3Com Switch 8800 Advanced Software V5 Configuration Guide

672 CHAPTER 46: IPV6 PIM CONFIGURATION
Based on the forwarding mechanism, IPv6 PIM falls into two modes:
■ Protocol Independent Multicast-Dense Mode for IPv6 (IPv6 PIM-DM), and
■ Protocol Independent Multicast-Sparse Mode for IPv6 (IPv6 PIM-SM).
n
To facilitate description, a network comprising IPv6 PIM routers or IPv6 PIM routing
switches is referred to as an "IPv6 PIM domain" in this document.
Introduction to IPv6
PIM-DM
IPv6 PIM-DM is a type of dense mode IPv6 multicast protocol. It uses the "push
mode" for IPv6 multicast forwarding, and is suitable for small-sized networks with
densely distributed IPv6 multicast members.
The basic implementation of IPv6 PIM-DM is as follows:
■ IPv6 PIM-DM assumes that at least one IPv6 multicast group member exists on
each subnet of a network, and therefore IPv6 multicast data is flooded to all
nodes on the network. Then, branches without IPv6 multicast forwarding are
pruned from the forwarding tree, leaving only those branches that contain
receivers. This "flood and prune" process takes place periodically, that is,
pruned branches resume IPv6 multicast forwarding when the pruned state
times out and then data is re-flooded down these branches, and then are
pruned again.
■ When a new receiver on a previously pruned branch joins an IPv6 multicast
group, to reduce the join latency, IPv6 PIM-DM uses the graft mechanism to
resume IPv6 multicast data forwarding to that branch.
Generally speaking, the IPv6 multicast forwarding path is a source tree, namely a
forwarding tree with the IPv6 multicast source as its "root" and IPv6 multicast
group members as its "leaves". Because the source tree is the shortest path from
the IPv6 multicast source to the receivers, it is also called shortest path tree (SPT).
How IPv6 PIM-DM
Works
The working mechanism of IPv6 PIM-DM is summarized as follows:
■ Neighbor discovery
■ SPT building
■ Graft
■ Assert
Neighbor discovery
In a IPv6 PIM domain, a PIM router discovers IPv6 PIM neighbors, maintains IPv6
PIM neighboring relationships with other routers, and builds and maintains SPTs by
periodically multicasting IPv6 PIM hello messages (hereinafter referred to as "hello
messages") to all other IPv6 PIM routers.
n
Every activated interface on a router sends hello messages periodically, and thus
learns the IPv6 PIM neighboring information pertinent to the interface.
SPT building
1 In an IPv6 PIM-DM domain, an IPv6 multicast source first floods IPv6 multicast
packets when it sends IPv6 multicast data to an IPv6 multicast group G: The packet
is subject to an RPF check. If the packet passes the RPF check, the router creates an