Reference Guide

42
PIM Sparse-Mode (PIM-SM)
PIM-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
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 S5000 supports a maximum of 96 PIM interfaces and 2K multicast entries including (*,G), and
(S,G) entries. There is no limit on the number of PIM neighbors the S5000 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.
• Dell Networking OS reduces the number of control messages sent between multicast routers by
bundling Join and Prune requests in the same message.
• Dell Networking OS supports PIM-SM on physical, virtual local area network (VLAN), and port-channel
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
714
PIM Sparse-Mode (PIM-SM)