OSPF LSA types: the database is the protocol
OSPF's magic trick is that every router in an area holds the same map and runs the same math on it. The map is the link-state database, and LSAs are its pieces. Learn to read them and every OSPF question becomes an open-book exam.
The cast of characters
| LSA | Name | Who generates it | What it says |
|---|---|---|---|
| 1 | Router | Every router | "Here are my links inside this area" |
| 2 | Network | The DR | "Here's everyone on this multi-access segment" |
| 3 | Summary | ABRs | "I can reach this prefix in another area" |
| 4 | ASBR Summary | ABRs | "Here's how to find the ASBR" |
| 5 | External | ASBRs | "Here's a route from outside OSPF" |
| 7 | NSSA External | ASBRs in an NSSA | "Type 5, but wearing an NSSA disguise" |
Types 1 and 2 never leave their area. Everything you know about other areas comes secondhand through the ABR — which is exactly why OSPF areas scale.
Reading the database
R3# show ip ospf database
OSPF Router with ID (3.3.3.3) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 214 0x8000000B 0x00D2A1 2
3.3.3.3 3.3.3.3 190 0x80000007 0x00A3C4 3
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
10.2.0.0 2.2.2.2 120 0x80000003 0x00B871
That summary LSA from 2.2.2.2 tells you R2 is an ABR and area 0 learned
10.2.0.0 from it. No topology diagram needed — the database is the diagram.
Why this matters for stub areas
Every stub area type is just a rule about which LSAs are allowed in:
- Stub: no type 5s — externals become a default route.
- Totally stubby: no type 3s either — everything becomes a default route.
- NSSA: no type 5s, but locals can inject type 7s.
Once you see stub areas as LSA filters instead of magic keywords, the
area 1 stub no-summary command practically explains itself.
Next up in this series: SPF itself — what actually happens when the topology changes, and why OSPF loves a stable backbone.