mediumMultiple Choice
300-410 Practice Question: Which LSA type is used by OSPF to advertise…
Which LSA type is used by OSPF to advertise prefixes from other routing protocols (redistribution) and has a default metric of 20?
⚠ Common exam trap
300-410 often tests LSA types and their specific roles; candidates confuse Type 4 (ASBR location) with Type 5 (external prefixes) or assume Type 3 handles redistribution, but only Type 5 (and Type 7 in NSSA) carries 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 LSA (AS-external LSA)
Type 5 LSAs (AS-external LSAs) are generated by an ASBR to advertise routes redistributed from other routing protocols or static routes into OSPF. They are flooded throughout the OSPF domain (except stub areas) and carry an external metric with a default value of 20 for redistributed routes.
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 LSA (Router LSA)
Why it's wrong here
Type 1 LSAs describe a router's own links and interfaces within its area, carrying no external prefix information, so they cannot represent redistributed routes. They are the correct choice when documenting intra-area topology, not for flooding external destinations with a default metric of 20.
- ✗
Type 3 LSA (Summary LSA)
Why it's wrong here
Type 3 LSAs advertise inter-area prefixes generated by ABRs, not redistributed external routes, and carry no default metric of 20. They would be correct when summarising internal networks between OSPF areas, but redistribution into OSPF requires a different LSA type.
- ✗
Type 4 LSA (ASBR Summary LSA)
Why it's wrong here
Type 4 LSAs identify the ASBR's router ID to other areas; they carry no prefix or metric information at all. They are correct when telling remote areas how to reach an ASBR, but they cannot advertise redistributed prefixes with a default metric of 20.
- ✓
Type 5 LSA (AS-external LSA)
Why this is correct
Type 5 AS-external LSAs carry redistributed prefixes from other routing protocols throughout the OSPF domain. Their default metric of 20 applies to E2 routes unless the metric is changed, and they are flooded across area boundaries by ABRs.
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 and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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