Administrator Guide

PIM Sparse-Mode (PIM-SM)
Implementation Information
The following information is necessary for implementing PIM-SM.
• The Dell EMC Networking implementation of PIM-SM is based on IETF Internet Draft draft-ietf-pim-sm-v2-new-05.
• The platform supports a maximum of 95 IPv4 and IPv6 PIM interfaces and 2000 multicast entries including (*,G), and (S,G) entries.
The maximum number of PIM neighbors is the same as the maximum number of PIM-SM interfaces.
• 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 first multicast packet.
• Dell EMC Networking OS reduces the number of control messages sent between multicast routers by bundling Join and Prune
requests in the same message.
• Dell EMC Networking OS supports PIM-SM on physical, virtual local area network (VLAN), and port-channel interfaces.
NOTE: Multicast routing is supported across default and non-default VRFs.
Protocol Overview
PIM-SM initially uses unidirectional shared trees to forward multicast traffic; that is, all multicast traffic must flow only from the
rendezvous point (RP) to the receivers.
After a receiver receives traffic from the RP, PM-SM switches to SPT to forward multicast traffic. Every multicast group has an RP and a
unidirectional shared tree (group-specific shared tree).
Requesting Multicast Traffic
A host requesting multicast traffic 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 traffic.
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.
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.
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