Shortest-path tree (SPT)
SPTs are especially useful in high data-rate applications where reducing unnecessary traffic concentrations and
throughput delays are significant. In the default PIM-SM configuration, SPT operation is automatically enabled.
(The software includes an option to disable SPT operation.
Shortest-path tree operation
In the default PIM-SM configuration, after an edge router receives the first packet of traffic for a multicast group
requested by a multicast receiver on that router, it uses Reverse Path Forwarding (RPF) to learn the shortest path
to the group source. The edge router then stops using the RPT and begins using the shortest path tree (SPT)
connecting the multicast source and the multicast receiver. In this case, when the edge router begins receiving
group traffic from the multicast source through the SPT, it sends a prune message to the RP tree to terminate
sending the requested group traffic on that route. (This results in entries for both the RP path and the STP in the
routing table.) When completed, the switchover from the RPT to a shorter SPT can reduce unnecessary traffic
concentrations in the network and reduce multicast traffic throughput delays.
Note that the switchover from RPT to SPT is not instantaneous. For a short period, packets for a given multicast
group may be received from both the RPT and the SPT. Also, in some topologies, the RPT and the SPT to the
same edge router may be identical.
Figure 21: Example PIM-SM domain with SPT active to support a host that has joined a multicast group
Chapter 4 PIM-SM (Sparse Mode)
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