Using Multicast in Your Network
19-10 Configuring Multicast
• A new dependent downstream device appears on a pruned branch.
• A dependent downstream device on a pruned branch restarts.
• A graft retransmission timer expires before a graft ACK is received.
Graft messages are sent upstream hop-by-hop until the multicast tree is reached. Since there is no
way to tell whether a graft message was lost or the source has stopped sending, each graft
message is acknowledged hop-by-hop.
When sending grafts, the downstream device does the following:
1. Verifies a prune exists for the source network and group.
2. Verifies that the upstream device is capable of receiving prunes (and therefore grafts).
3. Adds the graft to the retransmission timer list awaiting an acknowledgment.
4. Formulates and transmits the graft packet.
When receiving grafts, the upstream device does the following:
1. Verifies whether the neighbor is known.
• If unknown, discards the received graft.
• If known, proceeds as follows.
2. Ensures the graft message contains at least the correct amount of data.
3. Sends back a graft ACK to the sender.
4. If the sender was a downstream dependent neighbor from which a prune had previously been
received:
• Removes the prune state for this neighbor.
• If necessary, updates any forwarding cache entries based on this (source, group) pair to
include this downstream interface.
Figure 19-3 on page 19-11 shows the DVMRP pruning and grafting process.
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