OSPF Area Types
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Every non-backbone area answers one question: how much external noise are we willing to store?
| Area type | External routes (Type 5) | ABR injects | Typical use |
|---|---|---|---|
| Standard | Yes | — | Default behavior |
| Stub | No | Default route | Branches that just need "go up" |
| Totally stubby | No, and no inter-area | Default route only | Simplest possible branch |
| NSSA | Via Type 7 only | Optional default | Stub that still redistributes (e.g. a local ISP link) |
- Stub trades the full external table for one default route — the LSDB in a branch office shrinks dramatically.
- Totally stubby (Cisco flavor) goes further and blocks other areas' prefixes too: the ABR's default route is all you get.
- NSSA exists for the awkward case: "this area is a stub, but it also imports a few external routes." Those enter as Type 7 and convert to Type 5 at the ABR.
Restrictions worth remembering
Area 0 can't be stub. No virtual links through stub areas. Every router in the area must agree on the stub flag — it's carried in hellos, so a mismatch prevents adjacency (see Neighbor States).