Subject: Common label

From: Khuzema Pithewan <[email-masked]>
Date: Mon, 23 Jul 2001 17:49:15 +0530

Hi,

In "draft-ietf-mpls-rsvp-lsp-tunnel-08" section 2.4.3 says...

" Multipoint-to-point LSPs may be used when path messages do not carry the
EXPLICIT_ROUTE object, or when Path messages have identical EXPICIT_ROUTE
objects.  In either of these cases a common label may be assigned."

My question is, why it should check if EXPLICIT_ROUTE object should be same
to provide common label? I think instead of checking EXPLICIT_ROUTE object,
if I just check for the incoming and outgoing interfaces of the Path
messages on the LSR, even then I can assign the common label if they are
same.  

Regards,
Khuzema.
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---

From: David Charlap <[email-masked]>
Date: Mon, 23 Jul 2001 10:11:28 -0400

Khuzema Pithewan wrote:
> " Multipoint-to-point LSPs may be used when path messages do not
> carry the EXPLICIT_ROUTE object, or when Path messages have
> identical EXPICIT_ROUTE objects.  In either of these cases a
> common label may be assigned."
> 
> My question is, why it should check if EXPLICIT_ROUTE object should
> be same to provide common label? I think instead of checking
> EXPLICIT_ROUTE object, if I just check for the incoming and
> outgoing interfaces of the Path messages on the LSR, even then I
> can assign the common label if they are same.

Think about what will happen on the data plane.  Remember that data
packets are forwarded only according to their labels, and not according
to the actual content of the packets.

If you assign the same label to two different LSPs, then you effectively
merge them together.  Packets from both LSPs will necessarily follow the
same path to the egress, because they will have the same label.

If there is no ERO, or if the two LSPs have identical EROs, this merging
is not a problem - the packets are all going the same way anyway.  If
they have different EROs, however, then the data from one LSP will not
be able to follow the ERO, because it is sharing a label with an LSP
that has a different ERO.

-- David