3Com Switch 8800 Advanced Software V5 Configuration Guide

IPv6 PIM Overview 675
3 If there is a tie in the route metric to the source, the router with a higher IP address
of the local interface wins.
Introduction to IPv6
PIM-SM
IPv6 PIM-DM uses the "flood and prune" principle to build SPTs for IPv6 multicast
data distribution. Although an SPT has the shortest path, it is built with a low
efficiency. Therefore the PIM-DM mod is not suitable for large- and medium-sized
networks.
IPv6 PIM-SM is a type of sparse mode IPv6 multicast protocol. It uses the "pull
mode" for IPv6 multicast forwarding, and is suitable for large- and medium-sized
networks with sparsely and widely distributed IPv6 multicast group members.
The basic implementation of IPv6 PIM-SM is as follows:
■ IPv6 PIM-SM assumes that no hosts need to receive IPv6 multicast data. In the
IPv6 PIM-SM mode, routers must specifically request a particular IPv6 multicast
stream before the data is forwarded to them. The core task for IPv6 PIM-SM to
implement IPv6 multicast forwarding is to build and maintain rendezvous point
trees (RPTs). An RPT is rooted at a router in the IPv6 PIM domain as the
common node, or rendezvous point (RP), through which the IPv6 multicast
data travels along the RPT and reaches the receivers.
■ When a receiver is interested in the IPv6 multicast data addressed to a specific
IPv6 multicast group, the router connected to this receiver sends a join message
to the RP corresponding to that IPv6 multicast group. The path along which the
message goes hop by hop to the RP forms a branch of the RPT.
■ When a multicast source sends an IPv6 multicast packet to an IPv6 multicast
group, the router directly connected with the multicast source first registers the
multicast source with the RP by sending a register message to the RP by
unicast. The arrival of this message at the RP triggers the establishment of an
SPT. Then, the multicast source sends subsequent IPv6 multicast packets along
the SPT to the RP. Upon reaching the RP, the IPv6 multicast packet is duplicated
and delivered to the receivers along the RPT.
n
IPv6 multicast traffic is duplicated only where the distribution tree branches, and
this process automatically repeats until the IPv6 multicast traffic reaches the
receivers.
How IPv6 PIM-SM Works The working mechanism of IPv6 PIM-SM is summarized as follows:
■ Neighbor discovery
■ DR election
■ RP discovery
■ Embedded RP
■ Extracting an embedded RP address
■ RPT building
■ IPv6 Multicast source registration
■ Switchover from RPT to SPT
■ Assert