Areas and LSA types
Every router in an area holds the whole map of that area, and every change triggers SPF on every router in it. Past a few hundred routers, or with unstable links, that gets expensive. Areas split the network so that detailed topology stays local.
- Area 0 is the backbone. Every other area must attach to it, and traffic between areas always crosses area 0. This hub-and-spoke rule is what keeps inter-area routing loop-free.
- An Area Border Router (ABR) has interfaces in area 0 and at least one other area. It keeps one LSDB per area.
- An Autonomous System Boundary Router (ASBR) injects routes from outside OSPF (static, connected or BGP) via redistribution.
LSA types you need to know
| Type | Name | Originated by | Flooded |
|---|---|---|---|
| 1 | Router | Every router | Within its area |
| 2 | Network | The DR on a broadcast segment | Within its area |
| 3 | Summary (network) | ABR | Into other areas — prefixes, not topology |
| 4 | ASBR-Summary | ABR | Tells other areas how to reach an ASBR |
| 5 | AS-External | ASBR | Every normal area |
| 7 | NSSA-External | ASBR inside an NSSA | Only the NSSA; the ABR converts it to Type 5 |
In show ip ospf route you'll see these as N (intra-area), N IA (inter-area, from Type 3) and N E1/N E2 (external). E2, the default, keeps the external metric fixed. E1 adds the internal cost to reach the ASBR.
Special area types
| Area type | Blocks | Gets instead |
|---|---|---|
| Stub | Type 4, 5 | A default route (Type 3) |
| Totally stubby | Type 3, 4, 5 | Only a default route |
| NSSA | Type 4, 5 | Can still have its own ASBR, using Type 7 |
Stub areas are ideal for branch sites with one way out: their routers keep a tiny database and still reach everything through the default route.
- Type 1 stays inside its area. Router LSAs (Type 1) — and Network LSAs (Type 2) on LAN segments — are flooded only within their own area. R3 sits in both areas, so it holds a separate LSDB for each and runs SPF per area.
- The ABR summarises. R3 does not pass area 1's map into area 0. It advertises reachability instead: Summary LSAs (Type 3) saying "10.4.0.0/16 is reachable via me at cost 20". With `area 1 range` this is a single aggregate.
- The ASBR injects external routes. R2 redistributes routes from outside OSPF as AS-External LSAs (Type 5), flooded to every normal area. ABRs add an ASBR-Summary (Type 4) so routers in other areas can find R2.
- Stub area: a default instead. Area 1 is a stub, so R3 blocks Type 5 (and Type 4) LSAs and sends a default route as a Type 3 instead. R4 keeps a tiny database: its own area plus summaries plus 0.0.0.0/0.
Check your understanding
A branch router in a stub area needs to reach a prefix that the ASBR redistributes from BGP. What happens?
- It can't reach it — Type 5 LSAs are blocked
- It reaches it via the default route the ABR injects
- The ABR converts the Type 5 into a Type 7 for the stub area
- The adjacency to the ABR resets