OSPF LSA Types Explained (Type 1–7)

Type 1: Router LSA

What it describes: A router's directly connected links
Generated by: Every OSPF router
Flooded: Within the local area only
Key fields: Router ID, link types, costs

Example:

R1# show ip ospf database router 1.1.1.1

  LS Type: Router Links
  Link State ID: 1.1.1.1
  Advertising Router: 1.1.1.1
  Number of Links: 3

    Link connected to: a Stub Network
     (Link ID) Network/subnet number: 1.1.1.1
     (Link Data) Network Mask: 255.255.255.255
      Number of TOS metrics: 0
       TOS 0 Metrics: 1

    Link connected to: another Router (point-to-point)
     (Link ID) Neighboring Router ID: 2.2.2.2
     (Link Data) Router Interface address: 10.0.12.1
      Number of TOS metrics: 0
       TOS 0 Metrics: 1

What this tells us:

• R1 has a loopback (stub network)
• R1 has a P2P link to R2 with cost 1

Type 2: Network LSA

What it describes: Multi-access network (Ethernet) and which routers are attached
Generated by: Designated Router (DR) only
Flooded: Within the local area only
Key fields: DR's IP, list of attached routers

Example:

R1# show ip ospf database network 192.168.1.1

  LS Type: Network Links
  Link State ID: 192.168.1.1 (address of Designated Router)
  Advertising Router: 2.2.2.2
  Network Mask: /24
        Attached Router: 1.1.1.1
        Attached Router: 2.2.2.2
        Attached Router: 3.3.3.3

What this tells us:

• DR is 2.2.2.2
• Three routers on this segment

Note: Point-to-point links don't generate Type 2 LSAs (no DR).

Type 3: Summary LSA

What it describes: Routes to networks in other areas
Generated by: Area Border Routers (ABRs)
Flooded: Into adjacent areas
Key fields: Network address, mask, cost

Purpose: Summarize inter-area routes

Example:

R4# show ip ospf database summary 192.168.200.0

  LS Type: Summary Links (Network)
  Link State ID: 192.168.200.0 (summary Network Number)
  Advertising Router: 1.1.1.1 (ABR)
  Network Mask: /24
        TOS: 0  Metric: 3

What this tells us:

• ABR 1.1.1.1 is advertising 192.168.200.0/24 from another area
• Cost is 3

Routing table entry:

O IA 192.168.200.0/24 [110/3] via 172.16.14.1

O IA = Inter-Area (came from Type 3 LSA)

Type 4: ASBR Summary LSA

What it describes: How to reach an ASBR (router redistributing external routes)
Generated by: ABRs
Flooded: Into adjacent areas
Key fields: ASBR's Router ID, cost to reach it

Purpose: Tell routers in other areas where the ASBR is

Example:

R4# show ip ospf database asbr-summary 3.3.3.3

  LS Type: Summary Links (AS Boundary Router)
  Link State ID: 3.3.3.3 (AS Boundary Router address)
  Advertising Router: 1.1.1.1 (ABR)
  Network Mask: /0
        TOS: 0  Metric: 2

What this tells us:

• ASBR is 3.3.3.3
• Cost to reach it from this area is 2
• ABR 1.1.1.1 is advertising this

Why it matters: Routers need to know how to reach the ASBR to get external routes.

Type 5: External LSA

What it describes: Routes from outside OSPF (redistributed routes)
Generated by: ASBRs
Flooded: Throughout the entire OSPF domain (except stub areas)
Key fields: External network, metric type (E1/E2)

Example:

R1# show ip ospf database external 0.0.0.0

  LS Type: AS External Link
  Link State ID: 0.0.0.0 (External Network Number)
  Advertising Router: 3.3.3.3 (ASBR)
  Network Mask: /0
        Metric Type: 2 (Larger than any link state path)
        TOS: 0
        Metric: 1
        Forward Address: 0.0.0.0
        External Route Tag: 0

What this tells us:

• ASBR 3.3.3.3 is advertising default route (0.0.0.0/0)
• Metric type E2 (cost doesn't increase)
• Metric is 1

Routing table entry:

O*E2 0.0.0.0/0 [110/1] via 10.0.23.3

O*E2 = External Type 2

Type 7: NSSA External LSA

What it describes: External routes in an NSSA (Not-So-Stubby Area)
Generated by: ASBRs in NSSAs
Flooded: Within the NSSA only
Converted to: Type 5 LSA at the ABR (when leaving NSSA)

Purpose: Allow limited external route injection into stub-like areas

Example:

R4# show ip ospf database nssa-external 172.16.50.0

  LS Type: Type-7 AS External Link
  Link State ID: 172.16.50.0 (External Network Number)
  Advertising Router: 4.4.4.4
  Network Mask: /24
        Metric Type: 2
        TOS: 0
        Metric: 20
        Forward Address: 0.0.0.0
        External Route Tag: 0

What happens:

1. ASBR in NSSA generates Type 7
2. Type 7 floods within NSSA
3. ABR converts Type 7 to Type 5
4. Type 5 floods to rest of OSPF domain

LSA Type Summary Table

*Except stub areas

LSA Flow Example

Multi-Area Network with External Routes

Area 10 (NSSA)    Area 0        Area 20
    [R4]------[R1]----[R2]----[R3]------Internet
   ASBR        ABR           ABR        (ASBR)

LSA flow:

Within Area 10:

• Type 1: All routers advertise their links
• Type 7: R4 injects static routes

R1 (ABR) generates:

• Type 3: Summarizes Area 10 into Area 0
• Type 5: Converts R4's Type 7 to Type 5 for Area 0

Within Area 0:

• Type 1: R1, R2, R3 advertise links
• Type 3: From Area 10 and Area 20
• Type 5: External routes

R3 (ASBR) generates:

• Type 5: Default route from Internet

R2 (ABR) generates:

• Type 3: Summarizes Area 0 into Area 20
• Type 4: Tells Area 20 how to reach R3 (ASBR)

Viewing LSAs

Show All LSAs

R1# show ip ospf database

Show Specific LSA Types

R1# show ip ospf database router
R1# show ip ospf database network
R1# show ip ospf database summary
R1# show ip ospf database asbr-summary
R1# show ip ospf database external
R1# show ip ospf database nssa-external

Show LSA for Specific Network

R1# show ip ospf database summary 192.168.1.0

LSAs and Area Types

Standard Area

• Allows: Type 1, 2, 3, 4, 5

Stub Area

• Allows: Type 1, 2, 3
• Blocks: Type 4, 5
• ABR injects: Default route as Type 3

Totally Stubby Area

• Allows: Type 1, 2
• Blocks: Type 3 (except default), Type 4, 5

NSSA

• Allows: Type 1, 2, 3, 7
• Blocks: Type 5
• ABR converts: Type 7 → Type 5

Totally NSSA

• Allows: Type 1, 2, 7
• Blocks: Type 3 (except default), Type 5

Summary

Now you know:

✅ Type 1 (Router LSA) — Every router's links
✅ Type 2 (Network LSA) — DR advertises multi-access segment
✅ Type 3 (Summary LSA) — ABR advertises inter-area routes
✅ Type 4 (ASBR Summary) — ABR advertises path to ASBR
✅ Type 5 (External LSA) — ASBR advertises external routes
✅ Type 7 (NSSA External) — External routes within NSSA
✅ LSA flooding scope — Area vs domain-wide

Next Step:
LSAs populate the LSDB. How does SPF use the LSDB? Read How OSPF SPF Algorithm Works next.

Screenshot Suggestions:

1. show ip ospf database overview showing all LSA types
2. show ip ospf database router detailed output
3. LSA flow diagram in multi-area network
4. Table comparing LSA types and flooding scope

Internal Links:

• ← OSPF Stub Areas (Article 14)
• ← OSPF Areas Explained (Article 4)
• → How OSPF SPF Algorithm Works (Article 27)
• → OSPF Redistribution (Article 29)