This is in the overview, section 3. The DR waits for the SPT data before
pruning off the RPT.
.. When the first traffic starts to arrive from
the SPT, the DR or
upstream router starts to drop the packets for G from S
that arrive via
the RP tree. In addition, it sends an (S,G) Prune
message towards the
RP. This is known as an (S,G,rpt) Prune.
RP will have keepalive timer KAT(S,G) from the Register messages, plus
(*,G,I) and (S,G,rpt,I) state on the downstream interface from the Join(*,G) and
Prune(S,G,rpt) it has received, and continues to receive
No. PIM is pretty much hop-by-hop. PIM routers don't try
to work out the state of remote routers across the network; just adjacent
No. If the RP joins the SPT, it gets multicast data hop-by-hop just like
anyone else. When it gets the Prune(S,G,rpt), it'll leave the SPT again
and not forward any (S,G) data at all.
searching for this stuff in the PIM spec. It's all in
My next question, this is the
When a source DR gets multicast
packets it encapsulates them and sends it to the RP by unicast. The RP will
check, if there doesn’t exist receivers for the group, will send a Register-Stop
message and also starts within itself for the group. The source DR as to keep
sending null-register packets to the RP before the timer expires thereby
maintaining the state for the group. Now if a receiver joins the group the RP
will send a (S, G, spt) Join to the source DR. Now the source DR sends multicast
data to the RP, which in turn will forward it to the receiver’s DR through the
(*, G, rpt) Join that the receiver’s DR had sent earlier. Once the Receiver’s DR
learns the address of the source, it sends a (S, G, spt) Join directly to the
source DR and the source DR will forward multicast traffic directly to the
Now does the receiver’s
DR send the prune to the RP tree first? And when does the source DR send a prune
to the RP?
Now if this was the only
receiver for the group & the traffic is flowing through the source tree then
what states will be maintained by the RP for the group, assuming that the source
And does the RP know that
there is direct forwarding of traffic between the source DR and receiver
Is the multicast data
forwarded to the RP always encapsulated, even if the RP sends (S, G, spt) Join
to the source DR as above???