Administrator Guide
PIM Sparse-Mode (PIM-SM)
Protocol-independent multicast sparse-mode (PIM-SM) is a multicast protocol that forwards multicast trac to a subnet only after a
request using a PIM Join message; this behavior is the opposite of PIM-Dense mode, which forwards multicast trac to all subnets until a
request to stop.
Implementation Information
The following information is necessary for implementing PIM-SM.
• The Dell 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 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 rst multicast packet.
• Dell Networking OS reduces the number of control messages sent between multicast routers by bundling Join and Prune requests in
the same message.
• Dell 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 trac; that is, all multicast trac must ow only from the rendezvous
point (RP) to the receivers.
After a receiver receives trac from the RP, PM-SM switches to SPT to forward multicast trac. Every multicast group has an RP and a
unidirectional shared tree (group-specic shared tree).
Requesting Multicast Trac
A host requesting multicast trac 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 trac.
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.
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