Reference Guide

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.
Send Multicast Traffic
With PIM-SM, all multicast traffic must initially originate from the RP. A source must unicast traffic to the RP so that the
RP can learn about the source and create an SPT to it. Then the last-hop DR may create an SPT directly to the source.
1. The source gateway router (first-hop DR) receives the multicast packets and creates an (S,G) entry in its multicast
routing table. The first-hop DR encapsulates the initial multicast packets in PIM Register packets and unicasts
them to the RP.
2. The RP decapsulates the PIM Register packets and forwards them if there are any receivers for that group. The RP
sends a PIM Join message towards the source. All routers between the RP and the source, including the RP,
create an (S,G) entry and list the interface on which the message was received as an outgoing interface, thus
recreating a SPT to the source.
3. After the RP starts receiving multicast traffic via the (S,G), it unicasts a Register-Stop message to the first-hop DR
so that multicast packets are no longer encapsulated in PIM Register packets and unicast. After receiving the first
multicast packet from a particular source, the last-hop DR sends a PIM Join message to the source to create an
SPT to it.
4. There are two paths, then, between the receiver and the source, a direct SPT and an RPT. One router receives a
multicast packet on two interfaces from the same source in this case; this router prunes the shared tree by
sending a PIM Prune message to the RP that tells all routers between the source and the RP to remove the
outgoing interface from the (*,G) entry, and tells the RP to prune its SPT to the source with a Prune message.
FTOS Behavior: When the router creates an SPT to the source, there are then two paths between the receiver and the
source, the SPT and the RPT. Until the router can prune itself from the RPT, the receiver receives duplicate multicast
packets which may cause disruption. Therefore, the router must prune itself from the RPT as soon as possible. FTOS
optimizes the shared to shortest-path tree switchover latency by copying and forwarding the first (S,G) packet received
on the SPT to the PIM task immediately upon arrival. The arrival of the (S,G) packet confirms for PIM that the SPT is
created, and that it can prune itself from the shared tree.
Important Point to Remember
If you use a Loopback interface with a /32 mask as the RP, you must enable PIM Sparse-mode on the interface.
Configuring PIM-SM
Configuring PIM-SM is a three-step process.
1. Enable multicast routing (refer to the followint step).
2. Select a rendezvous point.
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