Refuse Multicast TrafficA host requesting to leave a multicast group sends an IGMP Leave message to the last-hop DR. If the host is the only remaining receiverfor 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 ofthe (*,G) entry. If the (*,G) entry has no remaining outgoing interfaces, multicast traffic for that group is no longer forwarded to thatsubnet.2 If the (*,G) entry has no remaining outgoing interfaces, the last-hop DR sends a PIM Prune message to towards the RP. All routersalong the way remove the interface on which the message was received from the outgoing interface list of the (*,G) entry. If on anyrouter there is at least one outgoing interface listed for that (*,G) entry, the Prune message is not forwarded.Send Multicast TrafficWith PIM-SM, all multicast traffic must initially originate from the RP. A source must unicast traffic to the RP so that the RP can learnabout 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. Thefirst-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 Joinmessage towards the source. All routers between the RP and the source, including the RP, create an (S,G) entry and list the interfaceon 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 multicastpackets are no longer encapsulated in PIM Register packets and unicast. After receiving the first multicast packet from a particularsource, 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 ontwo interfaces from the same source in this case; this router prunes the shared tree by sending a PIM Prune message to the RP thattells all routers between the source and the RP to remove the outgoing interface from the (*,G) entry, and tells the RP to prune itsSPT to the source with a Prune message.Dell Networking OS Behavior: When the router creates an SPT to the source, there are then two paths between the receiver and thesource, the SPT and the RPT. Until the router can prune itself from the RPT, the receiver receives duplicate multicast packets which maycause disruption. Therefore, the router must prune itself from the RPT as soon as possible. Dell Networking OS optimizes the shared toshortest-path tree switchover latency by copying and forwarding the first (S,G) packet received on the SPT to the PIM task immediatelyupon 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 RememberIf you use a Loopback interface with a /32 mask as the RP, you must enable PIM Sparse-mode on the interface.Configuring PIM-SMConfiguring PIM-SM is a three-step process.1 Enable multicast routing (refer to the following step).2 Select a rendezvous point.3 Enable PIM-SM on an interface.Enable multicast routing.CONFIGURATION modePIM Sparse-Mode (PIM-SM) 683
