On Sun, Aug 03, 2003, Quentin Garnier wrote:
> I can't speak on the behalf of my ISP, but AFAIK, the ISP only sees PPPoA
> sessions. The BAS is the PPPoE server, and transmits the PPP packets it
> doesn't handle over an ATM link.

PPPoA sessions are recognized because the ATM line rate is visible
in customer's virtual-interface.

Virtual-Access16 is up, line protocol is up 
  Hardware is Virtual Access interface
  Interface is unnumbered. Using address of Loopback1 (62.4.16.253)
  MTU 1500 bytes, BW 608 Kbit, DLY 100000 usec, 
                  ^^^^^^^^^^^

When we see:

Virtual-Access32 is up, line protocol is up 
  Hardware is Virtual Access interface
  Interface is unnumbered. Using address of Loopback1 (62.4.16.253)
  MTU 1500 bytes, BW 155520 Kbit, DLY 100000 usec, 
                  ^^^^^^^^^^^^^^

we know the session is not PPPoA but "other" (PPPoE, PPTP, etc.). So
we see the line rate between BAS and DSLAM, which is STM-1.

But this is just an information when session is established, it is
not possible to switch between different profiles when the session
is coming.

> What makes it broken here is that it is the BAS that negociates the MRU,
> although it should be the LNS, which is the real peer of the PPP
> connection.
> 
> Raphael, can you confirm I'm not mistaken here?

The LNS does not know there is a PPPoE (MTU = 1492) link between
customer's machine and the network. Only the BAS knows it, but
as a part of a VPN network, it cannot return ICMPs to tell a
remote server that it must fragment packets.

-- 
Raphael Bouaziz.
Administrateur Systèmes & Réseaux
Nerim -- Fournisseur d'accès à Internet
URL: <http://www.nerim.net/>