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DES-7200 Configuration Guide Chapter 7 PIM-SMv6
Configuration
7-2
PIM-SM forwards multicast data packets by setting up a multicast distribution tree.
There are two kinds of multicast distribution tree: the shared tree using G’s RP as
root and the shortest path tree using the multicast source as root. With the explicit
join/prune mechanism, PIM-SM sets up and maintains the multicast distribution
tree.
As illustrated in the above figure:
1 The DR on the receiving side receives the MLD (*,G) report message from a
host on the same side.
2 If the DR on the receiving side is not the RP of G, it will send a (*.G) Join
message to the RP. Upon receiving this message, the upstream router will send
the (*.G) Join message to the RP without the forwarding entry of the
corresponding group. In this way, the (*,G) Join message is transmitted hop to
hop until G’s RP receives the (*,G) Join message, indicating that the DR joins the
shared tree.
3 When the source host sends multicast data to a group, the data is encapsulated
in the registration message and sent by the DR on the source side to the RP in
unicast form. Then, the RP decapsulates the registration message, extracts the
data and forwards the data to every member of the group along the shared tree.
4 The RP sends the (S,G) Join message to the DR on the source side to join its
shortest path tree.
5 After the shortest path tree from the RP to the DR on the source side is
established, data packets are sent to the RP along this STP without
encapsulation.
6 When the first multicast data arrives along the SPT, the RP sends the
registration stop message to the DR on the source side, notifying the DR of
stopping registration and encapsulation. Upon receiving the registration stop
message, the DR no longer registers and encapsulates data packets. In stead, it
sends data packets to the RP along the shortest path tree, which then forwards
data packets to every group member along the shared tree.
7 When a receiving end does not need multicast packets, it sends the MLD leave
message.
8 The DR on the receiving side sends the prune message in multicast form hop
by hop to G’s RP, the prune message arrives at the RP or the router along the
way to the RP which has other (*,G) receiver so that data packets are not sent to
this receiving side any more.
9 If the RP has no downstream receiver at present, it sends (S,G) prune message
to the DR of the data source hop by hop. Consequently, the DR on the source
side prunes the interface receiving this (S,G ) prune message. In this way, data
packets are filtered on the DR of the data source.
RP election is also involved in PIM-SM. when there is one or more candidate
BSRs configured in the PIM-SM domain, some rule is applied to elect BSR.
Candidate RPs are also configured in the PIM-SM domain, which send the
packets containing their addresses and serviceable multicast groups to BSRs in
unicast form. BSRs generate bootstrap messages with a series of candidate RPs
and the addresses of corresponding multicast groups periodically. These
bootstrap messages are transmitted in the overall domain hop by hop. Devices
will receive and save these bootstrap messages. Upon receiving the member
relation report of a multicast group from the directly connected device, the DR will
use a hash algorithm and map the multicast group address to a candidate RP
who can serve this group if it has not the routing entry of this group. Then the DR
will send the join/prune message in multicast form hop by hop along the way to
the RP. On the other hand, upon receiving multicast packets from the directly
connected device, the DR will use a hash algorithm and map the multicast group
address to a candidate RP who can serve this group, and then encapsulate these