[c-nsp] QoS on 837 using PPPoE

[Dean Smith]

In addition when working on QoS on PPPoA we found...

With the SP on the physical vbr-nrt is indeed required. But that breaks the 
automatic tracking of the PVC size to the negotiated upstream rate on rate 
adaptive DSL. So we had to use TCL to track the DSL speed and change the 
VBR-NRT size to match ( we do this hourly in some cases / nightly in others)

Also we usually mark DSCP via the same SP as the policing - but with the SP 
on the ATM interface this was broken. So we have to mark inbound on the LAN 
and shape outbound on the ATM giving 2 policies to maintain.

If you run MLPPP - then you can do everything on one SP on the bundle 
interface but that gave 2 further issues - scaling the head end for MLPPP 
performance and we had random drop outs of the Multilink leaving us with no 
QoS at all.

All in all - not exactly simple and took months and months of lab work to 
get right. Cisco should be taking a long hard look at the architecture for 
future xDSL variants - and some decent doumentation wouldnt go amiss either.