The answer is to configure the ABR with an OSPF area filter-list to filter Type 3 LSAs. This feature works by applying a prefix-list to the ABR, which controls which inter-area summary LSAs are allowed into or out of area 0, directly reducing LSA flooding and shrinking the Link-State Database (LSDB) in the backbone. On the ENCOR 350-401 exam, this concept tests your understanding of OSPF route summarization and filtering without resorting to stub areas or redistribution, which are common distractors. A frequent trap is confusing the area filter-list with the distribute-list, but remember that filter-lists operate specifically on Type 3 LSAs at the area boundary, not on routing table entries. For a quick memory tip: think of the filter-list as a bouncer at the area 0 door, checking which inter-area prefixes get past the ABR.
CCNP OSPF Practice Question
This 350-401 practice question tests your understanding of ospf. 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.
A network engineer is configuring OSPF in a multi-area design. The engineer wants to reduce the amount of LSA flooding and the size of the LSDB in area 0. Which OSPF feature should be implemented on the ABR to achieve this goal?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure the ABR with an area filter-list to filter type 3 LSAs.
Option B is correct because configuring an area filter-list on the ABR allows the engineer to filter Type 3 summary LSAs entering or leaving area 0. This directly reduces LSA flooding and shrinks the LSDB in area 0 by preventing specific inter-area prefixes from being advertised into the backbone, without altering the area type or requiring additional redistribution.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Configure area 0 as a stub area.
Why it's wrong here
Incorrect because area 0 cannot be configured as a stub area; OSPF backbone area must be capable of carrying all LSA types.
✓
Configure the ABR with an area filter-list to filter type 3 LSAs.
Why this is correct
Correct because area filter-list can be used on an ABR to filter type 3 LSAs between areas, reducing LSDB size in area 0.
Related concept
Read the scenario before looking for a memorised answer.
✗
Configure OSPF database overflow protection.
Why it's wrong here
Incorrect because database overflow protection limits the number of LSAs but does not reduce flooding or LSDB size proactively.
✗
Configure the ABR as an ASBR.
Why it's wrong here
Incorrect because an ASBR redistributes external routes, which increases LSDB size, not reduces it.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume area 0 can be made a stub area to reduce LSAs, but Cisco tests the fact that area 0 is a transit area and cannot be a stub, making the filter-list the correct tool for this specific goal.
Detailed technical explanation
How to think about this question
The area filter-list uses a prefix list to filter Type 3 LSAs at the ABR, which is applied in either the 'in' or 'out' direction for a specific area. This feature is particularly useful in large multi-area designs where you want to summarize or restrict routes between areas without converting an area to a stub or NSSA, which would also block Type 5 LSAs and potentially break connectivity to external networks. Under the hood, the ABR simply does not originate the filtered Type 3 LSAs, so they never enter the backbone's LSDB.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
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
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
OSPF — This question tests OSPF — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Configure the ABR with an area filter-list to filter type 3 LSAs. — Option B is correct because configuring an area filter-list on the ABR allows the engineer to filter Type 3 summary LSAs entering or leaving area 0. This directly reduces LSA flooding and shrinks the LSDB in area 0 by preventing specific inter-area prefixes from being advertised into the backbone, without altering the area type or requiring additional redistribution.
What should I do if I get this 350-401 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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