PPPoE over L2TP over GigE questions

Jean-François Mezei jfmezei at vaxination.ca
Wed Jun 11 15:45:09 UTC 2008

Pardon my ignorance on the subject, but I would need to know how packets
between a BAS/LAC and an ISP's router are transported (this is within
Bell Canada ADSL territory).

Bell uses L2TP to link each BAS/LAC to the ISP.  Some of the ISPs get a
Gigabit Ethernet link to the Bell cloud.

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

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...

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.

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 ?

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 ?

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

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