H12-821_V1.0 · Question #636
Which of the following mechanisms does the PIM-DM mode include in the process of building a multicast forwarding tree? (Multiple choice)
The correct answer is A. State refresh C. Pruning D. Grafting. PIM-DM (Dense Mode) builds multicast trees using a flood-and-prune model: it first floods traffic everywhere, then uses Pruning (C) to cut back branches with no interested receivers, and Grafting (D) to quickly re-attach pruned branches when new receivers appear - avoiding the…
Question
Options
- AState refresh
- BRPT switching
- CPruning
- DGrafting
How the community answered
(65 responses)- A82% (53)
- B18% (12)
Explanation
PIM-DM (Dense Mode) builds multicast trees using a flood-and-prune model: it first floods traffic everywhere, then uses Pruning (C) to cut back branches with no interested receivers, and Grafting (D) to quickly re-attach pruned branches when new receivers appear - avoiding the wait for prune-state expiry. State Refresh (A) is also PIM-DM-specific: it periodically re-sends prune state to prevent pruned branches from timing out and triggering another costly re-flood cycle.
Why B is wrong: RPT (Rendezvous Point Tree) switching is a PIM-SM (Sparse Mode) mechanism - it describes the process of switching from a shared RP tree to a source-specific shortest-path tree. PIM-DM has no concept of a Rendezvous Point; it only builds source-based trees.
Memory tip: Think of PIM-DM as "Flood, Prune, Graft, Refresh" - four operations that form a complete lifecycle. If you see anything involving an RP (Rendezvous Point) or shared trees, that's always PIM-SM territory, not DM.
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