TP-Link T3700G-28TQ, Руководство пользователя

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Grafting
When a new receiver on a previously pruned branch of the tree joins a multicast group, the PIM
DM takes the Graft mechanism to actively resume this node’s function of forwarding multicast
data, thus reducing the time it takes to resume to the forwarding state. The process is
illustrated as below:
(1) The branch that needs to receive the multicast data again will send a graft message to its
upstream node up the distribution tree towards the source hop-by-hop, applying to rejoin
the SPT;
(2)
The upstream node turns the downstream node into forwarding state after receiving the
graft message, and responses with a Graft-Ack message to confirm;
(3)
If the downstream node sending the graft message doesn’t receive the Graft-Ack
message from its upstream node, it will keep sending graft messages until being
confirmed.
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Assert Mechanism
If there are multiple multicast routers in one network segment, these routers may send the
same multicast packets to this network segment repeatedly. To avoid this kind of situation, the
Assert Mechanism is applied to select the exclusive router to forward the multicast data.
Figure 11-10 Assert Mechanism
As shown in Figure 11-12, the downstream node Router C will receive the same two (S, G)
multicast packets from Router A and Router B in the local network after they receive them from
the upstream nodes. Router A and Router B will also receive the multicast packets on their local
interfaces sent from each other.
Meanwhile, Router A and Router B will send the Assert Messages through their local interfaces
to all the PIM routers with the multicast address 224.0.0.13. The Assert Message contains the
following information: the multicast source address S, the multicast group address G, the
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