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sends a join message to the upstream node in the RP direction. Each routers between the
leaf router and the RP will created a (*, G) entry in their forwarding table, indicating
packets sent by any source to multicast group G applies to this entry. When RP receives a
packet sending to multicast group G, the packet will move along the established route to
reach the leaf router and the host. This completes a RP-rooted RPT.
(2) Multicast source registration.
When multicast source S sends a multicast packet to multicast group G, the PIM-SM
multicast router directly connected to it will packet the multicast packet as a registration
packet and unicast to the appropriate RP. If multiple PIM-SM multicast routers exist in the
network, the designated router (DR) is responsible for the forwarding of this multicast
packet.
(3) SPT toggle
When multicast router finds the multicast packets from RP destined to G in a speed
exceeding the threshold, the multicast router will send a join message to the upstream
node in the source S direction and cause the toggling from RPT to SPT.
2. Pre-PIM-SM configuration work
(1) Configure candidate RP
In PIM-SM networks, multiple RPs are allowed, they are referred to as the candidate RP
(C-RP). Each C-RP is responsible for the forwarding of multicast packet destined to a
certain range of addresses. Configuring multiple C-RP enables RP load balance. All
C-RPs are of the same priority. On receiving BSR advertised C-RP message, multicast
routers will calculate the RP corresponding to a certain multicast group with a same
algorithm.
It should be noted that one RP can service multiple multicast groups or all multicast
groups. Each multicast group in any time can have only one corresponding RP, multiple
association is forbidden.
2) Configure BSR
BSR is the core of management in PIM-SM networks; it is responsible for gathering
information from C-RP and broadcasting the information gathered.
Each network can have one BSR, and several Candidate-BSRs (C-BSRs). This way, once
a BSR fails, another BSR will quickly take its place. BSR will be decided by the
auto-election between C-BSRs.
16.4.2 PIM-SM Configuration
16.4.2.1 PIM-SM Configuration Task Sequence
1. Enable PIM-SM (required)
2. Configure PIM-SM sub-parameters