The MPLS WG Archive

["Alexander Marhold" <alexander@marhold.at>]

to add my points into the discussion:

Let us assume that the middle router does aggregation !

Now the packet enters the middle router labelled and he cannot have a direct
outgoing labelled path as he has not got labels for the summary but only for
the specific destinations for forwarding the packet.

Therefore whenever he receives a labelled packet and does not have any
outgoing label for that he has to remove the label and do a normal layer-3
lookup in his routing table.

If that lookup fails he drops the packet and tries sending back an ICMP
destination unrechable.

Assuming ordered LSP control it would be nearly impossible to detect such
situation and if the path is now complete or not. Therefore in normal
MPLS-frame based routing environments  it is not implemented.

-----------------
I agree that for MPLS/VPN where the middle routers have no knowledge about
the detsinations this is not optimal, and an "ordered LSP control" could
prevent that, but it is up to the implementer of the MPLS code to decide on
this.

with best regards

Alexander Marhold
Senior Consultant
PRO IN

> -----Original Message-----
> From: owner-mpls@UU.NET [mailto:owner-mpls@UU.NET]On Behalf
> Of Olle Pers
> Sent: Tuesday, January 30, 2001 11:11 AM
> To: 'Giles Heron'
> Cc: mpls@UU.NET; David Charlap
> Subject: RE: LDP - question about partially setup LSPs
>
>
>
> *
> *  I would agree with you on this one.
> *
> Well, here is one that would not agree, without more
> convincing arguments.
>
>
> *  Your second point is key.  How can you know that the
> *  destination address in the packet is an IGP prefix?
> *  I suspect that many networks will want to run BGP
> *  edge-to-edge, but to have only IGP routes in the core.
> *
> The IGP knew the prefix, and B had sent out a label mapping
> for it. That was the scenario in David's question.
> So the router B knows how to forward unlabeled packets with
> this prefix. And
> it did send out a label mapping for exactly the same prefix,
> knowing that it
> didn't have any corresponding downstream label yet. Why? I
> don't want to
> think B did that just in order to drop these packets.
>
>
> *  In addition to this you have the issue of MPLS VPN and Layer
> *  2 over MPLS traffic.  Either one of these will generate a
> *  label stack in which the innermost label is assigned edge-to-
> *  edge and is not visible to the intermediate devices.
> *
> These applications require that there exists an edge-to-edge
> LSP which can
> be used as a tunnel. We cannot assume them to work, before
> the setup of this
> LSP is completed. And with unordered control, the ingress
> will not know when
> this has happened.
>
>
> But why use unordered control at all, if it leads to
> problems? What's the
> application?
>
>
> *
> *  Giles
> *
>
> Olle Pers
> Telia, Sweden
>