Users Guide

Root Path Tree (RTP)
An RPT is the path between the RP and receivers (hosts) in a multicast group (see figure). The RPT is built by means of a PIM
join message from a receiver DR.
● A receiver sends a request to join group (G) in an IGMP host membership report. A PIM sparse-mode router, the receiver
DR, receives the report on a directly attached subnet and creates an RPT branch for the multicast group of interest.
● The receiver DR sends a PIM join message to its RPF neighbor, the next-hop address in the RPF table, or the unicast routing
table.
● The PIM join message travels up the tree and each router in the tree finds its RPF neighbor by using either the RPF table or
the unicast routing table. This is done until the message reaches the RP and forms the RPT. Routers along the path set up
the multicast forwarding state to forward requested multicast traffic back down the RPT to the receiver.
PIM-SM source registration
An RPT is a unidirectional tree, permitting traffic to flow down from the RP to the receiver in one direction. For multicast traffic
to reach the receiver from the source, another branch of the distribution tree that is called the shortest-path tree, must be built
from the source DR to the RP.
The shortest-path tree is created in the following way:
1. The source becomes active, sending out multicast packets on the LAN to which it is attached. The source DR receives the
packets and encapsulates them in a PIM register message, which it sends to the RP router.
2. When the RP router receives the PIM register message from the source, it sends a PIM join message back to the source.
3. The source DR receives the PIM join message and begins sending traffic down the SPT toward the RP router.
4. Once the RP router receives traffic, it sends a register stop message to the source DR to stop the register process.
5. The RP router sends the multicast traffic down the RPT toward the receiver.
Multicast
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