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Neighbor discovery
In an IPv6 PIM domain, each IPv6 PIM interface periodically multicasts IPv6 PIM hello messages to all
other IPv6 PIM routers on the local subnet. Through the exchanging of hello messages, all IPv6 PIM
routers discover IPv6 PIM neighbors, maintain IPv6 PIM neighboring relationship with other routers, and
build and maintain SPTs.
SPT building
The process of building an SPT is the flood-and-prune process:
1. In an IPv6 PIM-DM domain, the IPv6 multicast data from the IPv6 multicast source S to the IPv6
multicast group G is flooded throughout the domain. A router performs an RPF check for the IPv6
multicast data. If the check succeeds, the router creates an (S, G) entry and forwards the data to
all downstream nodes in the network. In the flooding process, all the routers in the IPv6 PIM-DM
domain create the (S, G) entry.
2. The nodes without downstream receivers are pruned. A router that has no downstream receivers
multicasts a prune message to the upstream node to remove the interface that receives the prune
message from the (S, G) entry. In this way, the upstream stream node stops forwarding subsequent
packets addressed to that IPv6 multicast group down to this node.
NOTE:
n (S, G) entry contains an IPv6 multicast source address S, an IPv6 multicast
roup address G, an
outgoing interface list, and an incoming interface.
A prune process is initiated by a leaf router. As shown in Figure 85, the router interface that does not
have any downstream receivers initiates a prune process by sending a prune message toward the IPv6
multicast source. This prune process goes on until only necessary branches are left in the IPv6 PIM-DM
domain, and these necessary branches constitute an SPT.
Figure 85 SPT building
The pruned state of a branch has a finite holdtime timer. When the timer expires, IPv6 multicast data is
again forwarded to the pruned branch. The flood-and-prune cycle takes place periodically to maintain
the forwarding branches.
Source
Server
Host A
Host B
Host C
Receiver
Receiver
IPv6 multicast packets
SPT
Prune message
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