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
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.

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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Last reviewed: Jun 24, 2026

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