H3C S5500-EI - Page 386

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• IPv6 PIM-DM assumes that at least one IPv6 multicast group member exists on each subnet of a
network. 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
flooded again down these branches, and then the branches 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.
In general, the IPv6 multicast forwarding path is a source tree. That is, it is 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)."
The working mechanism of IPv6 PIM-DM is summarized as follows:
• Neighbor discovery
• SPT establishment
• Graft
• Assert
Neighbor discovery
In an 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 to all other IPv6 PIM routers on the local subnet.
NOTE:
Every IPv6 PIM enabled interface on a router sends hello messa
g
es periodically and, therefore, learns the
IPv6 PIM neighboring information pertinent to the interface.
SPT establishment
The process of constructing an SPT is the flood and prune process.
1. In an IPv6 PIM-DM domain, an IPv6 multicast source first floods IPv6 multicast packets when it
sends IPv6 multicast data to IPv6 multicast group G. The packet undergoes an RPF check. If the
packet passes the RPF check, the router creates an (S, G) entry and forwards the packet to all
downstream nodes in the network. In the flooding process, an (S, G) entry is created on all the
routers in the IPv6 PIM-DM domain.
2. The nodes without downstream receivers are pruned. A router that has no downstream receivers
sends a prune message to the upstream node to notify the upstream node to delete the
corresponding interface from the outgoing interface list in the (S, G) entry and stop forwarding
subsequent packets addressed to that IPv6 multicast group down to this node.
An (S, G) entry contains the multicast source address S, IPv6 multicast group address G, outgoing
interface list, and incoming interface.