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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,
the interface on which the message was received is added to the outgoing interface list associated with the (*,G) entry, and
the message is not (and does not need to be) forwarded towards the RP.
Refuse Multicast Traffic
A host requesting to leave a multicast group sends an IGMP Leave message to the last-hop DR. If the host is the only remaining
receiver for that group on the subnet, the last-hop DR is responsible for sending a PIM Prune message up the RPT to prune its
branch to the RP.
1. After receiving an IGMP Leave message, the gateway removes the interface on which it is received from the outgoing
interface list of the (*,G) entry. If the (*,G) entry has no remaining outgoing interfaces, multicast traffic for that group is no
longer forwarded to that subnet.
2. If the (*,G) entry has no remaining outgoing interfaces, the last-hop DR sends a PIM Prune message to towards the RP.
All routers along the way remove the interface on which the message was received from the outgoing interface list of the
(*,G) entry. If on any router there is at least one outgoing interface listed for that (*,G) entry, the Prune message is not
forwarded.
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.
PIM Sparse-Mode (PIM-SM)
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