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 EMC 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 EMC 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.
NOTE: With scaled groups, the convergence time for IPv6 multicast traffic takes around 2 to 3 minutes, when there is a
path change to source with CoPP settings. As a workaround, increase the CoPP rate.
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 IPv4 or IPv6 multicast routing (refer to the following step).
2. Select a rendezvous point.
3. Enable PIM-SM on an interface.
Enable multicast routing.
CONFIGURATION mode
{ip | ipv6} multicast-routing [vrf vrf-name]
Related Configuration Tasks
The following are related PIM-SM configuration tasks.
• Configuring S,G Expiry Timers
• Configuring a Static Rendezvous Point
• Configuring a Designated Router
• Creating Multicast Boundaries and Domains
Enable PIM-SM
You must enable PIM-SM on each participating interface.
1. Enable IPv4 or IPv6 multicast routing on the system.
CONFIGURATION mode
{ip | ipv6} multicast-routing [vrf vrf-name]
2. Enable PIM-Sparse mode.
PIM Sparse-Mode (PIM-SM)
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