H12-821_V1.0 · Question #755
RPF has the function of preventing multicast forwarding loops.
The correct answer is B. FALSE. B is correct because RPF's (Reverse Path Forwarding) primary function is to prevent duplicate multicast packet forwarding, not to prevent forwarding loops. RPF works by checking that an incoming multicast packet arrives on the interface that the router would use to reach the…
Question
Options
- ATRUE
- BFALSE
How the community answered
(36 responses)- A28% (10)
- B72% (26)
Explanation
B is correct because RPF's (Reverse Path Forwarding) primary function is to prevent duplicate multicast packet forwarding, not to prevent forwarding loops. RPF works by checking that an incoming multicast packet arrives on the interface that the router would use to reach the packet's source - if it doesn't, the packet is discarded. This ensures packets are forwarded only from the correct upstream direction, preventing redundant copies, not loops per se.
Why A is wrong: Loop prevention in multicast is handled by the multicast routing protocol itself (e.g., PIM building a loop-free distribution tree), not by the RPF check. Conflating RPF's role with loop prevention is the key trap here.
Memory tip: Think of RPF as a "sanity check on the source direction" - it asks "did this packet come from where the source actually is?" If yes, forward it; if no, drop it. Loop prevention = distribution tree protocol's job; duplicate delivery prevention = RPF's job.
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