Reference 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-SM
• Enable PIM-SM
• Configuring S,G Expiry Timers
• Configuring a Static Rendezvous Point
• Configuring a Designated Router
• 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.
● 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.
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