The answer is that the multicast stream is arriving on an interface that is not the RPF interface for the source. This is the most likely cause because Protocol Independent Multicast (PIM) enforces a Reverse Path Forwarding (RPF) check on every incoming multicast packet: the router verifies that the packet arrives on the same interface that the unicast routing table uses to reach the source IP. When the unicast route points to a different interface, the RPF check fails, and the packet is silently dropped—even though the (S,G) entry and Designated Router are correct. On the ENCOR 350-401 exam, this scenario tests your understanding of multicast forwarding fundamentals versus unicast routing, and a common trap is assuming a correct (S,G) entry guarantees delivery. Remember the core rule: multicast follows the unicast route back to the source, not the path the data arrives on. A helpful memory tip is "RPF = Reverse Path First"—if the incoming interface doesn't match the unicast reverse path, the packet is dropped.
CCNP IP Multicast Practice Question
This 350-401 practice question tests your understanding of ip multicast. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A network engineer is troubleshooting multicast video distribution across an enterprise campus. The multicast source is connected to a switch that is the PIM Designated Router (DR) on a multi-access segment. Receivers in a different VLAN report that they are not receiving the multicast stream, although the DR shows the correct (S,G) entry. The engineer checks the RPF neighbor for the source and notices that the unicast route to the source points to a different interface than the one where the multicast stream is received. What is the most likely cause of the issue?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The multicast stream is arriving on an interface that is not the RPF interface for the source.
Multicast forwarding requires RPF check: the incoming interface for multicast traffic must match the unicast reverse path to the source. If the unicast route points to a different interface, the RPF check fails and the multicast packet is dropped, even if the DR is correct.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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 DR is not configured as the RP (Rendezvous Point).
Why it's wrong here
Incorrect because the DR and RP are separate roles; the issue is RPF failure, not RP configuration.
✓
The multicast stream is arriving on an interface that is not the RPF interface for the source.
Why this is correct
Correct because multicast forwarding requires the incoming interface to match the unicast RPF interface; a mismatch causes the packet to be dropped.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The switchport connected to the source is not configured as a trunk.
Why it's wrong here
Incorrect because the scenario mentions a multi-access segment; trunking is not directly related to RPF failure.
✗
IGMP snooping is disabled on the receiver VLAN.
Why it's wrong here
Incorrect because IGMP snooping affects group membership, not the RPF check for the source.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Trap categories for this question
Scenario analysis trap
Incorrect because the scenario mentions a multi-access segment; trunking is not directly related to RPF failure.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.
TExam Day Tips
→Check area mismatch first when OSPF adjacency fails.
→Review passive interfaces when a network is advertised but no neighbour forms.
→Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
Got this wrong? Here's your next step.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-401 OSPF questions on adjacency and route selection.
IP Multicast — This question tests IP Multicast — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The multicast stream is arriving on an interface that is not the RPF interface for the source. — Multicast forwarding requires RPF check: the incoming interface for multicast traffic must match the unicast reverse path to the source. If the unicast route points to a different interface, the RPF check fails and the multicast packet is dropped, even if the DR is correct.
What should I do if I get this 350-401 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-401 OSPF questions on adjacency and route selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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