Subject: Can RSVP distribution of labels be driven by the IGP ?

From: [email-masked]
Date: Wed, 14 Mar 2001 07:50:49 -0000

LDP is 'classically' the method for distribution of labels for an MPLS
network where the LSPs are created to reflect the IGP running in the
network, for hop-by-hop, unicast routing.
I understand that there are (a number of) extensions to RSVP signalling
protocol to allow it to also distribute labels, most notably when trying to
overide the LSP path creation driven by the IGP, e.g. to realise  constraint
based routing.

Q1: Can RSVP be operated in a mode whereby it acts just like LDP (with the
same scalability i.e. no hand-crafting of LSPs) in that it is used purely
for the distribution of labels that reflect the IGP (i.e. needs to match the
basic functionality of LDP) ?

Q2: If the answer to Q1 above is "yes", then would this permit a
architecture to support 2547bis IP VPNs where MP-iBGP is used to propagate
the inner label and RSVP extensions are used to propagate the outer (PE to
PE) label, (again with the same order of magnitude of scalability) as a
MP-iBGP+LDP architecture ?
[I am not so concerned about whether 2547bis mandates or not the use of LDP
towards the PE].
 
The thinking here is that if LSP 'protection' is required from PE to PE
(i.e. certain VPNs contained mission-critical traffic), then RSVP extensions
plus could provide this mechanism, however I have heard of some
architectures where "LDP is tunnelled across an RSVP TE core" and this would
imply to me that such an architecture could not provide LSP protection from
PE to PE.

Any clarifications on the above welcome.

Richard Crouch,
Core Network Development (Technology), 
Vodafone Limited,
United Kingdom.

tel +44(0)1635 676478
fax +44(0)1635 231792
vodafone +44(0)7899 728963
[email-masked]

---

From: David Charlap <[email-masked]>
Date: Wed, 14 Mar 2001 11:28:46 -0500

[email-masked] wrote:
> 
> I understand that there are (a number of) extensions to RSVP
> signalling protocol to allow it to also distribute labels, most
> notably when trying to overide the LSP path creation driven by the
> IGP, e.g. to realise  constraint based routing.

draft-ietf-mpls-rsvp-lsp-tunnel-08.txt

> Q1: Can RSVP be operated in a mode whereby it acts just like LDP
> (with the same scalability i.e. no hand-crafting of LSPs) in that it
> is used purely for the distribution of labels that reflect the IGP
> (i.e. needs to match the basic functionality of LDP) ?

Not without some major redesign of RSVP.  And IMO, I think it would be
far better to just use LDP rather than to reinvent the wheel with yet
another set of RSVP extensions.

RSVP, like CR-LDP, is a tunnel-setup protocol, not a label distribution
protocol.  It's not possible for it to just derive labels from the IGP
and advertise them.  Instead, an ingress node must request an LSP from
itself to an egress node.

An explicit route object is optional.  If not present, each router will
consult the local routing table to determine the next-hop to the egress
node.

Determination of what routers should be ingress and egress nodes, and
computation of explicit paths is beyond the scope of RSVP.  It may be
done through manual configuration, as you have described.  It may also
be done by off-line traffic engineering software or by some software
component of the router.  But it is not a part of RSVP.

-- David