Subject: PIM and MPLS

From: Ali Boudani <[email-masked]>
Date: Fri, 05 Jan 2001 16:40:50 +0100

hello,
In the draft "Using PIM to Distribute MPLS Labels for Multicast Routes"

In page 4 section 4 we can see the following

Suppose an LSR Rd, as the result of receiving, from a
         downstream neighbor on the shared tree, a Join/Prune message
         such as described in 3, creates (S,G)RPT-bit state with a non-
         null output interface list.  In this case, it may send a
         Join/Prune message upstream on the shared tree, containing the
         group address followed by a prune-list.  An element of the
         prune list will contain the address S and a corresponding
         label.  However, a special bit (the "Label Only" bit, or "L-
         bit") in the element will be set indicating to the upstream LSR

         that the source S is not really to be pruned from the shared
         tree.  The result is that the upstream LSR Ru, will still send
         packets from S to G to Rd, and will label those packets as
         specified.  When Rd receives such packets, it forwards them
         according to the output interface list of the (S,G)RPT-bit
         entry.  Intuitively, this label represents a route along the
         shared tree, but only for packets from the specified source.

what does that mean ??
how the label represents a route along the shared tree , but only for
packets from the specified source.
As I know, if there is a prune message and the SPT is established , that
mea there is no need to use the shared tree for that source anymore??
why do we need to put this L-bit ??
Thanx

---

From: Yiqun Cai <[email-masked]>
Date: Fri, 5 Jan 2001 18:41:05 -0800 (PST)

> why do we need to put this L-bit ??

Consider the following scenario:

                  2RP
                   |
                   |
                   R1
                   |
                   |
       R2 -------- Ru
                   |
                   |
                   Rd -------- R3 ------ 2SRC

Rd has joined the source tree via R3, but R2 stays on the shared tree. 
Therefore, Ru will have an (S,G,RPT) entry with non-null olist. 

When using label switching, R1 has to use one label for packets sourced 
by S and another label for the rest of the packets on the shared tree so 
that Ru can make the right forwarding decision without seeing the IP 
header. This is achieved by Ru sending the message as described in the 
draft. Without the L bit, R1 will not maintain such a state.

-- 
Yiqun