The answer is the OSPF Type 5 LSA, also known as the AS-external LSA. This LSA type is correct because it is originated exclusively by an Autonomous System Boundary Router (ASBR) to advertise external routes that have been redistributed into OSPF from another routing protocol or domain. Unlike Type 3 or Type 4 LSAs, Type 5 LSAs have an AS-wide flooding scope, meaning they are flooded throughout the entire OSPF domain, including all areas, without being blocked by any area border router. On the Cisco CCNP ENARSI 300-410 exam, this concept tests your understanding of OSPF route redistribution and LSA propagation; a common trap is confusing Type 5 with Type 4 LSAs, but remember that Type 4 only advertises the location of the ASBR, not the external routes themselves. For a quick memory tip, think “5 for External” — the number five rhymes with “arrive,” as external routes arrive from outside the OSPF domain and are flooded everywhere.
300-410 Device Access Control Practice Question
This 300-410 practice question tests your understanding of device access control. 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.
Which OSPF LSA type is used to advertise external routes and is flooded throughout the entire OSPF domain?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Type 5 (AS-external LSA)
Type 5 (AS-external LSA) is correct because it is originated by an ASBR to advertise external routes redistributed into OSPF from another routing domain. These LSAs are flooded throughout the entire OSPF domain, including all areas, and their flooding scope is AS-wide, as defined in RFC 2328.
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.
✗
Type 1 (Router LSA)
Why it's wrong here
Router LSAs describe the state of a router's interfaces and are flooded only within the area.
✗
Type 3 (Summary LSA)
Why it's wrong here
Summary LSAs are generated by ABRs to advertise inter-area routes and are flooded between areas, not throughout the entire domain.
✗
Type 4 (ASBR Summary LSA)
Why it's wrong here
Type 4 LSAs advertise the location of an ASBR and are flooded within an area, not the entire domain.
✓
Type 5 (AS-external LSA)
Why this is correct
Correct. Type 5 LSAs are flooded throughout the entire OSPF domain (except stub areas) and advertise external routes.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between Type 3 and Type 5 LSAs, where candidates mistakenly think Type 3 LSAs carry external routes because they are also 'summary' LSAs, but Type 3 LSAs only carry inter-area routes, not external routes.
Detailed technical explanation
How to think about this question
Type 5 LSAs have a flooding scope of AS-wide, meaning they are flooded unchanged into all areas except stub areas and NSSAs, where they are blocked by default. The forwarding address field in a Type 5 LSA can be set to a non-zero value to redirect traffic to a different next-hop, which is a subtle behavior that can affect routing decisions in multi-homed ASBR scenarios. In real-world deployments, Type 5 LSAs are critical for redistributing BGP or static routes into OSPF, and their handling in stub areas often requires careful planning to avoid routing black holes.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Device Access Control — This question tests Device Access Control — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Type 5 (AS-external LSA) — Type 5 (AS-external LSA) is correct because it is originated by an ASBR to advertise external routes redistributed into OSPF from another routing domain. These LSAs are flooded throughout the entire OSPF domain, including all areas, and their flooding scope is AS-wide, as defined in RFC 2328.
What should I do if I get this 300-410 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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