Administrator Guide

Send Multicast Traffic
With PIM-SM, all multicast traffic must initially originate from the RP. A source must unicast traffic to the RP
so that the RP can learn about the source and create an SPT to it. Then the last-hop DR may create an SPT
directly to the source.
1 The source gateway router (first-hop DR) receives the multicast packets and creates an (S,G) entry in its
multicast routing table. The first-hop DR encapsulates the initial multicast packets in PIM Register
packets and unicasts them to the RP.
2 The RP decapsulates the PIM Register packets and forwards them if there are any receivers for that
group. The RP sends a PIM Join message towards the source. All routers between the RP and the source,
including the RP, create an (S,G) entry and list the interface on which the message was received as an
outgoing interface, thus recreating a SPT to the source.
3 After 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. After
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
receives 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.
Dell Networking 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. The 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.
Important Point to Remember
If you use a Loopback interface with a /32 mask as the RP, you must enable PIM Sparse-mode on the
interface.
Configuring PIM-SM
Configuring PIM-SM is a three-step process.
1 Enable multicast routing (refer to the following step).
2 Select a rendezvous point.
3 Enable PIM-SM on an interface.
Enable multicast routing.
CONFIGURATION mode
PIM Sparse-Mode (PIM-SM) 686