White Papers

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.
Topics:
• Implementation Information
• Protocol Overview
• Configuring PIM-SM
• Configuring S,G Expiry Timers
• Configuring a Static Rendezvous Point
• Configuring a Designated Router
• Creating Multicast Boundaries and Domains
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,
41
640 PIM Sparse-Mode (PIM-SM)