The MPLS WG Archive

[Mukul Katiyar <mukul@cplane.com>]

Jeremy,

> 
> >3. the VCID case. Again an overlay model where there exists CLIP or LANE
> 
> Same answer as #2: similar but not identical to CLIP, nothing to
> do with LANE.
> 
> >except here MPLS coordinates with ATM signaling to create SVCs as needed. L3
> >& ATM routing protocols are run.
> 
> That's not really true. The routing situation is actually very similar
> to #2. The ATM switches, or most of them, are not LSRs, and just run an
> SVC routing protocol (typically PNNI). The LSRs run IP routing,
> but don't really participate in PNNI, except to request SVC
> set-ups.
> 
> The major difference between #2 and #3 is that VCID adds some hooks to
> support SVCs as well as PVCs. Otherwise, they're fairly similar.
> 

I think #2 and #3 are similar only from the point
of view of mix of ATM and MPLS signalling & routing
protocols used for setting up the channels,
but they differ fundamentally with respect to their models. 
While #2 is a classical overlay model, the MPLS network
remains unaware of the existence of
ATM switched connections, and multiple LSPs "share" the same ATM
virtual connection using it as a virtual link. 

The #3 is a peer model whereby the LSP is tunneled through the
ATM network using ATM signalling. The edge LSRs in this case are
certainly aware of existance of the ATM switched connections and
trigger the tunnel establishment for the LSP.
The encapsulation used for #3 is similar to #1 as specified in 
draft-ietf-mpls-atm-04.txt, rather then that of #2 which shall
use what ever has been established by CLIP or LANE.
I think this makes the case #3 to be similar to #1 rather then #2.
please correct me if I am missing something.

regards
Mukul

> Here's the other one:
> 
> 4. Virtual Trunks. This is similar to #2, except that PVPs or SPVPs
> are used instead of (S)PVCs. This means several things:
> - The LSRs at the ends of the PVPs use ATM MPLS encapsulation, with
>   distinct VCs within the PVP signifying distinct labels.
> - The PVPs are 'virtual trunks', performing much the same function
>   as a physical ATM link between the LSRs terminating the PVPs.
> - Again, not all ATM switches are LSRs.
> - The PVPs may terminate on ATM-LSRs, which may switch between
>   ordinary ATM MPLS links (scheme #1) and virtual trunks. These
>   ATM-LSRs run both IP routing and whatever the switch uses for
>   (S)PVPs, but there is no interaction between the two.
> 
> For interworking between CLIP or LANE and ATM MPLS, any of these
> schemes can be used, with the edge LSRs having some CLIP or LANE
> interfaces and some ATM MPLS. CLIP or LANE and ATM MPLS are
> quite different IP-over-ATM schemes, and a full layer 3 routing
> function (e.g. in an edge LSR) is usually required to interwork
> between them.
> 
> Regards,
> 
> Jeremy Lawrence