The most likely cause is that the unicast routing table does not have a route back to the source. This is because the Reverse Path Forwarding (RPF) neighbor for a multicast (S,G) entry is determined by consulting the unicast routing table for the best path toward the source IP address; if that table lacks a route to the source, or points to a different next-hop due to asymmetric routing or a misconfigured static route, the RPF check will fail and the multicast tree will not build correctly. On the Cisco SPCOR 350-501 exam, this concept tests your understanding of how multicast relies on unicast topology for loop prevention, often appearing in troubleshooting scenarios where `show ip mroute` reveals an unexpected RPF neighbor. A common trap is assuming the issue is with multicast protocols like PIM, when the root cause is actually a missing or incorrect unicast route. Memory tip: “RPF follows the unicast path back to the source—if the route is wrong, the tree is gone.”
350-501 Networking Practice Question
This 350-501 practice question tests your understanding of networking. 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.
During multicast troubleshooting, the command 'show ip mroute' shows that the RPF neighbor for a particular (S,G) entry is incorrect. What is the most likely cause?
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 unicast routing table does not have a route back to the source.
The RPF neighbor is determined by the unicast routing table towards the source. If the unicast route points to a different next-hop than the actual multicast path (e.g., due to asymmetric routing or incorrect unicast route), the RPF check fails and the multicast tree is not built correctly.
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 unicast routing table does not have a route back to the source.
Why this is correct
RPF check uses unicast route towards source; missing or incorrect route leads to wrong RPF neighbor.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The multicast routing protocol is not enabled globally.
Why it's wrong here
If not enabled, no multicast entries would exist.
✗
TTL threshold is set too high on the incoming interface.
Why it's wrong here
TTL threshold would drop packets, not affect RPF neighbor.
✗
IGMP snooping is disabled on the switch.
Why it's wrong here
IGMP snooping is layer 2; does not affect RPF on routers.
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.
Networking — This question tests Networking — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The unicast routing table does not have a route back to the source. — The RPF neighbor is determined by the unicast routing table towards the source. If the unicast route points to a different next-hop than the actual multicast path (e.g., due to asymmetric routing or incorrect unicast route), the RPF check fails and the multicast tree is not built correctly.
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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