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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?
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 AS-external LSAs are used to advertise external routes redistributed into OSPF. The default metric for these LSAs is 20, as defined by Cisco IOS.
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
Incorrect. Router LSAs describe the router's interfaces and cost within an area.
- ✗
Type 3 LSA (Summary LSA)
Why it's wrong here
Incorrect. Summary LSAs advertise inter-area routes, not external routes.
- ✗
Type 4 LSA (ASBR Summary LSA)
Why it's wrong here
Incorrect. Type 4 LSAs advertise the location of an ASBR, not external prefixes.
- ✓
Type 5 LSA (AS-external LSA)
Why this is correct
Correct. Type 5 LSAs carry external routes with a default metric of 20.
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
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