VPLS model for L2VPN Framework document

Ali's been  trying to help me understand  just what Mick and  Norm have been
objecting to with respect to the  VPLS model in the framework.  Let's see if
I've improved my understanding of the issue.

In a  pure bridged  environment, a set  of bridges  is connected via  a LAN.
Each bridge has a  single port to a particular LAN.  Each  LAN can contain a
set of  VLANs, and  a packet  on that LAN  belongs to  a particular  VLAN by
virtue of  carrying a .1Q tag.   The LAN can also  contain untagged packets,
which do not belong to any of the VLANs. 

A  multi-VLAN spanning  tree protocol  can  be run  using untagged  packets.
This protocol  will determine, for each  VLAN, whether a  particular port is
blocked FOR THAT VLAN. 

If we want to  allow the PE bridges to be part  of a larger bridged network,
it would be desirable for this  model to be supported by the VPLS framework.
This would require:

- the PE contains a single bridge entity

- that bridge entity contains a single "port" to the emulated LAN

- each distinct  VLAN on  the emulated  LAN is replaced  by a  distinct VPLS
  instance (an "emulated VLAN")

- a further distinct  VPLS instance is used to  carry the "untagged packets"
  of the emulated LAN. 

The  VPLS  instance used  to  carry the  untagged  packets  would, from  the
perspective of discovery and setup,  be an independent VPLS instance.  It is
only the  use of  this VPLS instance  that makes  it any different  than any
other  VPLS  instance. Let's  call  this  VPLS  instance the  "control  VPLS
instance".   Then we  need to  allow a  PE bridge  to attach,  via  a single
emulated LAN port,  to one control VPLS instance and  to an arbitrary number
of additional VPLS instances.

A set of PEs that attach to  a common VPLS control instance may be termed an
"island".  Note that there is no requirement that two PE bridges in the same
island support the same set of  VPLS instances, nor is there any requirement
that  a VPLS  instance  (other than  the  control instance)  stay within  an
island. 

The  framework model,  on the  other hand,  suggests that  a PE  may contain
multiple bridges,  each with a port  to a single VPLS  instance.  This would
seem  to rule  out  the possibility  of  using a  single  spanning tree  for
multiple VLANs.

To eliminate this problem, we could state  that a PE may contain one or more
bridges,  each of  which  has a  single  emulated LAN  port.  Further,  each
emulated LAN port  may attach to one or more VPLS  instances.  When a single
emulated  LAN  port  attaches to  more  than  one  VPLS instance,  the  VPLS
instances may be thought of as "emulated VLANs", and no more than one of the
VPLS instances may be used to carry traffic that would be "untagged traffic"
if  the emulated  LAN port  were a  real LAN  port.  We  could also  add the
"island" stuff from two paragraphs back. 

Should we go further, and state that  a PE must contain a single bridge with
a single  emulated port that attaches  to all the VPLS  instances?  The "one
bridge" model seems to be required if and  only if the PEs are to be part of
a  larger  bridged  network.  Since  this  won't  always  be the  case,  I'm
reluctant  to make the  "multiple bridges"  model illegal  unless it  can be
shown that there are no conceivable advantages to it.

Comments?