33PIM Sparse-Mode (PIM-SM)Protocol-independent multicast sparse-mode (PIM-SM) is supported on Dell Networking OS.PIM-SM is a multicast protocol that forwards multicast traffic to a subnet only after a request using a PIMJoin message; this behavior is the opposite of PIM-Dense mode, which forwards multicast traffic to allsubnets until a request to stop.Implementation InformationBe aware of the following PIM-SM implementation information.• The Dell Networking implementation of PIM-SM is based on IETF Internet Draft draft-ietf-pim-sm-v2-new-05.• The SPT-Threshold is zero, which means that the last-hop designated router (DR) joins the shortestpath tree (SPT) to the source after receiving the first multicast packet.• Dell Networking OS reduces the number of control messages sent between multicast routers bybundling Join and Prune requests in the same message.• Dell Networking OS supports PIM-SM on physical, virtual local area network (VLAN), and port-channelinterfaces.NOTE: Multicast routing is supported across default and non-default VRFs.Protocol OverviewPIM-SM initially uses unidirectional shared trees to forward multicast traffic; that is, all multicast trafficmust 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. Everymulticast group has an RP and a unidirectional shared tree (group-specific shared tree).Requesting Multicast TrafficA 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 canreceive the requested traffic.1. After receiving an IGMP Join message, the receiver gateway router (last-hop DR) creates a (*,G) entryin its multicast routing table for the requested group. The interface on which the join message wasreceived 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 wasreceived becomes the outgoing interface associated with the (*,G) entry. This process constructs anRPT branch to the RP.PIM Sparse-Mode (PIM-SM) 605
