Subject: [mpls] draft-decraene-mpls-ldp-interarea-04.txt clarification.

From: Luca Martini <[email-masked]>
Date: Thu, 22 Mar 2007 03:17:44 -0600

Bruno,

If my interpretation is correct, I think that you need to make it very 
clear that the original LDP FEC is installed because the LSR found a 
longest prefix matching IP RIB entry resulting in a NH, and output 
interface, then the original LDP FEC will be advertised with a label for 
that LSR.
One could mistakenly assume that a new aggregate FEC would be 
advertised, which should not be the case.

I am also assuming that if an LSR not supporting this procedure is in 
the path , we should have no adverse effect other then that the LSP will 
stop at the LSR that does not support this procedure.
( it might be nice to explains this )

Luca

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From: "DECRAENE Bruno RD-CORE-ISS" <[email-masked]>
Date: Thu, 22 Mar 2007 19:31:27 +0100

Luca,

> Luca Martini wrote:

You are perfectly correct. There is no FEC aggregation and LDP keeps
distributing the exact original FEC (typically a /32 loopback address)
in all IGP areas of the network, just like today with RFC 3036.
What the draft would allow is for the IP routing protocol to perform IP
routing aggregation between IGP area boundaries. This is important to
scale link-state IGP both in terms of number of prefixes and in terms of
routing dynamics and this is standard IGP practice.

> stop at the LSR that does not support this procedure.

You're right. If a LSR does not support this longest match procedure,
then the FEC is not propagated any further from the egress toward the
ingress router. The result is a subset of the required LSP but the LSP
is not even broken in the middle.

In the following example where PE2 is the egress and PE4 the ingress, if
ABR2 does not support the procedure, we would only have a LSP from P3 to
PE2.

PE4 --- ABR2 ------- P3 ------  ABR1 ----  PE2

> ( it might be nice to explains this )

Thanks for clarifying these two points. We will update the draft
accordingly.

Bruno