- A
The distribute-list is applied outbound, but OSPF distribute-lists only filter routes in the routing table, not LSAs, so the routes are still advertised.
Why wrong: This is incorrect because although distribute-list outbound does filter routes from being installed in the routing table, it does not prevent LSA advertisement for intra-area and inter-area routes. However, on an ABR, if an 'area range' is configured, the summary LSA can be filtered by the distribute-list, which would prevent advertisement—contradicting the statement that routes are still advertised.
- B
The BFD session is flapping due to a mismatch in the 'bfd interval' command, causing OSPF to withdraw routes.
Why wrong: The BFD session is up per the scenario, so flapping is unlikely. BFD mismatch would cause the session to go down, not just withdraw routes while the session is up.
- C
The distribute-list references an ACL that denies the network, but OSPF uses type 3 LSAs that are not affected by ACLs.
Why wrong: OSPF distribute-lists can reference ACLs to filter specific routes, but ACLs do filter based on network addresses. However, the statement that type 3 LSAs are not affected by ACLs is false; ACLs can match the network in the summary LSA. But the key point is that the distribute-list outbound on an ABR can filter the summary LSA generated by 'area range'.
- D
The 'area range' command is configured on the ABR, summarizing the route, and the distribute-list is filtering the summary.
This is correct. When an ABR has an 'area range' command summarizing a network, it creates a type 3 LSA for that summary. If a distribute-list outbound is applied that denies that summary network, the ABR will not advertise the type 3 LSA to neighbors in other areas. This explains why routes are not being advertised despite the BFD session being up.
300-410 OSPF distribute-list outbound Practice Question
This 300-410 practice question tests your understanding of bidirectional forwarding detection (bfd). This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. A key principle to apply: oSPF distribute-list outbound. 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 configures BFD on an OSPF link and also applies a distribute-list outbound under the OSPF process. The BFD session is up, but OSPF routes are not being advertised to the neighbor. The engineer verifies that the distribute-list is correctly configured. 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 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
The 'area range' command is configured on the ABR, summarizing the route, and the distribute-list is filtering the summary.
OSPF distribute-list outbound only filters routes that are being redistributed into OSPF, not intra-area or inter-area LSAs. On an ABR, the 'area range' command generates a summary LSA (type 3), which can be filtered by a distribute-list outbound if it references that specific summary route. In this scenario, the distribute-list is correctly configured to deny the network, and the ABR summarizes that network via 'area range', causing the summary LSA to be filtered and not advertised to the neighbor. BFD does not affect this behavior.
Key principle: OSPF distribute-list outbound
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 distribute-list is applied outbound, but OSPF distribute-lists only filter routes in the routing table, not LSAs, so the routes are still advertised.
Why it's wrong here
This is incorrect because although distribute-list outbound does filter routes from being installed in the routing table, it does not prevent LSA advertisement for intra-area and inter-area routes. However, on an ABR, if an 'area range' is configured, the summary LSA can be filtered by the distribute-list, which would prevent advertisement—contradicting the statement that routes are still advertised.
- ✗
The BFD session is flapping due to a mismatch in the 'bfd interval' command, causing OSPF to withdraw routes.
Why it's wrong here
The BFD session is up per the scenario, so flapping is unlikely. BFD mismatch would cause the session to go down, not just withdraw routes while the session is up.
- ✗
The distribute-list references an ACL that denies the network, but OSPF uses type 3 LSAs that are not affected by ACLs.
Why it's wrong here
OSPF distribute-lists can reference ACLs to filter specific routes, but ACLs do filter based on network addresses. However, the statement that type 3 LSAs are not affected by ACLs is false; ACLs can match the network in the summary LSA. But the key point is that the distribute-list outbound on an ABR can filter the summary LSA generated by 'area range'.
- ✓
The 'area range' command is configured on the ABR, summarizing the route, and the distribute-list is filtering the summary.
Why this is correct
This is correct. When an ABR has an 'area range' command summarizing a network, it creates a type 3 LSA for that summary. If a distribute-list outbound is applied that denies that summary network, the ABR will not advertise the type 3 LSA to neighbors in other areas. This explains why routes are not being advertised despite the BFD session being up.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF distribute-list outbound
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap is that distribute-list outbound under OSPF does not filter intra-area or inter-area LSAs—only redistributed routes. However, on an ABR with 'area range', the summary LSA can be filtered.
Trap categories for this question
Scenario analysis trap
The BFD session is up per the scenario, so flapping is unlikely. BFD mismatch would cause the session to go down, not just withdraw routes while the session is up.
Detailed technical explanation
How to think about this question
Treat this as a scenario question. Identify the problem, the constraint, and the best action. Then compare each option against those facts.
KKey Concepts to Remember
- OSPF distribute-list outbound
- ABR area range
- Type 3 LSA filtering
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
OSPF distribute-list outbound
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.
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
What to study next
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FAQ
Questions learners often ask
What does this 300-410 question test?
Bidirectional Forwarding Detection (BFD) — This question tests Bidirectional Forwarding Detection (BFD) — OSPF distribute-list outbound.
What is the correct answer to this question?
The correct answer is: The 'area range' command is configured on the ABR, summarizing the route, and the distribute-list is filtering the summary. — OSPF distribute-list outbound only filters routes that are being redistributed into OSPF, not intra-area or inter-area LSAs. On an ABR, the 'area range' command generates a summary LSA (type 3), which can be filtered by a distribute-list outbound if it references that specific summary route. In this scenario, the distribute-list is correctly configured to deny the network, and the ABR summarizes that network via 'area range', causing the summary LSA to be filtered and not advertised to the neighbor. BFD does not affect this behavior.
What should I do if I get this 300-410 question wrong?
Review oSPF distribute-list outbound, then practise related 300-410 questions on the same topic to reinforce the concept.
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 distribute-list outbound
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Last reviewed: Jun 18, 2026
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