R21xx-HP FlexFabric 11900 IP Multicast Configuration Guide

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In PIM-DM, the multicast forwarding paths for a multicast group constitutes a source tree, which is rooted
at the multicast source and has multicast group members as its "leaves." Because the source tree consists
of the shortest paths from the multicast source to the receivers, it is also called a "shortest path tree (SPT)."
The operating mechanism of PIM-DM is summarized as follows:
• Neighbor discovery
• SPT building
• Graft
• Assert
Neighbor discovery
In a PIM domain, each interface that runs PIM on a router periodically multicasts PIM hello messages to
all other PIM routers (identified by the address 224.0.0.13) on the local subnet to discover PIM neighbors,
maintain PIM neighboring relationship with other routers, and build and maintain SPTs.
SPT building
The process of building an SPT is the flood-and-prune process:
1. In a PIM-DM domain, when the multicast source S sends multicast data to the multicast group G,
the multicast data is flooded throughout the domain. A router performs an RPF check for the
multicast data. If the RPF check succeeds, the router creates an (S, G) entry and forwards the data
to all downstream nodes in the network. In the flooding process, all the routers in the PIM-DM
domain create the (S, G) entry.
2. The nodes without downstream receivers are pruned. A router that has no downstream receivers
sends a prune message to the upstream node to remove the interface that receives the prune
message from the (S, G) entry. In this way, the upstream stream node stops forwarding subsequent
packets addressed to that multicast group down to this node.
NOTE:
A
n (S, G) entry contains a multicast source address S, a multicast group address G, an out
g
oin
g
interface
list, and an incoming interface.
A prune process is initiated by a leaf router. As shown in Figure 24, the router interface that does not
have any downstream receivers initiates a prune process by sending a prune message toward the
multicast source. This prune process goes on until only necessary branches are left in the PIM-DM domain,
and these necessary branches constitute an SPT.