OSPF LSA Types Explained: Complete Guide with CLI, EVE-NG Lab & Real-World Use Cases [CCNP ENTERPRISE]

Today we’re going to tackle one of the most crucial yet often misunderstood parts of OSPF—LSA Types. When I train CCNA and CCNP candidates, many get confused with when and where to use Type 1 vs Type 3 or what the heck Type 7 LSAs really do in a stub area.

So, I created this blog to simplify things. Whether you’re prepping for certifications or dealing with real production networks, this post will give you clarity on OSPF LSAs, complete with CLI commands, a real-world lab, and troubleshooting tips. Let’s go!

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Theory in Brief – What are OSPF LSAs?

OSPF (Open Shortest Path First) is a link-state routing protocol that uses Link State Advertisements (LSAs) to exchange routing and topology information.

LSAs are building blocks of OSPF—they tell routers about:

• Links and neighbors
• External routes
• Network topology

Each LSA Type serves a unique purpose and is confined to specific areas based on OSPF design.

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Key LSA Types (0–7)

1. Type 1 (Router LSA) – Advertises a router’s interfaces and cost, stays within the area.
2. Type 2 (Network LSA) – Generated by the DR, represents a multi-access segment like Ethernet.
3. Type 3 (Summary LSA) – Sent by ABRs to advertise routes between areas.
4. Type 4 (ASBR Summary LSA) – Tells other areas about the ASBR’s location.
5. Type 5 (External LSA) – Advertises routes redistributed from other protocols like BGP or EIGRP.
6. Type 6 – Multicast (Not commonly used)
7. Type 7 (NSSA External LSA) – Used in NSSA areas instead of Type 5s.

Types 1–2 are intra-area, 3–4 are inter-area, and 5/7 are external routes.

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Summary – OSPF LSA Types, Scope & Use Cases

LSA Type	Name	Generated By	Scope	Purpose
1	Router LSA	All routers	Area-local	Describes router interfaces and costs
2	Network LSA	DR only	Area-local	Lists routers in a broadcast segment
3	Summary LSA	ABR	Area to area	Advertises routes between areas
4	ASBR Summary LSA	ABR	Area to area	Points to ASBR location
5	External LSA	ASBR	Entire OSPF domain	Redistributed routes from outside OSPF
6	Multicast LSA	(Not used)	N/A	Specialized – not widely implemented
7	NSSA External LSA	ASBR in NSSA	NSSA area only	External routes in NSSA instead of Type 5

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Essential CLI Commands (Show, Debug, Verify)

Task	Command	Description
View OSPF LSA database	show ip ospf database	Shows all LSA types in the database
View Type 1 LSAs	show ip ospf database router	Lists router LSAs
View Type 2 LSAs	show ip ospf database network	Lists network LSAs
View inter-area summary LSAs	show ip ospf database summary	Lists Type 3/4 LSAs
View external routes (Type 5)	show ip ospf database external	Lists redistributed routes
View Type 7 LSAs in NSSA	show ip ospf database nssa-external	NSSA-specific LSAs
Debug LSA updates	debug ip ospf lsa-generation	Watch LSA generation

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Real-World Use Cases

Scenario	LSA Types Involved	Reason
Basic OSPF single area	Type 1, 2	Internal routers and multi-access segments
ABR between Area 0 and Area 1	Type 1, 2, 3	Routes summarized from one area to another
Redistributing BGP into OSPF	Type 5, Type 4	Injecting external routes via ASBR
NSSA with external routes	Type 7 (converted to 5)	Type 7 LSAs used to avoid full Type 5 flooding
DR/BDR election and segment updates	Type 2	Generated by DR to list segment participants

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EVE-NG Lab – OSPF LSA Types Demonstration

Lab Objective:

• Configure OSPF with multiple areas and observe LSA types exchanged between routers.
• Simulate NSSA and external route redistribution.

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Topology Diagram

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Configuration Snippets

R1 (Backbone Router):

router ospf 1
 network 10.0.0.0 0.0.0.255 area 0

R2 (ABR):

router ospf 1
 network 10.0.0.0 0.0.0.255 area 0
 network 10.1.0.0 0.0.0.255 area 1
 network 10.2.0.0 0.0.0.255 area 2

R3 (Area 1):

router ospf 1
 network 10.1.0.0 0.0.0.255 area 1

R4 (NSSA + Redistribution):

router ospf 1
 network 10.2.0.0 0.0.0.255 area 2 nssa
 redistribute static subnets

After configuration, use show ip ospf database to observe Type 1, 3, 5, and 7 LSAs.

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Troubleshooting Tips

Problem	Likely Cause	Fix
Missing external route	ASBR not generating Type 5/7	Check redistribute and route map configs
Type 3 LSA not received	ABR not configured correctly	Verify ABR interface area membership
Incomplete topology	Router not forming neighbor	Check OSPF network and hello timers
No Type 7 in NSSA	Area not defined as NSSA	Use area 2 nssa on both ends
LSAs stuck in database	Database not aging	Use clear ip ospf process to refresh

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FAQ – OSPF LSA Types

1. What is an LSA in OSPF and why is it important?

Answer:
LSA stands for Link-State Advertisement. It’s the core data structure used in OSPF to share routing and topology information between routers. LSAs are flooded within areas, forming the Link-State Database (LSDB). Routers then run the Shortest Path First (SPF) algorithm on this LSDB to calculate optimal paths. Without LSAs, OSPF cannot function.

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2. How many LSA types exist in OSPF, and are all of them used commonly?

Answer:
There are 11 types of LSAs in OSPFv2. However, in most enterprise networks, you’ll frequently encounter:

• Type 1 – Router LSA
• Type 2 – Network LSA
• Type 3 – Summary LSA
• Type 4 – ASBR Summary LSA
• Type 5 – External LSA
• Type 7 – NSSA External LSA

Types 6, 8, 9, 10, and 11 are rarely used and mostly appear in special scenarios like multicast OSPF or Opaque LSAs for TE extensions.

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3. What is a Type 1 LSA (Router LSA)?

Answer:
A Type 1 LSA is generated by every OSPF router for each area it belongs to. It contains:

• The router’s connected interfaces.
• IP addresses.
• Link types and cost.

These LSAs are flooded within the area and used by other routers to build a map of that area’s topology.

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4. What is a Type 2 LSA (Network LSA)?

Answer:
A Type 2 LSA is generated by the Designated Router (DR) on broadcast and non-broadcast multi-access (NBMA) networks. It:

• Represents the multi-access network as a pseudo-node.
• Lists all routers connected to that network.

It simplifies SPF calculations by reducing the number of neighbor relationships each router has to manage.

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5. What is a Type 3 LSA (Summary LSA), and how does it work across areas?

Answer:
A Type 3 LSA is generated by ABRs to advertise inter-area routes. It:

• Summarizes routes from one area and injects them into another.
• Does not carry topology details — only reachability info.

This helps OSPF scale by minimizing LSDB sizes in large, multi-area networks.

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6. What is a Type 4 LSA (ASBR Summary LSA)?

Answer:
Type 4 LSAs are also generated by ABRs, but they advertise the reachability of an ASBR. This is essential because:

• When an ASBR injects external routes (via Type 5 or 7 LSAs), routers in other areas need to know how to reach the ASBR.
• Type 4 LSAs provide this path information.

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7. What is a Type 5 LSA (External LSA)?

Answer:
A Type 5 LSA is generated by an ASBR to advertise external routes from other protocols (like BGP, static, EIGRP, etc.). It:

• Is flooded throughout the OSPF domain (except stub areas).
• Carries two types of external metrics: E1 (cumulative cost) and E2 (default, external cost only).

Use Type 5 LSAs when you need OSPF to carry non-OSPF routes.

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8. What is a Type 7 LSA and where is it used?

Answer:
A Type 7 LSA is generated by an ASBR inside an NSSA area (Not-So-Stubby Area). Since Type 5 LSAs aren’t allowed in NSSA, Type 7 is used instead and:

• Is converted to Type 5 by the ABR when leaving the NSSA.
• Allows route redistribution while preserving the stub nature of the area.

Type 7 LSAs provide flexibility in hybrid designs.

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9. How can I view LSA information using CLI on Cisco routers?

Answer:
Use the following commands:

• View all LSAs: bashCopyEditshow ip ospf database
• View specific types: bashCopyEditshow ip ospf database router show ip ospf database network show ip ospf database summary show ip ospf database external

These commands are crucial for troubleshooting and understanding OSPF behavior in labs or production.

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10. Can you summarize LSA types, their sources, and scopes in a table?

Answer:

LSA Type	Name	Generated By	Scope	Purpose
Type 1	Router LSA	All OSPF Routers	Within the same area	Advertises router links
Type 2	Network LSA	DR	Within the same area	Advertises multi-access networks
Type 3	Summary LSA	ABR	Inter-area	Advertises inter-area networks
Type 4	ASBR Summary LSA	ABR	Inter-area	Advertises ASBR location
Type 5	External LSA	ASBR	Throughout the OSPF domain	Advertises external routes
Type 7	NSSA External LSA	ASBR in NSSA	NSSA area only	External routes in NSSA (converted)

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YouTube Link

Watch the Complete CCNP Enterprise: OSPF LSA Types Explained: Complete Guide with CLI, EVE-NG Lab Demo & Explanation on our channel:

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Final Note

Understanding how to differentiate and implement OSPF LSA Types is critical for anyone pursuing CCNP Enterprise (ENCOR) certification or working in enterprise network roles. Use this guide in your practice labs, real-world projects, and interviews to show a solid grasp of architectural planning and CLI-level configuration skills.

If you found this article helpful and want to take your skills to the next level, I invite you to join my Instructor-Led Weekend Batch for:

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