A new member joins a multicast group in PIM-DM, which of the following descriptions are correct? (Multiple choice)

Question

Options

• AIn PIM-DM mode, after using the "diffusion-pruning" method to establish a multicast distribution tree, the status refresh mechanism prevents the downlink interface from entering the pruned state due to periodic reports.
• BYou can use the grafting mechanism to proactively establish a multicast distribution path in reverse direction to help new users receive multicast information.
• CThe multicast network needs to update the multicast distribution tree before it can send multicast data to group members.
• DAfter a member joins the multicast group, the multicast routing table no longer needs to be updated.

• A
  7% (3)
• B
  82% (36)
• D
  11% (5)

Explanation

B and C are correct because PIM-DM's graft mechanism allows a downstream router to immediately send a Graft message upstream (reverse path) when a new host joins a previously pruned branch, restoring forwarding without waiting for the prune timer to expire (B). Consequently, the multicast distribution tree must be updated - the pruned branch restored - before the network can actually forward traffic to that new member (C).

Option A is wrong because the state refresh mechanism does the opposite of what's described: it maintains (extends) the pruned state by periodically refreshing prune timers, preventing unnecessary re-flooding. It keeps interfaces pruned longer, not preventing them from being pruned.

Option D is wrong because when a new member joins, the multicast routing/forwarding table absolutely must be updated - the outgoing interface list for the (S,G) entry needs to reflect the new branch, which is precisely what the graft procedure triggers.

Memory tip: Think of PIM-DM as "flood, then prune dead ends." When a newcomer shows up on a pruned branch, the Graft message says "add me back!" upstream - the tree must re-grow to reach them before any data flows. State refresh = "stay pruned," not "stop pruning."

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