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