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PIM Sparse-Mode (PIM-SM)
Protocol-independent multicast sparse-mode (PIM-SM) is a multicast protocol that forwards multicast
traffic to a subnet only after a request using a PIM Join message; this behavior is the opposite of PIM-
Dense mode, which forwards multicast traffic to all subnets until a request to stop.
Implementation Information
The Dell Networking implementation of PIM-SM is based on IETF Internet Draft draft-ietf-pim-sm-v2-
new-05
.
• There is no limit on the number of PIM neighbors can have.
• The SPT-Threshold is zero, which means that the last-hop designated router (DR) joins the shortest
path tree (SPT) to the source after receiving the first multicast packet.
• The Dell Networking OS reduces the number of control messages sent between multicast routers by
bundling Join and Prune requests in the same message.
• The system supports PIM-SM on physical, virtual local area network (VLAN), and port-channel
interfaces.
• The system supports 2000 IPv6 multicast forwarding entries, with up to 128 PIM-source-specific
multicast (SSM) neighbors/interfaces.
• IPv6 Multicast is not supported on synchronous optical network technologies (SONET) interfaces.
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.
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PIM Sparse-Mode (PIM-SM)