PPPoE over L2TP over GigE questions

Robert E. Seastrom rs at seastrom.com
Wed Jun 11 19:37:42 UTC 2008

Jean-François Mezei <jfmezei at vaxination.ca> writes:

> 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.

Actually, they don't set up connections directly from the BASes and
SMSes anymore.  I'm quite sure they've got some old Redback kit still
out there too, as well as perhaps some other ancient stuff.

You're going to be talking to a tunnel switch (TSW2-TORONTO63 for
instance).  These are all Juniper ERXes to the best of my knowledge.

N number of BAS/SMS devices talk to a TSW, which talks to your LNS.
This cuts down drastically on the number of tunnels that you have to
manage (Bell has a couple of hundred BASes out there last I checked).
Brings the number of tunnels (and VLANs) down to a couple of hundred.
The tunnel switch is smart enough to look inside the authentication
packets at session start time and switch you properly based on the
realm the customer is logging into.

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

My recollection is that it includes the src/dst MAC addresses and the
rest of the ethernet header in the L2TP frame.

> 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...

I'd call them VPN endpoints for a layer 2 VPN; thus the functionality
they're providing is more like a bridge than a router, notwithstanding
their peeking into layer 5.

> 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.

Yes, and at that point it's PPPoEoATM.  Which gets turned into
PPPoEoATMoL2TP on the upstream side of 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 ?

Nothing that's visible on the upstream side of the BAS - it's all
src/dst mac address at that point.

> 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 ?

the ADSL router has 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.)

These other services are VLAN-per-customer and don't use PPPoE or L2TP
at all.  I think we looked at these and decided not to use 'em.

You may be thinking too deeply about this though.  Contact me offline
if you want a working redacted config for Cisco kit talking to Bell
Canada.  :-)


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