102
this way, the upstream stream node stops forwarding subsequent packets addressed to that
multicast group down to this node.
NOTE:
An (S, G) entry contains a multicast source address S, a multicast group address G, an outgoing
interface list, and an incoming interface.
A prune process is initiated by a leaf router. As shown in
, the router interface that does not
have any downstream receivers initiates a prune process by sending a prune message toward the
multicast source. This prune process goes on until only necessary branches are left in the PIM-DM
domain, and these necessary branches constitute an SPT.
Figure 38 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
A previously pruned branch might have new downstream receivers. To reduce the latency for
resuming the forwarding capability of this branch, a graft mechanism is used 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 on an interface, the upstream node adds the receiving
interface to the outgoing interface list of the (S, G) entry. It also sends a graft-ack message to
the graft sender.
3.
If the graft sender receives a graft-ack message, the graft process finishes. Otherwise, the graft
sender continues to send graft messages at a graft retry 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
