Reference Guide

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PIM Sparse-Mode (PIM-SM)
Protocol-independent multicast sparse-mode (PIM-SM) is supported on the Z9000 platform.
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
Be 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 shortest path tree
(SPT) to the source after receiving the first multicast packet.
• Dell Networking operating system (FTOS) reduces the number of control messages sent between multicast
routers by bundling Join and Prune requests in the same message.
• FTOS supports PIM-SM on physical, virtual local area network (VLAN), and port-channel interfaces.
• FTOS 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.
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.
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