The MPLS WG Archive

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

Mayank Kumar wrote:
> but then the answer to my second ques, does not give me a good reason. Why
> is it necessary to have interface specific labels for atm or frame relay???
> how does it matter that , we have the label encapuslated in the L2 header in
> case of atm or frame-relay.Am i missing something in saying that ,we can
> always assign a label from the platform wide label space , as long as that
> label confirms to be a valid dlci (for frame-relay) or a valid vpi/vci
> air( for atm).

The nature of ATM is to assign VCIDs on a per-interface basis.  No 
existing ATM switch supports the concept of per-platform VCIDs, because 
doing so would break all of the ATM protocols.

Since ATM switches, and by extensions ATM interfaces, all use 
per-interface VCIDs, it makes no sense to specify per-platform labels 
for these interfaces.  You'll just end up specifying something that will 
never be implemented.

I assume a similar argument holds for FR.

And, as was already said, label spaces for ATM and FR are fundamentally 
different from shim-label label spaces.  Shim-header label spaces use a 
20-bit integer for a label.  FR uses two different size DLCIs, neither 
of which is 20 bits - and there are per-interface range limits.  ATM 
uses the two-dimensional VPI/VCI values, where VPI may be two different 
widths, and there are per-interface range limits on each dimension.

Trying to combine all three of these incompatible technologies into one 
huge pool of labels is asking for disaster.

-- David