TP-Link T2700G-28TQ T2700G-28TQ User Guide V1 - Page 256

Grafting, Assert Mechanism, Assert Mechanism

Page 256 highlights

 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.  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 11-12 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 priority and cost of the unicast route to the multicast source. The router to forward the multicast packets of (S, G) is elected based on the following rules and in the order listed: (1) The router with the unicast route of the higher priority to the multicast source; 244

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244
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.
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 11-12 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 priority
and cost of the unicast route to the multicast source. The router to forward the multicast packets of
(S, G) is elected based on the following rules and in the order listed:
(1)
The router with the unicast route of the higher priority to the multicast source;