The answer is that the PE does not have the multicast VRF enabled. In an MVPN environment, the MVRF (Multicast VRF) is the critical construct that allows the PE to recognize and forward multicast traffic within a specific VRF context; without it, the PE will silently drop any incoming multicast packets from the CE, even if PIM neighbors are up and the RP is reachable. This question tests your understanding of the MVPN control plane on the Cisco SPCOR 350-501 exam, where a common trap is assuming that PIM adjacency alone guarantees multicast forwarding—it does not, because the MVRF must be explicitly configured to bind the multicast routing table to the VRF. A helpful memory tip is: "No MVRF, no multicast VRF—traffic gets the curb."
350-501 Architecture Practice Question
This 350-501 practice question tests your understanding of architecture. 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.
In a multicast environment, a PE router is not receiving multicast traffic from a source behind a CE. The PIM neighbors are established. The RP is reachable. What is the most likely 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 PE does not have the multicast VRF enabled
For MVPN, the PE must have multicast VRF (MVRF) enabled to forward multicast traffic. Without it, traffic is not accepted. Option A: IGMP joins are sent by hosts, not CE; Option C: source address in VRF is handled by MVRF; Option D: RP is configured per VRF, but if reachable, it's likely configured. So B 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 CE is not sending IGMP joins
Why it's wrong here
IGMP joins are sent by hosts, not the CE router.
✗
The source address is not in the VRF
Why it's wrong here
The source address is behind the CE, so it is in the VRF. This is not a typical issue.
✓
The PE does not have the multicast VRF enabled
Why this is correct
Without multicast VRF, the PE cannot forward multicast traffic in the VRF.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The stem states the RP is reachable, so it is likely configured.
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.
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 350-501 OSPF questions on adjacency and route selection.
Architecture — This question tests Architecture — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The PE does not have the multicast VRF enabled — For MVPN, the PE must have multicast VRF (MVRF) enabled to forward multicast traffic. Without it, traffic is not accepted. Option A: IGMP joins are sent by hosts, not CE; Option C: source address in VRF is handled by MVRF; Option D: RP is configured per VRF, but if reachable, it's likely configured. So B is correct.
What should I do if I get this 350-501 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 350-501 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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