The answer is that the RP is not configured on the last-hop router. In PIM sparse mode, when a last-hop router receives an IGMP join from a receiver, it must initiate a PIM join toward the RP to build the shared tree; if the router has no knowledge of the RP address or the group-to-RP mapping, it cannot send this join, leaving the outgoing interface list (OIL) empty despite the (S,G) entry being present. This scenario tests your understanding of the PIM sparse mode state machine and the critical role of RP configuration—a common trap on the ENCOR 350-401 exam is assuming reachability alone is sufficient, when in fact the router must explicitly know the RP to generate the join. Remember the mnemonic: “No RP, no join, no OIL.”
CCNP IP Multicast Practice Question
This 350-401 practice question tests your understanding of ip multicast. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 deploying IP multicast in an OSPF-based enterprise network. The network uses PIM sparse mode with a static RP. The engineer notices that multicast traffic from a source to a group is not reaching receivers in a remote subnet, even though the RP is reachable and the receivers have sent IGMP joins. The engineer checks the multicast routing table on the last-hop router and sees that the (S,G) entry is present, but the outgoing interface list (OIL) is empty. What is the most likely reason for the empty OIL?
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 RP is not configured on the last-hop router.
In PIM sparse mode, the last-hop router must send a PIM join toward the RP to join the shared tree. If the RP is reachable but the join is not being sent because the router does not know the RP or the group-to-RP mapping, the OIL remains empty.
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 RP is not configured on the last-hop router.
Why this is correct
Correct because without the RP configured, the router cannot send a PIM join to the RP, so the OIL remains empty.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The multicast source is not registered with the RP.
Why it's wrong here
Incorrect because the source registration is done by the first-hop router, not the last-hop router; the issue is on the receiver side.
✗
PIM dense mode is enabled on the last-hop router.
Why it's wrong here
Incorrect because dense mode would flood traffic, not cause an empty OIL.
✗
The TTL of the multicast packets is too low.
Why it's wrong here
Incorrect because TTL affects packet forwarding, not the OIL population; the OIL is built by PIM joins.
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 350-401 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 RP is not configured on the last-hop router. — In PIM sparse mode, the last-hop router must send a PIM join toward the RP to join the shared tree. If the RP is reachable but the join is not being sent because the router does not know the RP or the group-to-RP mapping, the OIL remains empty.
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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