The MPLS WG Archive

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

Nagabhushana wrote:
> 
> 1. In the MPLS architecture draft it is said that
> 
> " 3.26.1. Non-merging LSRs
> 
>  The MPLS forwarding procedures is very similar to the forwarding
>  procedures used by such technologies as ATM and Frame Relay. That is,
>  a unit of data arrives, a label (VPI/VCI or DLCI) is looked up in a
>  "cross-connect table", on the basis of that lookup an output port is
>  chosen, and the label value is rewritten. In fact, it is possible to
>  use such technologies for MPLS forwarding; a label distribution
>  protocol can be used as the "signalling protocol" for setting up the
>  cross-connect tables. "
> 
> what I understand from this is we can use the LDP itself for SVC
> signalling in a frame relay network. So in this case the Q.933
> signalling is obsolete here.

Assuming you don't want backwards compatibility with legacy FR devices,
of course.

> So my question now is do we have to reserve any particular DLCI for
> LDP messages itself? Is there any standard already defined for this?

On ATM hardware, one VC is reserved for unlabeled IP traffic.  This is
usually VPI=0 and VCI=32, although it may be configured to other
values.  IP signalling packets (LDP or RSVP), being unlabeled IP
packets, are sent and received on this VC.

Since MPLS router must be able to forward unlabeled traffic in addition
to labeled traffic, I would assume that some similar kind of mechanism
must also be in place on FR hardware.  I don't know which DLCI is the
usual default, but that should be an easy thing to change once you have
the mechanism working.

> secondly, assuming LDP as the signalling protocol, we have to run our
> LDP stack over each of the UNI and NNI elements. So this would mean
> that we have to support TCP/IP stack(to maintain the integrity of the
> LDP sessions) and also IP encapsulatio/decapsulation at each of the
> NNI elements which is not necessary in normal FR networks.

Yes.  There must be a way to get IP packets from the FR stream into
whatever processor is running the protocol.  For RSVP, you only have to
send and receive IP datagrams (RSVP runs either as raw IP or
encapsulated in UDP).  For LDP, you must have a working TCP stack.

-- David