- A
Strict uRPF drops packets if the source IP is not reachable via the receiving interface, which fails in asymmetric routing scenarios.
Strict mode requires the source IP to be in the routing table with the same interface as the incoming packet.
- B
The uRPF 'allow-default' option was not configured, so default routes are ignored.
Why wrong: Allow-default affects default routes, but the issue is asymmetric routing, not default routes.
- C
The neighbor's BGP updates have a source IP that is not in the routing table.
Why wrong: If the source IP is not in the routing table, it would be dropped, but the scenario describes asymmetric routing.
- D
Loose mode should be used instead, but strict mode was configured by mistake.
Why wrong: Loose mode would allow asymmetric routing, but the question asks for the explanation of the drop.
300-410 BGP Troubleshooting Practice Question
This 300-410 practice question tests your understanding of bgp troubleshooting. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
An engineer enables unicast RPF (uRPF) in strict mode on an interface. Afterward, some legitimate traffic from a BGP neighbor is dropped. The neighbor has two paths to the router, and traffic may arrive on a different interface than the return path. What is the most likely explanation?
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 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
Strict uRPF drops packets if the source IP is not reachable via the receiving interface, which fails in asymmetric routing scenarios.
Strict uRPF checks that the source IP of incoming packets matches the routing table entry for the interface it arrived on. If asymmetric routing occurs, packets from a valid neighbor may arrive on a different interface than the return path, causing them to be dropped.
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.
- ✓
Strict uRPF drops packets if the source IP is not reachable via the receiving interface, which fails in asymmetric routing scenarios.
Why this is correct
Strict mode requires the source IP to be in the routing table with the same interface as the incoming packet.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
The uRPF 'allow-default' option was not configured, so default routes are ignored.
Why it's wrong here
Allow-default affects default routes, but the issue is asymmetric routing, not default routes.
- ✗
The neighbor's BGP updates have a source IP that is not in the routing table.
Why it's wrong here
If the source IP is not in the routing table, it would be dropped, but the scenario describes asymmetric routing.
- ✗
Loose mode should be used instead, but strict mode was configured by mistake.
Why it's wrong here
Loose mode would allow asymmetric routing, but the question asks for the explanation of the drop.
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
If the source IP is not in the routing table, it would be dropped, but the scenario describes asymmetric routing.
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 300-410 OSPF questions on adjacency and route selection.
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FAQ
Questions learners often ask
What does this 300-410 question test?
BGP Troubleshooting — This question tests BGP Troubleshooting — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: Strict uRPF drops packets if the source IP is not reachable via the receiving interface, which fails in asymmetric routing scenarios. — Strict uRPF checks that the source IP of incoming packets matches the routing table entry for the interface it arrived on. If asymmetric routing occurs, packets from a valid neighbor may arrive on a different interface than the return path, causing them to be dropped.
What should I do if I get this 300-410 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 300-410 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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Last reviewed: Jun 18, 2026
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