Administrator Guide

Protocol Overview
PIM-SM initially uses unidirectional shared trees to forward multicast traffic; that is, all multicast traffic must
flow only from the rendezvous point (RP) to the receivers.
After a receiver receives traffic from the RP, PM-SM switches to SPT to forward multicast traffic. Every
multicast group has an RP and a unidirectional shared tree (group-specific shared tree).
Requesting Multicast Traffic
A host requesting multicast traffic for a particular group sends an Internet group management protocol
(IGMP) Join message to its gateway router.
The gateway router is then responsible for joining the shared tree to the RP (RPT) so that the host can receive
the requested traffic.
1 After receiving an IGMP Join message, the receiver gateway router (last-hop DR) creates a (*,G) entry in
its multicast routing table for the requested group. The interface on which the join message was
received becomes the outgoing interface associated with the (*,G) entry.
2 The last-hop DR sends a PIM Join message to the RP. All routers along the way, including the RP, create
an (*,G) entry in their multicast routing table, and the interface on which the message was received
becomes the outgoing interface associated with the (*,G) entry. This process constructs an RPT branch
to the RP.
3 If a host on the same subnet as another multicast receiver sends an IGMP report for the same multicast
group, the gateway takes no action. If a router between the host and the RP receives a PIM Join message
for which it already has a (*,G) entry, the interface on which the message was received is added to the
outgoing interface list associated with the (*,G) entry, and the message is not (and does not need to be)
forwarded towards the RP.
Refuse Multicast Traffic
A host requesting to leave a multicast group sends an IGMP Leave message to the last-hop DR. If the host is
the only remaining receiver for that group on the subnet, the last-hop DR is responsible for sending a PIM
Prune message up the RPT to prune its branch to the RP.
1 After receiving an IGMP Leave message, the gateway removes the interface on which it is received from
the outgoing interface list of the (*,G) entry. If the (*,G) entry has no remaining outgoing interfaces,
multicast traffic for that group is no longer forwarded to that subnet.
2 If the (*,G) entry has no remaining outgoing interfaces, the last-hop DR sends a PIM Prune message to
towards the RP. All routers along the way remove the interface on which the message was received from
the outgoing interface list of the (*,G) entry. If on any router there is at least one outgoing interface listed
for that (*,G) entry, the Prune message is not forwarded.
PIM Sparse-Mode (PIM-SM) 685