R21xx-HP FlexFabric 11900 IP Multicast Configuration Guide

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Figure 24 SPT building
The pruned state of a branch has a finite holdtime timer. When the timer expires, multicast data is again
forwarded to the pruned branch. The flood-and-prune cycle takes place periodically to maintain the
forwarding branches.
Graft
To reduce the join latency when a new receiver on a previously pruned branch joins a multicast group,
PIM-DM uses a graft mechanism to turn the pruned branch into a forwarding branch, as follows:
1. The node that needs to receive the multicast data sends a graft message to its upstream node,
telling it to rejoin the SPT.
2. After receiving this graft message, the upstream node adds the interface that received the graft
message into the outgoing interface list of the (S, G) entry for the multicast group, and then sends
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 continues to send graft messages at a configurable interval until it receives an acknowledgment
from its upstream node.
Assert
On a subnet with more than one multicast router, the assert mechanism shuts off duplicate multicast flows
to the network. It does this by electing a unique multicast forwarder for the subnet.
Source
Server
Host A
Host B
Host C
Receiver
Receiver
Multicast packets
SPT
Prune message