Accton Technology ES4626 - Manual (Page 693)

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interface
17.2 PIM-SM6
17.2.1 Introduction to PIM-SM6
PIM-SM6（Protocol Independent Multicast, Sparse Mode）is the IPv6 version of
Protocol Independent Multicast Sparse Mode. It is a multicast routing protocol in sparse
mode and mainly used in large network with group members distributed relatively sparse
and wide. It is no difference from the IPv4 version PIM-SM except the addresses it uses
are IPv6 addresses. Thus we don’t differentiate between PIM-SM and PIM-SM6 in this
chapter. All PIM-SM in the text without specific explanation is IPv6 version PIM-SM.
Unlike the Flooding-Prune of Dense Mode, PIM-SM Protocol assumes no host needs
receiving multicast data packets. PIM-SM router forwards multicast data packets to a
host only on definite request.
By setting RP (Rendezvous Point) and BSR (Bootstrap Router), PIM-SM announce
multicast packet to all PIM-SM routers and establish, using Join/Prune message of
routers, RPT (RP-rooted shared tree) based on RP. Consequently the network bandwith
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