PIM Sparse Mode
A PIM sparse mode (PIM-SM) domain uses Reverse Path Forwarding (RPF) to create a path from a data source to the receiver requesting the data. When a receiver issues an explicit Join request, an RPF check is triggered. A (*,G) PIM Join message is sent towards the RP from the receiver's DR. (By definition, this message is actually called a Join/Prune message, but for clarity in this description, it is called either Join or Prune, depending on its context.) The Join message is multicast hop by hop upstream to the ALL-PIM-ROUTERS group (224.0.0.13) by means of each router's RPF interface until it reaches the RP. The RP router receives the (*,G) PIM Join message and adds the interface on which it was received to the OIL of the RPT forwarding state entry. This builds the rendezvous point tree (RPT) connecting the receiver with the RP. The RPT remains standing, even if no active sources generate traffic.
When a source becomes active, the source's DR encapsulates multicast data packets into a PIM Register message and sends them by means of unicast to the RP router.
If the RP router has interested receivers in the PIM-SM domain, the RP router sends a PIM Join message towards the source to build a shortest path tree (SPT) back to the source. The source sends multicast packets out on the LAN, and the source's DR encapsulates the packets in a PIM Register message and forwards it towards the RP router by means of unicast. The RP router receives PIM Register messages back from the source, and thus adds a new source to the distribution tree, keeping track of sources in a PIM table. Once an RP router receives packets natively (with S,G), it sends a Register Stop to stop receiving the register messages by means of unicast.
In actual application, many receivers with multiple SPTs are involved in a multicast traffic flow. To simply illustrate the process, we'll follow the multicast traffic from the RP router to one receiver. In such a case, the RP router begins sending multicast packets down the RPT towards the receiver's DR for delivery to the interested receivers. When the receiver's DR gets the first packet from the RPT, the DR sends a PIM Join message towards the source's DR to start building an SPT back to the source. When the source's DR receives the PIM Join from the receiver's DR, it starts sending traffic down all SPTs. When the first multicast packet is received by the receiver's DR, the receiver's DR sends a PIM Prune message to the RP router to stop duplicate packets being sent through the RPT. In turn, the RP router stops sending multicast packets to the receiver's DR, and sends a PIM Prune message for this source over the RPT toward's the source's DR to halt multicast packet delivery to the RP router from that particular source.
If the RP router receives a PIM register message from an active source, but has no interested receivers in the PIM-SM domain, it still adds the active source into the PIM table. However, after adding the active source into the PIM table, the RP router sends a Register Stop message. The RP router knows of the active source's existence and no longer needs to receive advertisement of the source (which utilizes resources).
This section contains more information about the routers and PIM-SM functions briefly described above: