PLANET XGS3-24042 - Manual (Page 462)

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PIM-SM routers and establish, using Join/Prune message of routers, RPT (RP-rooted shared tree) based on
RP. Consequently the network bandwidth occupied by data packets and control messages is cut down and
the transaction cost of routers is reduced. Multicast data get to the network segment where the multicast
group members are located along the shared tree flow. When the data traffic reaches a certain amount,
multicast data stream can be switched to source-based SPT (Shortest Path Tree) to shorten network delay.
PIM-SM doesn’t rely on any specific unicast routing protocol but make RPF examination using existing
unicast routing table.
1. PIM-SM Working Principle
The working process of PIM-SM mainly includes neighbor discovery, creation of RPT, registration of
multicast source, SPT switch and so on. The neighbor discovery mechanism is the same with the
mechanism of PIM-DM. We won’t introduce any more.
(1) Creation of RP Shared Tree (RPT)
When a host joins a multicast group G, the leaf router directly connected with the host finds out through
IGMP message that there is a receiver of multicast group G, then it works out the corresponding
Rendezvous Point RP for multicast group G, and send join message to upper level nodes in RP
direction. Every router on the way from the leaf router to RP will create a (*, G) table item, indicating the
message from any source to multicast group G is suitable for this item. When RP receives the
message sent to multicast group G, the message will get to the leaf router along the established path
and then reach the host. In this way, the RPT with RP as root is created.
(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 take charge of sealing the multicast packet into registered message and
unicast it to corresponding RP. If there are more than one PIM-SM multicast routers on a network
segment, then DR (Designated Router) takes charge of forwarding the multicast packet.
(3) SPT Switch
Once the multicast router finds that the rate of the multicast packet from RP with destination address G
exceeds threshold, the multicast router will send Join message to the upper lever nodes in the source
direction, which results in the switch from RPT to SPT.
2. Preparation before PIM-SM configuration
(1) Configuration Candidate RP
More than one RPs (candidate RP) are permitted in PIM-SM network and each C-RP (Candidate RP)
takes charge of forwarding multicast packets with destination address in a certain range. To configure
more than one candidate RPs can achieve RP load balancing. There is no master or slave difference
among RPs. All multicast routers work out the RP corresponded with certain multicast group based on
the same algorithm after receiving the candidate RP message announced by BSR.
Note that one RP can serve more than one multicast groups, even all multicast groups. But each
multicast group can only correspond with one unique RP at any moment. It can’t correspond with more
RPs at the same time.
(2) BSR Configuration
As the management core of PIMSM network, BSR is in charge of collecting messages sent by
candidate RPs and broadcast them..
There may be only one BSR within a network. However, there may be several candidate BSRs to be
configured. With such arrangement, once a BSR fails, another may be switched to. C-BSR determines
BSR through automatic selection.