The MPLS WG Archive

[David Charlap <david.charlap@marconi.com>]

Philip Matthews wrote:
> 
> However, there are routers with ATM cards that use a fourth technique:
> send all packets over a single VC and use the top label stack entry to
> carry the label.

AFAIK, this is not a legal technique.  The only router I know that does
this is only doing so because it doesn't actually claim support MPLS on
ATM interfaces.  What you're seeing is simply a side effect of running
MPLS on an unsupported interface - when you do so, it takes POS-
formatted data and blindly shoves it out a VC, just like it does for
control traffic.

(Note that this technique is not mentioned in the
draft-ietf-mpls-arch-06.txt spec.)

Even if would be legal, this fourth technique will not be very efficient
on many hardware platforms.  ATM backplanes are usually optimized for
forwarding cells based on VP/VC numbers.  They are not deisgned to run
efficiently in a situation where they must reassemble (from the cells)
each and every packet prior to making a forwarding decision.

See also draft-ietf-mpls-atm-03.txt.

-- David