06 - LSA Types & the LSDB

What this note covers

The LSDB (Link-State Database) is the collection of all LSAs a
router has received — its complete picture of the topology. Different
LSA types describe different pieces of that topology, and each is
generated by a specific role and flooded within a specific scope.
Once the LSDB is synchronized, SPF runs on it.

The LSDB

The database SPF runs on

• The LSDB must be identical on all routers within an area.
• It’s built from received LSAs, exchanged during adjacency
  formation (DBD → LSR → LSU → LSAck).
• Once every router in the area holds the same LSDB, each one runs
  SPF independently on that shared data.
• A topology change floods the affected LSA and triggers a partial
  SPF re-run — the database is updated, never wiped.

Transit vs Stub network

Two ways a link is described in a Router LSA

• Transit network — a segment with a DR election (multiple
  OSPF routers share it, traffic transits through it). Described using
  the DR’s IP.
• Stub network — a segment with no OSPF neighbors (e.g. a LAN
  with only hosts, or a loopback). Nothing transits it; it’s just an
  endpoint. Described by its logical network, mask and cost.

Note this “stub network” is a link descriptor — not the same as a
stub area (see 08 - Special Area Types (Stub - TS - NSSA)).

The LSA types

Type	Name	Generated by	Flooding scope
1	Router LSA	Every OSPF router	Within its area only
2	Network LSA	The DR (multi-access only)	Within its area only
3	Summary LSA	ABR	Into other areas
4	ASBR Summary	ABR	Into areas outside the ASBR’s
5	AS External	ASBR	Entire OSPF domain
7	NSSA External	ASBR inside an NSSA	Within the NSSA (→ translated to 5)

Type 1 — Router LSA

Note

• Generated by every OSPF device; describes all of that router’s
  links. Flooded only within the area where it was created.
• Describes four kinds of link: Transit (DR network),
  Stub (no neighbors), Point-to-Point (direct link to another
  OSPF device), and Virtual Link.
• Verify: show ip ospf database router.

Type 2 — Network LSA

Note

• Generated by the DR on multi-access segments.
• Lists the routers attached to the segment and the segment’s logical
  network. Flooded only within the area.
• No DR (point-to-point) → no Type 2 LSA.
• Verify: show ip ospf database network.

Type 3 — Summary LSA

Note

• Generated by the ABR to advertise the networks of one area
  into another (carries network, cost and next hop).
• Receiving this LSA does not trigger a full SPF run — inter-area
  routes are computed more simply (distance-vector-like between areas).
• Verify: show ip ospf database summary.

Type 4 — ASBR Summary

Note

• Generated by the ABR to tell routers in other areas how to
  reach the ASBR.
• Inside the ASBR’s own area, its identity is known via the Type 1
  LSA; outside that area, a Type 4 is needed.
• Verify: show ip ospf database asbr-summary.

Type 5 — AS External LSA

Note

• Generated by the ASBR for routes redistributed from outside
  the OSPF domain (another protocol, a default route, or directly
  connected external networks).
• Flooded through the entire OSPF domain, regardless of area
  (except into stub/NSSA areas, which block it).
• Verify: show ip ospf database external.

Type 7 — NSSA External LSA

Note

• Used inside an NSSA, where Type 5 is not allowed. An ASBR in the
  NSSA originates Type 7 for its external routes.
• The ABR translates Type 7 into Type 5 as the routes leave the
  NSSA toward the rest of the domain.

Redistribution uses two LSA types

When external routes enter OSPF, they’re carried as Type 5
normally, or as Type 7 when the ASBR sits inside an NSSA. That’s
the “redistribution via two LSA types” idea in one line.

Types you’ll see listed but rarely use

Obsolete / advanced — know they exist

• Type 6 (Group Membership / MOSPF) — multicast extension,
  obsolete, not used on modern networks.
• Type 8 / 9 (Opaque LSAs in OSPFv2) — generic containers for
  extensions like MPLS Traffic Engineering. In OSPFv3 the
  numbers 8 and 9 mean something different (Link LSA and Intra-Area
  Prefix LSA) — see 11 - OSPFv3 (IPv6).
• Type 10 / 11 (Opaque) — area-scope and AS-scope opaque LSAs,
  also for extensions like MPLS-TE.

These appear in the full LSA table but are not core to CCNA/early
CCNP study — recognize the names, don’t sink time here yet.

The SPF (Dijkstra) algorithm

Added — how the best path is actually computed

Once the LSDB is complete, each router runs Dijkstra’s SPF:

1. The router places itself as the root of the tree.
2. It examines the LSDB and adds the lowest-cost link to reach
   each neighbor, then the next lowest, building outward.
3. It keeps selecting the cheapest not-yet-added path until every
   network is placed on the shortest-path tree.
4. The results become intra-area routes in the routing table.

Key points:

• Every router computes the tree from its own perspective — the
  root is always “me,” so each router gets a different tree from the
  same LSDB.
• Cost is the metric it minimizes (see
  09 - Metric, Path Selection & Route Types).
• SPF is CPU-intensive, which is the whole reason areas exist: to
  limit how big the LSDB — and therefore the SPF calculation — gets.

LSA scope summary

Which LSAs cross area boundaries

• Type 1 and 2 never leave their area.
• Type 3 and 4 are how the ABR carries information between
  areas.
• Type 5 floods the whole domain (blocked only by stub/NSSA).