PIM-SM (Sparse Mode)
PIM-SM Operation and Router Types
intermediate PIM-SM routers leading to the PIM-SM edge router(s) for the
multicast receiver(s) requesting the traffic. (If the RP has no current join
requests for the group, then the traffic is dropped at the RP.)
PIM-SM
Router “A”
PIM-SM
Router “B”
PIM-SM
Router “C”
PIM-SM
Router “D”
Source of
Multicast
Group “X”
RPT Path
Host “Y”
Designated Router
(DR) for Unicast Source of
Multicast Group “X”
Rendezvous Point
(RP) Elected To Support
Multicast Group “X”
Edge Router
Intermediate
Router for
RPT Path for
Group “X”
In default PIM-SM operation, the RPT path
forms to deliver the first multicast packet
from Group “X” to Host “Y”.
(Note that any router configured in the
domain as a BSR candidate can be elected
as the BSR.
Figure 4-1. Example PIM-SM Domain with RPT Active To Support a Host Joining a
Multicast Group
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. Refer to “Changing
the Shortest-Path Tree (SPT) Operation” on page 4-42.)
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 Forward-
ing (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. Refer to “Routing Table Entries” on
4-10
