PLANET XGS3-24040 - Manual (Page 408)

Chapter 44 IPv6 Multicast Protocol
44-6
route shall be guaranteed above all. If all attempts fail to solve the problems on PIM-DM, then use debug
commands such as debug ipv6 pim, copy DEBUG information in 3 minutes and send to Technology Service
Center.
44.2 PIM-SM6
44.2.1 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 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.