Administrator Guide

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.
Topics:
• Implementation Information
• Protocol Overview
• Configuring PIM-SSM
• Enable PIM-SM
• Configuring S,G Expiry Timers
• Configuring a Static Rendezvous Point
• Configuring a Designated Router
• Electing an RP using the BSR Mechanism
• Creating Multicast Boundaries and Domains
• Enabling PIM-SM Graceful Restart
Implementation Information
The Dell Networking implementation of PIM-SM is based on IETF Internet Draft draft-ietf-pim-sm-v2-new-05.
• The maximum number of PIM interfaces is 95.
• 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 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
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