Reference Guide

718 | PIM Sparse-Mode (PIM-SM)
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Requesting Multicast Traffic
A host requesting multicast traffic for a particular group sends an 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. Upon 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.
Refusing 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. Upon 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.
Sending 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