PPPoE over L2TP over GigE questions

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.)

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

Jean-Fran´┐Żois Mezei <jfmezei@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. :slight_smile:

-r

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 tunnels.

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

f.

That's some really good news... does it mean they're getting rid of
the ATM network and the *&()&* Newbridges too?

It's been a year and a half since I've even logged into the LNSes in
question, and over two years since doing any meaningful
reconfiguration... but it's good to hear my friends in Canada are
getting improved service from Bell in some areas, even if they offset
it by doing stupid stuff in other areas. :-/

-r

Francois Menard <francois@menards.ca> writes:

This has been a very informative thread. All sorts of acronyms to
research and so forth. :slight_smile:

The mention of TR-101 took me down another rabbit hole, and I discovered
http://www.dslforum.org/trlist/trlist.php.

Very interesting info.

Charles

How about a couple more rabbit holes to dig into:

Try to find the intersection between:

DSL Forum TR-101 unbundling
Bell replacing tunnel switching with AGAS
802.3AH PBB
QinQ
NENA i2
Wiremap update protocol
LIS
SIP Location Conveyance
PIDF-LO transmission over the NENA i2 V0 interface
Carrying the originating DSLAM port over the RADIUS accounting interface
Bitstream unbundling of the multicast plane
Denial of access to subloop unbundling
Phase II costing of DSL access and capping mark-ups to 15%
Capping the mark-up for conditional-essential services
Throttling
DPI
Net Neutrality
Dark fibre forbearance
Phase out of non-essential services

You mix this alphabet soup, eat a good portion daily - this keeps your brain sharp and the doctor away

We're trying to figure out how the next 10 years of DSL unbundling will be done, such as that it will support prioritized VoIP, triple play and E-9-1-1, and how ISPs will be able to secure their own DSL aggregation out of dark fibre being available at tariffed rates.

Comments due June 25 at the CRTC.

F.