How does the BAS address ATM packets back to an individual subscriber ?
> Do each subscribers get their own VPI/VCI that points to the right port
> on the right DSLAM ?

That is done via ppp(oe) authentication.

> And in cases where the telcos are extending the ethernet to the DSLAM,
> with the fragmentation into multiple ATM frames limited to the ADSL link
> itself, how does the BAS address invididual customers ? Does each ADSL
> port on the DSLAM get its own ethernet address ?

pppoe is ethernet, so they use the mac adress of the pppoe source (client 
pc, adsl modem, whatever)

> (since some services do not use PPPoE, I have to assume that the DSLAM
> doesn't base its packet switching on PPPoE session IDs.)

pppoe is commonly used for large scale setups.
but you can also build a network without pppoe and plain ethernet.

Kind regards,
 	Ingo Flaschberger