The MPLS WG Archive

[Jeremy Lawrence <jlawrenc@cisco.com>]

At 11:29 08/24/2000 -0400, Paul Tasillo wrote:
>Thank you for the insight.
>So if I understand correctly, there are three possible MPLS over ATM
>schemes:

Actually four: I'll add one at the end.

>1. ATM-LSR: Here the the LSR (maybe all of the links) has a IP address and
>participates in L3 routing protocols. MPLS controls signaling and assigns
>VPI/VCIs in a meaningful way to create VC to next hops.

True.

>2. The ATM cloud has a mesh of PVC which MPLS uses as ptp links. This is
>frame based not cell based. This is an overlay model where MPLS works on top
>of CLIP or LANE? 

Yes and no. This mode is vaguely similar to CLIP: MPLS just runs over
RFC2684 (successor to RFC1483) over a PVC. However MPLS-over-PVCs is
not MPLS-over-CLIP (i.e. MPLS over IP over ATM), it is something
different: frame-based MPLS over ATM. It has nothing to do with LANE.

>And the LSR still participate in L3 routing protocols? (as
>well as ATM routing protocols being run).

In this model, the ATM switches are not LSRs. The LSRs are the
routers at the end of the (S)PVCs, which do run IP routing protocols.
The ATM switches do whatever they do to route (S)PVCs, but that is
invisible to MPLS. 

>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.


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