From: Donny MATEO (donny.mateo@sg.ca-indosuez.com)
Date: Fri Oct 04 2002 - 00:05:50 GMT-3
Hi Group,
a bit of confusion here on sparse mode PIM with the receiver joining the on a router that is along
the SPT from the source to the RP
src1------rtrA---------s1-rtrB -s0-------------s0-rtrC (RP) -s1---------rtrD
e0
|
new rcvr
Before any of you flame me of NDA, this not NDA stuff, i took it out from Developing IP Multicast
Network of Beau Williamson page 276. You could double check if you like.
The confusion is that as we know RP will send a join (S1, G) to the source and join an SPT tree (S1,
G). Now in this case the new receiver is along the tree. When he send an IGMP join message to rtrB.
Rtrb will send a (* , G) join to the RP ( rtrC) and also will put interface E0 in the outgoing list
on (S1, G) entry and (* , G) entry.
Now in the RP (rtrC) there are two entries one is the shared tree and another one is the SPT to the
source. rtrB will send a join (* , G) to rtrC and when rtrC received this is will put interface s0
to outgoing list on the (*, G ) entries and this is replicated as well to the (S1, G) entries Since
on (S1, G) entries the outgoing will be the same interface as the incoming interface (from source1),
so this out going interface will be deleted, leaving only S1 as the outgoing interface on the list
of (S1, G) entry.
Ok here is the question. Beau conclude that RP will not send the multicast traffic to new receiver,
new receiver will receive the traffic from rtrB (because of the (S1, G) entry in router B put e0 on
the outgoing list). the (S1, G) entry in the rtrC for S0 is pruned because it's the same as the
incoming interface.
But here is the conflicting fact, on a Sparse mode, the (*, G) entry is the one used to foward the
traffic, and the entry of the (*, G) indicated two outgoing interface (S0, and S1) since this is the
RP and it has no incoming interface so this is valid.
My question is why does Beau conclude that traffic is not going to be sent to rtrB ?It's true that
the (S, G) entry did not list S0 as the outgoing interface the RP won't foward the multicast
traffic. I aggree if this is Dense mode only. but this is sparse so RP should foward traffic from
source down the Shared tree. The shared tree is indicated by (* , G) entry and in this case RP will
foward traffic to rtrB based on the fact that S0 is an outgoing list for (*, G) entry. In another
words both interface s1 and s0 is on the shared tree so multicast traffic should be fowarded
accordingly.
Donny
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