OSPF LSA Types

🌐 OSPF LSA Types – The Complete Guide (OSPFv2 & OSPFv3)

For Interviewers

Learners

Network Engineers

Troubleshooters

🔍 What are LSAs in OSPF?

LSA (Link-State Advertisement) is the core building block of OSPF (Open Shortest Path First). Each router generates LSAs to describe its local view of the network, which is then stored in the Link-State Database (LSDB). Routers use these LSAs to build a full topology map and compute shortest paths using Dijkstra’s SPF algorithm.

📘 OSPFv2 LSA Types (IPv4)

OSPFv2 uses 6 primary LSA types to describe the network. Here’s a detailed technical breakdown.

LSA Type	Name	Role/Analogy	Generated By	Contains	Flooding Scope
Type 1	Router LSA	🧭 Local Scout	Every OSPF router	Interfaces, cost, link types	Within originating area
Type 2	Network LSA	🏢 DR’s Mandate	Designated Router (DR)	List of routers on a multi-access network	Within originating area
Type 3	Summary LSA	🌐 Area Translator	Area Border Router (ABR)	Routes to networks in other areas	Between areas (interarea)
Type 4	ASBR Summary	📍 ASBR Locator	ABR	Route to the ASBR (not external routes)	Throughout OSPF domain (except stub areas)
Type 5	AS External	📡 Outside Messenger	ASBR	External prefixes (from redistribution)	Entire OSPF domain (except stub areas)
Type 7	NSSA External	🛡️ Stub Exception	ASBR in NSSA	External prefixes (NSSA redistribution)	Only in NSSA; converted to Type 5 by ABR

🧠 Technical Notes:

• Type 1 & 2: Form the intra-area SPF tree.
• Type 3 & 4: Used for interarea routing.
• Type 5 & 7: Used for external routes (redistributed).

⚙️ Route Calculation & LSA Behavior

✅ Intra-area vs. Interarea Routes

• Intra-area (Type 1 & 2) always preferred over interarea (Type 3).
• SPF is computed per area for intra-area routes.

🌐 External Routes (Type 5 & 7)

• Two types:
  • E1/N1: Includes cost to ASBR + external metric.
  • E2/N2 (default): Only includes external (seed) metric; doesn’t increase with distance.

🧱 Stub Area Rules

Area Type	Blocks	Allows	Injected Default Route
Stub	Type 5, 4	Type 3	Yes (via Type 3)
Totally Stubby	Type 5, 4, 3	None	Yes (Type 3)
NSSA	Type 5	Type 7	Optional (ABR converts Type 7 to Type 5)
Totally NSSA	Type 5, 3	Type 7	Yes (Type 3)

🧬 OSPFv3 LSA Types (IPv6 Support)

OSPFv3 is modular and supports both IPv4 and IPv6, with prefixes separated from topology data.

LSA Type (Hex)	Name	Equivalent (OSPFv2)	Purpose	Flooding Scope
0x2001	Device-LSA (Router LSA)	Type 1	Interface details (no IP info)	Area
0x2003	Inter-Area Prefix LSA	Type 3	IPv6 prefixes from other areas	Area
0x0008	Link-LSA	N/A	Link-local and IPv6 info for local interface	Link-local
0x2009	Intra-Area Prefix LSA	N/A	IPv6 prefixes associated with a router/network	Area

💡 Design Insight:

• OSPFv3 separates IP prefixes from topology, optimizing SPF recalculation.
• SPF is based on Router (0x2001) & Network LSA equivalents only.

🔎 Interview Questions & Revision Highlights

1. What LSA types form the SPF tree?
   • Type 1 and 2 (OSPFv2), 0x2001 (OSPFv3)
2. Which LSA types are used by ABRs?
   • Type 3 and Type 4
3. What’s the difference between E1 and E2?
   • E1 adds internal cost to ASBR; E2 only uses external metric.
4. Can Type 5 LSAs exist in stub areas?
   • No. Stub areas block Type 5 (and Type 4).
5. How is Type 7 converted in NSSA?
   • ABR translates Type 7 into Type 5 before flooding outside the NSSA.
6. Why does OSPFv3 move prefix info out of Router LSAs?
   • To decouple topology from IP addresses and optimize SPF calculations.

🧭 Quick Comparison Table: OSPFv2 vs OSPFv3

Feature	OSPFv2	OSPFv3
IP Support	IPv4 only	IPv4 & IPv6
Prefix in LSA	Inside Router LSA	Separate LSAs
Authentication	In protocol	IPsec (external)
LSA Types	Type 1–5, 7	0x2001, 0x2003, 0x0008, 0x2009, etc.

✅ Summary: Why LSA Types Matter

Understanding OSPF LSA types is critical for: - Designing scalable multi-area networks. - Diagnosing routing loops or blackholes. - Mastering OSPF behavior for certification (CCNA/CCNP/CCIE). - Performing high-level interviews or deep troubleshooting.

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