PIM-SM Packet Forwarding

PIM-SM Packet Forwarding

When a PIM-SM router receives the initial multicast packet from a source, the packet is flooded onto all interfaces in the output interface list (oilist). Recall that the oilist is populated with those interfaces that lead to downstream receivers which have indicated their desire to receive the traffic using IGMP. In PIM-DM, there is only one RPF interface for a particular source. With PIM-SM, there can be two RPF possibilities for a particular source, depending on whether the traffic is flowing down the shared tree or down the source tree (see Figure 7-7).

Figure 7-7: PIM-SM RPF check depends on the tree used.

Packet forwarding is similar to PIM-DM. If the group is in the oilist and it is not in the prune state, then the packet will be forwarded. One major difference between PIM-DM forwarding and PIM-SM forwarding is that in PIM-DM an interface is added to the oilist if a PIM-DM neighbor has been discovered on the interface or if a join has been received or forwarded from a neighbor. In PIM-SM, the interface will only be put in the list if the downstream neighbor has sent a join to this router, if there is a directly attached receiver for the group and a join has been received, or if the interface has been manually configured to join the group.

PIM-SM Joining  A leaf router will send a (*,G) Join message toward the RP if the leaf router has received a Join from a directly attached receiver or from a downstream neighbor. The router will forward the join to the RP along the unicast route, and each router along the path to the RP will process the Join. If a router does not have (*,G) state, then the state will be created and the Join will be sent toward the RP. If the router does have the state, then the Join message has reached the shared tree and the router does not have to do anything.

PIM-SM Registering  When a PIM-SM-enabled router initially receives a multicast packet from a sender, the router may or may not have the state for this source and group. A sender does not have to join the group it is sending to use IGMP. The router only needs to register with the RP using a PIM-SM register packet (see Figure 7-8).

Figure 7-8: PIM Sparse-Mode Register Packet Format

The Register packet is then sent as a unicast packet to the RP. The multicast packets that are received by the router directly attached to the source are encapsulated in Register messages, one per message. When the RP receives the Register message, the multicast packet will be extracted and sent down the shared tree toward the receivers. The RP will also send a (S,G) Join back toward the source in order to build the shortest path tree back to the source.

Once the path is established from the source to the RP, the source leaf router will begin to send multicast packets toward the RP as normal IP multicast packets. The source will also send the multicast packets encapsulated in Register messages, so the RP will receive them twice. When the RP detects that multicast packets from the source are being received as normal IP multicast packets, the RP sends a Register-Stop packet to the router directly attached to the source (see Figure 7-9).

Figure 7-9: PIM-Sparse Mode Register-Stop Packets

Upon reception of the Register-Stop message, the first-hop router will quit encapsulating the multicast traffic in Register messages and only send them to the RP as normal IP multicast packets. Figure 7-10 illustrates the registering process.