297
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Assert
Neighbor discovery
In an IPv6 PIM domain, a PIM router discovers IPv6 PIM neighbors, maintains IPv6 PIM neighboring
relationship 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.
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 the
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 to stop
forwarding subsequent packets addressed to that IPv6 multicast group down to this node.
NOTE:
•
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 82
SPT establishment in an IPv6 PIM-DM domain
Source
Server
Host A
Host B
Host C
Receiver
Receiver
IPv6 multicast packets
SPT
Prune message
