404
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
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 IPv6 multicast data sends a graft message toward its upstream
node, as a request to join the SPT again.
2.
After receiving this graft message, the upstream node puts the interface on which the graft was
received into 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 IPv6 multicast flows to the network. It does this by electing a unique IPv6 multicast
forwarder on the multi-access network.
Figure 117 Assert mechanism
As shown in
, after Router A and Router B receive an (S, G) IPv6 multicast 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, and both Router A and Router B, on their
own downstream interfaces, receive a duplicate IPv6 multicast packet that the other has forwarded.
After detecting this condition, both routers send an assert message to all IPv6 PIM routers 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 IPv6 unicast route/IPv6 MBGP route/IPv6 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) IPv6 multicast 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 to the source, the router with a smaller metric to
the source wins.
3.
If both routers have the same metric, the router with a higher IPv6 link-local address on the
downstream interface wins.
