mediumMultiple Choice
300-410 Practice Question: Which OSPF LSA type is used to advertise external…
Which OSPF LSA type is used to advertise external routes and is flooded throughout the entire OSPF domain?
⚠ Common exam trap
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.
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
✓
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.
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
Type 1 Router LSAs advertise router links and stub networks within a single area, so they never carry external prefixes. It is tempting because every OSPF router originates them, but external routes require Type 5 AS External LSAs flooded domain-wide by the ASBR.
- ✗
Type 3 (Summary LSA)
Why it's wrong here
Type 3 Summary LSAs advertise inter-area routes between areas via ABRs, carrying internal prefixes rather than external ones. It is tempting because they flood widely, but redistribution from other protocols is advertised by Type 5 AS External LSAs, not summaries.
- ✗
Type 4 (ASBR Summary LSA)
Why it's wrong here
Type 4 ASBR Summary LSAs advertise the location of an ASBR to other areas; they identify the router, not the external prefixes themselves. It is tempting because ASBRs relate to external routing, but the external routes are carried in Type 5 LSAs.
- ✓
Type 5 (AS-external LSA)
Why this is correct
Type 5 AS-external LSAs advertise routes redistributed into OSPF from outside the domain, such as from BGP or static routes. They are flooded throughout the entire OSPF domain, except into stub areas, satisfying the stem's requirement for domain-wide external route advertisement.
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.