Administrator Guide

PIM Sparse-Mode (PIM-SM)

PIM-sparse mode (PIM-SM) is a multicast protocol that forwards multicast trac to a subnet only after a request using a PIM Join

message; this behavior is the opposite of PIM-Dense mode, which forwards multicast trac to all subnets until a request to stop.

Topics:

• Implementation Information

• Protocol Overview

• Conguring PIM-SM

• Conguring S,G Expiry Timers

• Conguring a Static Rendezvous Point

• Conguring 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 rst 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 trac; that is, all multicast trac must ow only from the rendezvous

point (RP) to the receivers.

After a receiver receives trac from the RP, PM-SM switches to SPT to forward multicast trac. Every multicast group has an RP and a

unidirectional shared tree (group-specic shared tree).

Requesting Multicast Trac

A host requesting multicast trac 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 trac.

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.

688 PIM Sparse-Mode (PIM-SM)