Reference Guide

PIM Sparse-Mode (PIM-SM) | 685
3. Once the RP starts receiving multicast traffic via the (S,G) it unicasts a Register-Stop message to the
first-hop DR so that multicast packets are no longer encapsulated in PIM Register packets and unicast.
Upon receiving the first multicast packet from a particular source, the last-hop DR sends a PIM Join
message to the source to create an SPT to it.
4. There are two paths, then, between the receiver and the source, a direct SPT and an RPT. One router
will receive a multicast packet on two interfaces from the same source in this case; this router prunes
the shared tree by sending a PIM Prune message to the RP that tells all routers between the source and
the RP to remove the outgoing interface from the (*,G) entry, and tells the RP to prune its SPT to the
source with a Prune message.
Important Points to Remember
If a loopback interface with a /32 mask is used as the RP, you must enable PIM Sparse-mode on the
interface.
Configuring PIM-SM
Configuring PIM-SM is a two-step process:
1. Enable multicast routing using the command
ip multicast-routing from CONFIGURATION mode.
2. Select a Rendezvous Point.
3. Enable PIM-SM on an interface. See page 685.
Related Configuration Tasks
• Configuring S,G Expiry Timers
• Configuring a Static Rendezvous Point
• Configuring a Designated Router
• Creating Multicast Boundaries and Domains
Enabling PIM-SM
You must enable PIM-SM on each participating interface:
Dell Networking OS Behavior: When the router creates an SPT to the source, there are then two paths between
the receiver and the source, the SPT and the RPT. Until the router can prune itself from the RPT, the receiver
receives duplicate multicast packets which may cause disruption. Therefore, the router must prune itself from the
RPT as soon as possible. Dell Networking OS optimizes the shared to shortest-path tree switchover latency by
copying and forwarding the first (S,G) packet received on the SPT to the PIM task immediately upon arrival. The
arrival of the (S,G) packet confirms for PIM that the SPT is created, and that it can prune itself from the shared
tree.
Step Task Command Command Mode
1 Enable multicast routing on the system.
ip multicast-routing
CONFIGURATION