3Com Switch 8800 Advanced Software V5 Configuration Guide

PIM Overview 565
n
■ An (S, G) entry contains the multicast source address S, multicast group
address G, outgoing interface list, and incoming interface.
■ For a given multicast stream, the interface that receives the multicast stream is
referred to as "upstream", and the interfaces that forward the multicast stream
are referred to as "downstream".
A prune process is first initiated by a leaf router. As shown in Figure 174, a router
without any receiver attached to it (the router connected with Host A, for
example) sends a prune message, and this prune process goes on until only
necessary branches are left in the PIM-DM domain. These branches constitute the
SPT.
Figure 174 SPT building
The "flood and prune" process takes place periodically. A pruned state timeout
mechanism is provided. A pruned branch restarts multicast forwarding when the
pruned state times out and then is pruned again when it no longer has any
multicast receiver.
Graft
When a host attached to a pruned node joins a multicast group, to reduce the join
latency, PIM-DM uses a graft mechanism to resume data forwarding to that
branch. The process is as follows:
1 The node that need to receive multicast data sends a graft message hop
by hop toward the source, as a request to join the SPT again.
2 Upon receiving this graft message, the upstream node puts the interface
on which the graft was received into the forwarding state and responds
with a graft-ack message to the graft sender.
3 If the node that sent a graft message does not receive a graft-ack message
from its upstream node, it will keep sending graft messages at a
configurable interval until it receives an acknowledgment from its
upstream node.
Source
Server
Host A
Host B
Host C
Receiver
Receiver
Multicast packets
SPT
Prune message