Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 285
Types of Multicast Trees Using PIM-SM

Multicast trees are still built using PIM, but there are two types:

Shared Tree (Rendezvous Point (RP) rooted trees): Indicated by a (*,G) in the routing table, which indicates a shared tree for the multicast Group G.

Source-Rooted Tree (or SRT tree): Indicated by a (S,G) in the routing table. A source-rooted tree has been built for the multicast Group G and is sourced by the IP address(es).

Like all other multicast routing protocols, PIM conveys its messages in IGMP header data packets. If a host (receiver) wants to join a group, it will convey its membership information through IGMP. When a PIM router receives this IGMP message, the DR looks up the associated RP. The DR creates a “wildcard” entry for the group, which is written as (*,G). The DR creates a Join/Prune message (both Join and Prune entries are included in the same message). The flowchart for this process is shown in the slide. PIM works in conjunction with IGMP.

For a given (source, group) a multicast tree is initially built around the RP router. This initial tree is called a shared tree in that all members of the group converse using this single shared tree (albeit, it may not be the shortest path between a source and a host). It is easy to construct, reduces the amount of overhead in the router (tables, state information, etc.), and is easy to implement; however, it may not be efficient. Shared trees are built based on the center point rendezvous router. This shared tree may not allow for the shortest tree to be built between a source and some receiver hosts.

The PIM protocol can adapt here as well. Based on the data rate, after the shared tree is constructed (after meeting at the rendezvous point) between a host receiver and a source, it can change from being a shared tree to a shortest-path tree. The router sends a Join command directly to the source and a multicast tree is built. The original path through the rendezvous router is torn down.

An important point needs to be brought out here: PIM, like other multicast algorithms, uses RPF. Since PIM-SM uses both source-rooted trees and RP-rooted trees (explained later), the RPF check is done differently for source trees and shared trees. If a PIM router has a source-root tree state, it does the RPF check from the source IP address of the multicast packet. If the router has a shared-tree state (and no explicit source-tree state), it does the RPF check on the RP’s address (which is known when members join the group).

PIM-SM uses the RPF lookup function to determine where it needs to send Joins and Prunes.