LSA Types and LSDB

LSA Types and LSDB

Quick Answer: OSPF’s LSDB holds LSAs. For HCIA, master Types 1–5: 1 Router (every router, per area), 2 Network (DR, multi-access), 3 Summary (ABR, inter-area prefixes), 4 ASBR-Summary (ABR, how to reach an ASBR), 5 External (ASBR, redistributed routes). Type 1/2 stay inside an area; Type 3/4/5 implement hierarchy and external injection.

Neighbor state Full means synchronized databases with a peer. This section answers: synchronized what? Each LSA type has a different originator, flooding scope, and exam meaning. If you mix Type 3 with Type 5, multi-area and redistribution questions collapse.

LSDB fundamentals

• LSA (Link-State Advertisement): a record describing topology or reachability.
• LSDB (Link-State Database): the set of LSAs a router holds for OSPF.
• Area-scoped vs AS-scoped: Type 1 and Type 2 are flooded throughout a single area only. ABRs do not forward raw Type 1/2 into other areas; they translate reachability into Type 3 (and related) information. Type 5 externals flood across the AS except where special area types block them.
• SPF input: Within an area, Type 1 and Type 2 LSAs build the graph for Dijkstra. Inter-area and external info attach as leaves/summaries rather than full remote graphs.

When display ospf lsdb shows different Type 1 sets on two routers that should be in the same area, something is wrong (area split, adjacency failure, filtering, or corruption).

LSA Types 1–5 (HCIA core table)

Some materials mention Type 7 (NSSA external) and opaque LSAs; HCIA priority remains 1–5 clarity.

Type 1 — Router LSA

Each router originates a Type 1 LSA per area in which it has interfaces. Contents conceptually include:

• Links to transit networks, point-to-point neighbors, stub networks
• Metrics (costs) on those links
• Bits indicating ABR/ASBR roles when applicable

Type 1 answers: “What does this router attach to inside the area, and at what cost?” SPF stitches Type 1 data (with Type 2) into a shortest-path tree. Intra-area routes come from this graph.

Type 2 — Network LSA

On multi-access networks, listing every pairwise link would be messy. The DR originates a Type 2 LSA representing the network pseudo-node: which routers attach to this segment. Cost from router to network appears in Type 1; the network LSA lists members. On pure point-to-point links without DR, Type 2 may not appear the same way—another reason network type matters.

Exam tip: If a question asks who generates Type 2, answer DR, not “every router” and not “ABR.”

Type 3 — Summary LSA (inter-area prefixes)

ABRs do not flood all remote Type 1/2 LSAs into Area 0 or leaf areas. Instead, an ABR originates Type 3 LSAs that say, effectively: “Prefix X/Y is reachable via me (the ABR) at cost Z,” derived from the ABR’s knowledge of other areas.

Implications:

• Remote areas do not see full internal topology of each other—only prefixes (unless other mechanisms apply).
• Summarization at ABRs (when configured) reduces Type 3 count further.
• Type 3 is still OSPF-internal reachability (inter-area), not redistribution from BGP/static.

Type 3 vs Type 5 trap

Never describe a redistributed firewall static as “a Type 3 from the ABR” unless the ABR is merely summarizing an OSPF inter-area prefix. Redistribution → Type 5 (standard areas).

Type 4 — ASBR-Summary

Routers in a remote area may need a Type 5 external prefix, but Type 5 is advertised by an ASBR that might live in another area. Those routers also need to know how to reach the ASBR’s Router ID. ABRs generate Type 4 LSAs to announce a route to the ASBR into other areas.

Mental model:

1. ASBR injects external prefix → Type 5.
2. ABR advertises “ASBR is reachable here” → Type 4 into other areas.
3. Remote routers install external routes using path to ASBR + external metric rules.

Inside the ASBR’s own area, routers reach the ASBR via Type 1/2 topology, so Type 4 is about inter-area awareness of the ASBR.

Type 5 — External LSA

ASBRs originate Type 5 LSAs for routes redistributed into OSPF (static, RIP, BGP, connected as external, etc.). External metrics may be:

• E1 (Type 1 external): external cost plus internal cost to ASBR (rough conceptual teaching).
• E2 (Type 2 external): external cost only; often default style in many designs—internal cost to ASBR does not add into the external metric the same way as E1.

HCIA may not demand deep E1/E2 math on every item, but know Type 5 = external, and E1 vs E2 is about how cost is considered.

Flooding and reliability (exam-level)

• LSAs age; refresh keeps them alive; max-age flushes stale data.
• Updates use multicast 224.0.0.5 / 224.0.0.6 (AllDRouters) depending on context (DR-related flooding optimizations on multi-access).
• Sequence numbers help routers prefer newer instances of the same LSA.

You need not memorize every packet subtype name beyond Hello, DBD, LSR, LSU, LSAck at associate level, but know that after Exchange/Loading, LSUs carry the actual LSA bodies.

Reading the LSDB on Huawei VRP

[Router] display ospf lsdb
[Router] display ospf lsdb router
[Router] display ospf lsdb network
[Router] display ospf lsdb summary
[Router] display ospf lsdb asbr
[Router] display ospf lsdb ase
[Router] display ospf routing

Map commands conceptually:

Correlate display ospf routing entries with codes that distinguish intra-area, inter-area, and external routes (exact legend letters can vary by version; understand the categories).

Worked multi-area story

• R1 (Area 1 internal) and R2 (ABR: Area 1 + Area 0), R3 (Area 0), R4 ASBR in Area 0 redistributing 203.0.113.0/24.
• In Area 1, R1’s LSDB has Type 1 (and Type 2 if multi-access) for Area 1 devices.
• R1 learns other OSPF internal prefixes in Area 0/elsewhere as Type 3 from ABR R2—not as remote Type 1s.
• R1 learns 203.0.113.0/24 as Type 5; it uses Type 4 (from ABR) plus Area 1 topology to reach the ASBR direction.

If the redistribution is removed, Type 5 disappears; Type 3 for true OSPF inter-area subnets remains.

Stub area awareness (light touch)

Stub areas block Type 5 externals; ABRs may inject a default instead. Totally stubby (vendor feature set) also blocks Type 3 except default. NSSA allows Type 7 local externals that convert to Type 5 at the NSSA ABR. HCIA may mention stubs to reduce external LSAs; do not let stub vocabulary overwrite Types 1–5 definitions.

Exam checklist

• Name Types 1–5 with originator and purpose in one sentence each.
• State Type 1/2 are area-scoped; ABR converts inter-area reachability to Type 3.
• Type 2 originator = DR.
• Type 5 = external; Type 3 = inter-area OSPF prefixes.
• Type 4 exists so other areas can reach the ASBR for external routes.
• Use LSDB displays to verify presence of each class when troubleshooting missing routes.

Final section: DR/BDR election mechanics and VRP configuration/verification commands that implement everything above.