350-501 Networking Practice Question
Which TWO statements correctly describe differences between PIM dense mode and PIM sparse mode? (Choose two.)
⚠ Common exam trap
Cisco often tests the reversal of PIM-DM and PIM-SM behaviors, specifically confusing which mode uses explicit joins versus flood-and-prune, and which mode requires an RP.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
PIM-DM assumes all downstream routers want to receive multicast traffic, so it initially floods traffic.
PIM-DM operates on the assumption that every downstream router wants to receive multicast traffic for a given group. It initially floods multicast traffic out all PIM-enabled interfaces, then prunes back branches that do not have interested receivers. This flood-and-prune behavior is fundamental to PIM-DM's design for dense multicast environments.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
PIM-DM supports the use of a bootstrap router (BSR) for RP discovery.
Why it's wrong here
BSR is used in sparse mode, not dense mode.
- ✗
PIM-SM is more bandwidth efficient for high density groups.
Why it's wrong here
Dense mode is better for high density; sparse mode for sparse groups.
- ✗
PIM-DM uses explicit join messages, while PIM-SM uses flood and prune.
Why it's wrong here
Opposite: Dense mode floods and prunes; sparse mode uses explicit join.
- ✓
PIM-DM assumes all downstream routers want to receive multicast traffic, so it initially floods traffic.
Why this is correct
Dense mode floods to all interfaces and then prunes where not wanted.
- ✓
PIM-SM requires a Rendezvous Point (RP) to facilitate group membership.
Why this is correct
RP is a key component in sparse mode.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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