hardMultiple Select
350-401 Practice Question: Which two statements about multicast RPF check…
Which two statements about multicast RPF check are true? (Choose two.)
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
✓
The RPF check uses the unicast routing table (or a dedicated multicast routing table) to determine the best path back to the multicast source.
RPF check is a fundamental loop prevention mechanism in multicast. It verifies that the incoming interface of a multicast packet is the same interface the router would use to reach the source. If the check fails, the packet is dropped. RPF can be influenced by static mroutes and is performed on all multicast packets, not just data.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The RPF check uses the unicast routing table (or a dedicated multicast routing table) to determine the best path back to the multicast source.
Why this is correct
Correct because the RPF check compares the source IP address of the multicast packet against the routing table (RIB or MRIB) to find the outgoing interface toward the source; the packet is accepted only if it arrives on that interface.
- ✓
If the RPF check fails, the multicast packet is dropped to prevent routing loops.
Why this is correct
A failed RPF check means the multicast packet arrived on an interface that is not the optimal path back to the source; accepting it would allow duplicate or looping traffic to be re-flooded downstream. Multicast routers build forwarding state based on the source's location, so any packet that does not come from the expected upstream interface is immediately discarded. This per-packet drop is the core loop-prevention mechanism in multicast, since PIM does not maintain per-flow state like TCP.
- ✗
The RPF check is performed only on the first packet of a multicast stream; subsequent packets are forwarded without verification.
Why it's wrong here
Multicast forwarding is a soft-state, per-packet operation; there is no connection setup in which the first packet establishes a path and later packets trust it. Every incoming multicast packet is independently subjected to the RPF check against the current unicast routing table, because the network can change at any time due to convergence or link failure. If only the first packet were checked, a subsequent packet arriving on a new, non-RPF interface after a topology change could be forwarded, re-creating a loop. Thus, the RPF check runs on every packet for continuous loop prevention.
- ✗
The RPF check can be overridden by configuring a static multicast route (mroute) that points to a different incoming interface.
Why it's wrong here
Incorrect because while static mroutes can influence the RPF check by altering the path back to the source, they do not override the check; the check still uses the mroute to determine the expected incoming interface. The statement is misleading because the RPF check is still performed, just with a different routing table entry.
- ✗
The RPF check is only relevant for PIM-SM and not for PIM-DM.
Why it's wrong here
RPF is a foundational mechanism for all Protocol Independent Multicast modes, not just PIM-SM. In PIM-DM (dense mode), routers initially flood multicast traffic out all interfaces except the one toward the source; the RPF check at each router ensures that a packet arriving on any other interface is a duplicate from a downstream neighbor and is dropped to prevent a broadcast storm. PIM-SM builds shared and source trees using the same RPF concept to reach the RP and the source. Without RPF, neither dense-mode flood-and-prune nor sparse-mode tree construction could prevent loops.
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Same concept, more angles
1 more way this is tested on 350-401
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Variation 1. Which three statements about RPF check in IP multicast are true? (Choose three.)
hard- ✓ A.The RPF check ensures that multicast packets are forwarded only if they arrive on the interface that the router would use to send unicast traffic back to the source.
- ✓ B.If the RPF check fails, the multicast packet is dropped by the router.
- C.The RPF check is performed only on the first packet of a multicast stream to determine the forwarding path.
- D.The RPF check relies solely on the multicast routing table (MRIB) to determine the incoming interface.
- ✓ E.A multicast packet can fail the RPF check even if the unicast route to the source exists, if the packet arrives on a different interface than the one used for unicast return traffic.
Why A: Option A is correct because the RPF check verifies that a multicast packet arrives on the interface the router would use to reach the source via the unicast routing table (or the MRIB derived from it), which is the fundamental definition of the RPF check in IP multicast. Option B is correct because when the incoming interface does not match the RPF interface, the router considers the packet to have arrived via a suboptimal or looped path and silently drops it. Option E is correct because the RPF check compares the actual arrival interface against the RPF interface; even if a valid unicast route to the source exists, a packet arriving on any other interface fails the check and is discarded. Option C is not correct because the RPF check is performed on every multicast packet, not just the first packet of a stream, to continuously guard against loops and suboptimal paths. Option D is not correct because the RPF check can use the unicast routing table (or the MRIB, which is populated from unicast routing information) and, in some implementations, static mroutes, so it does not rely solely on the multicast routing table.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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