PPPoE over L2TP over GigE questions

Francois Menard francois at menards.ca
Tue Jun 17 11:20:14 UTC 2008

Actually, with AGAS, there are no tunnel switches anymore

multiple tunnels are set-up directly netween Juniper ERXes aggregating  
DSLAMs and acting as LAC's and the ISPs LNS's receiving the L2TP  

This is one giant step towards TR-101, but Bell won't accept to do this


On 11-Jun-08, at 3:37 PM, Robert E. Seastrom wrote:

> 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  
>> with the fragmentation into multiple ATM frames limited to the ADSL  
>> link
>> itself, how does the BAS address invididual customers ? Does each  
>> 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  
>> 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.  :-)
> -r

François D. Ménard
francois at menards.ca

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