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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 determine their IPv6 PIM neighbors, maintain IPv6 PIM neighboring relationships with
other routers, and build and maintain SPTs.
Every IPv6 PIM enabled interface on a router sends hello messages 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
routers in the IPv6 PIM-DM domain.
2.
Nodes without downstream receivers are pruned. A router that has no downstream receivers
sends a prune message to the upstream node. The message notifies 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.
NOTE:
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An (S, G) entry contains the multicast source address S, IPv6 multicast group address G,
outgoing interface list, and incoming interface.
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For a given IPv6 multicast stream, the interface that receives the IPv6 multicast stream is called
"upstream," and the interfaces that forward the IPv6 multicast stream are called "downstream."
A leaf router first initiates a prune process. As shown in
, a router without any receiver
attached to it (the router connected with Host A, for example) sends a prune message. This prune
process continues until only necessary branches remain in the IPv6 PIM-DM domain. These
branches constitute the SPT.
Figure 116 SPT establishment in an IPv6 PIM-DM domain
Source
Server
Host A
Host B
Host C
Receiver
Receiver
IPv6 multicast packets
SPT
Prune message
