[cisco-bba] how to simultaneously terminate L2TP sessions and provide differing levels of congestion between classes of user

[Tony Redstone]

Hello,

We're trying to solve a problem whereby when a 155Mb/s service from
our L2TP DSL supplier is congested, we want to give differing levels
of priority to, say, two, previously defined classes of end user.  We
want do this in a way which preserves the end users' dscp/prec bits.

To give a more specific example, say all users can be put into two
groups, "home" and "office."  Under congested conditions, office users
should be given a larger share of the 155Mb/s than home users.

The setup is this: 1x 155Mb L2TP service (from supplier) terminating
on an OC3 ATM interface in a Cisco 7301 running mainline 12.3(x) where
x>=13 (we have multiple NASes).  Sessions arrive from supplier and hit
the default vpdn group.  All sessions are terminated locally, assigned
an IP etc and given internet access out through one of the GE ports.

We've found a solution which appears to work by setting a policy-map
on the virtual-access inteface which sets the discard-class either to
1 or 2 depending on whether the user is "home" or "office,"
respectively.  The discard-class seems to be carried over correctly
after L2TP header encapsulation and so we can set a policy-map
outbound through the OC3 which gives more bandwidth to discard-class 2
than discard-class 1.  I'm not sure if this is an intended use of
discard-class and I've been unable to find any documentation which
says what we've tried should work (ie is supported).

We initially tried using qos-group but we get an error saying you
can't use set qos-group in a policy-map applied in an outbound
direction (which, generally speaking, of course makes sense).

I was wondering if anyone has tried to solve a similar problem and
what solution you came up with ?   any comments on using discard-class
 ?   I'm worried that if we implement this, we'll do an upgrade to
12.3(21) sometime in the future and it all breaks because it's not
supported ;-)

Regards,
Tony

tested on 12.3(13a):

In radius:
  set Cisco-Policy-Down == dc-home
   or  Cisco-Policy-Down == dc-office

on the NAS:
policy-map dc-home
  class class-default
    set discard-class 1
!
policy-map dc-office
  class class-default
    set discard-class 2
!
class-map dc1
  match discard-class 1
!
class-map dc2
  match discard-class 2
!
policy-map l2tp-out
  class dc1
    bandwidth X
  class dc2
    bandwidth Y
!
interface a1/0
   pvc a/b
      service-policy output l2tp-out
!