Which Multicast Mode Sends Prune Messages When No Members Exist?

350-401 By islever Editorial Last updated

Short answer

B PIM dense mode

Question

Which multicast operational mode sends a prune message to the source when there are no connected members or downstream neighbors?
  1. IGMPv3
  2. PIM dense mode Correct Answer
  3. PIM sparse mode
  4. IGMPv2

Key Takeaways

The exam tests your understanding of PIM Dense Mode's flood-and-prune behavior versus PIM Sparse Mode's explicit join mechanism, with the common trap being confusing prune messages (Dense) with join messages (Sparse).

PIM Dense Mode uses a push/flood-and-prune mechanism where multicast traffic is forwarded everywhere by default and pruned back when no receivers exist. Community consensus strongly confirms PIM Dense Mode (B) as the correct answer.

Common mistake: Many candidates incorrectly choose PIM Sparse Mode (C) because they associate 'no members' with Sparse Mode, failing to recognize that Sparse Mode uses explicit joins rather than prune messages to control traffic flow.

Expert Analysis

Why the Answer Is Correct

PIM Dense Mode operates on a push mechanism where multicast traffic is initially flooded to all interfaces in the network. As noted in comment [4], when a router has no directly connected members or PIM neighbors, it sends a prune message back toward the source to stop unwanted traffic. Comment [5] reinforces this by explaining that Dense Mode floods everywhere first, then prunes back branches without active receivers. This flood-and-prune behavior is the defining characteristic of PIM Dense Mode.

Why the Other Options Are Wrong

IGMPv2 and IGMPv3 (Options A and D) are host-to-router group management protocols, not multicast routing protocols that send prune messages to sources. Comment [2] clearly distinguishes PIM Sparse Mode as a pull mechanism that requires explicit joins and uses a Rendezvous Point (RP), making it fundamentally different from the prune-based Dense Mode. Comment [1] provides the clearest distinction: Dense mode forwards until told to stop, while Sparse mode doesn't forward until told to start.

Community Comment Notes

The community overwhelmingly supports answer B with 100% consensus. Comment [1] provides an excellent summary of the push vs. pull distinction between Dense and Sparse modes. Comment [4] directly quotes the exam scenario, confirming that Dense Mode sends prune messages when no members or downstream neighbors exist. Multiple comments ([2], [5], [6]) reinforce the push/pull mechanism distinction, which is crucial for exam success.

Official Reference

https://tools.ietf.org/html/rfc7761

This link opens the official vendor documentation in a new tab.

Exam Strategy

When you see 'prune message' in a multicast question, immediately think PIM Dense Mode and its flood-and-prune mechanism. Remember the mnemonic: Dense = Push (flood then prune), Sparse = Pull (explicit join only).

Community Votes

B
100%

100% of anonymous learners picked answer B. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 6 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 6

  1. babanaber 👍 5
    Here's the key difference between sparse and dense mode: Dense mode: we forward multicast traffic on all interfaces until a downstream router requests us to stop fowarding. Sparse mode: we don't forward multicast traffic on any interface until a downstream router requests us to forward it.
  2. masrur 👍 3 Answered B
    PIM dense mode (push mechanism) send multicast traffic to all interfaces until receivers ask to stop forward and then prune unwanted traffic, while PIM sparse mode (pull mechanism) start forwarding multicast traffic if requested to do it. PIM sparse mode is required RP (rendezvous point) from where downstream routers pulling traffics.
  3. slacker_at_work 👍 2 Answered B
    PIM (Protocol Independent Multicast) dense mode operates by flooding multicast traffic to all parts of the network, and routers prune back branches of the distribution tree where there are no active receivers. When there are no connected members or downstream neighbors, routers send prune messages upstream toward the source, informing it to stop sending traffic down the pruned branches.
  4. aatechler 👍 2 Answered B
    In dense mode, a router assumes that all other routers want to forward multicast packets for a group. If a router receives a multicast packet and has no directly connected members or PIM neighbors present, a prune message is sent back to the source. Subsequent multicast packets are not flooded to this router on this pruned branch. PIM builds source-based multicast distribution trees.
  5. 16561f6 👍 2
    C. PIM sparse mode Explanation: In PIM sparse mode, when there are no connected members or downstream neighbors for a multicast group, the router sends a prune message to the source. This mechanism helps to prevent unnecessary traffic from being forwarded to areas of the network that do not have interested receivers, thus optimizing bandwidth usage.
Show 1 more comment
  1. chiacche 👍 1 Answered B
    PIM Sparse Mode -> pull mechanism PIM Dense Mode -> push mechanism

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