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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 multicast data sends a graft message toward its upstream node,
as a request to join the SPT again.
2.
After receiving this graft message on an interface, the upstream node puts the receiving
interface to the forwarding state. It also responds with 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 keeps sending graft messages at a configurable interval until it receives an
acknowledgment from its upstream node.
Assert
On a multi-access network 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 on the
multi-access network.
Figure 41 Assert mechanism
As shown in
, after Router A and Router B receive an (S, G) packet from the upstream node,
both of them forward the packet to the local subnet. As a result, the downstream node Router C
receives two identical multicast packets. In addition, both Router A and Router B, on their
downstream interfaces, receive a duplicate packet forwarded by the other. After detecting this
condition, both routers send an assert message to all PIM routers (224.0.0.13) on the local subnet
through the interface that received the packet. The assert message contains the multicast source
address (S), the multicast group address (G), and the metric preference and metric of the unicast
route/MBGP route/multicast static route to the source. By comparing these parameters, either
Router A or Router B becomes the unique forwarder of the subsequent (S, G) packets on the
multi-access subnet. The comparison process is as follows:
1.
The router with a higher metric preference to the source wins.
2.
If both routers have the same metric preference, the router with a smaller metric wins.
3.
If both routers have the same metric, the router with a higher IP address on the downstream
interface wins.
PIM-SM
PIM-DM uses the flood-and-prune principle to build SPTs for multicast data distribution. Although an
SPT has the shortest path, it is built with a low efficiency. Therefore, the PIM-DM mode is not suitable
for large- and medium-sized networks.
