Subject: RE: Hierarchy between two LSRs

From: [email-masked]
Date: Thu, 17 May 2001 14:54:51 +0300

Hi,
 
Could you clarify the question, please?
 
It seems to me that you are asking that if two LSPs have an LDP peering, how
does LDP know that they have an LDP peering.
 
BR,
 
Peter White

-----Original Message-----
From: ext Asish Thomas Punnoose [mailto:[email-masked]]
Sent: 17 May, 2001 12:50
To: [email-masked]
Subject: Hierarchy between two LSRs

hello 
 
I am new to MPLS and i have a basic doubt. 
 
If hierarchy is introduced between two LSRs, how does LDP/ CR-LDP know
whether a hierarchy exists between the two.  
 
How does the label switching work in this situation.
 
Please do help and could someone send some pointers related to this topic.
 
Thanks in advance 
 
Asish Thomas

---

From: "Asish Thomas Punnoose" <[email-masked]>
Date: Thu, 17 May 2001 17:41:48 +0530

hello

Here i meant was that two LSRs are not directly connected, but there is a
subnetwork which interconnects these two LSRs, so in this case how does the
LDP help in distributing labels and forming a LSP

Hope i have made it clear.

regards
Asish

---

From: Abhijit M Gadgil <[email-masked]>
Date: Thu, 17 May 2001 17:43:13 +0530 (IST)

> LDP help in distributing labels and forming a LSP

LDP will make use of targetted hellos that would flow thr' the underlying
subnet.. Specification is quite elaborate wrt to that have a look at
rfc3036 section 2.4

-abhijit

---

From: Eric Gray <[email-masked]>
Date: Thu, 17 May 2001 10:50:54 -0400

Abhijit,

    This is slightly more complicated than has so far been noted.
Use of targetted hello messages is one approach.  Another is to
establish an LDP peer session using the LSP over which the
labels will be sent.  This would include sending 'hello' messages
over the LSP.   These could be targetted hello messages, or they
could be 'normal' hello mesages if the implementation does not
set TTL to equal '1' (this would not matter if the LSP were setup
using a 'pipe' model).  This is effectively using the LSP as a
seperate interface/link and is required if the targtted LSR wants
(for some reason) to use an interface specific label space in the
peer relationship.  This may not work if the same LSR relies on
PHP, but it would be just plain silly for such an LSR to do this in
that case.

    If LSRs use targetted hello messages - that are not sent over an
LSP directly to the 'remote' peer - to establish a session with that
peer, they need to be prepared to receive labels they distribute in
that session on any interface including any LSP that they terminate.

--
Eric Gray