What this note covers
Special area types shrink the LSDB and routing table of an area by
blocking certain LSA types at the ABR and replacing them with a
default route. Fewer LSAs means less memory, less SPF work, and
simpler routing for the routers inside — at the cost of losing
specific external/inter-area detail.
The four types at a glance
| Area type | Blocks Type 4 & 5 | Blocks Type 3 | Allows an ASBR (Type 7) |
|---|---|---|---|
| Stub | Yes | No | No |
| Totally Stubby | Yes | Yes | No |
| NSSA | Yes | No | Yes |
| Totally NSSA | Yes | Yes | Yes |
The pattern: “Stub” blocks external LSAs; “Totally” additionally
blocks inter-area (Type 3); “NSSA” re-allows a local ASBR via
Type 7.
Stub area
Behaviour
- Blocks Type 4 and Type 5 (external) LSAs from entering.
- The ABR injects a default route (0.0.0.0/0) via a Type 3
Summary LSA so internal routers can still reach the outside.- The same stub configuration must be applied on every router in
the area.- The E-bit (stub flag) in the Hello must match between neighbors
or the adjacency won’t form.- No ASBR can exist inside a stub area.
- Anchor:
area <area-id> stub.
Totally Stubby area
Behaviour
- Blocks Type 3, 4 and 5 — the area sees essentially nothing but
its own internal routes plus a default.- The ABR injects a single default route (via Type 3) covering
everything outside the area.- The ABR starts advertising that default into the totally stubby
area the moment one of its interfaces is assigned to area 0.- Cisco-proprietary. Anchor:
area <area-id> stub no-summary
(configured only on the ABR).
The problem NSSA solves
Why NSSA exists
Stub and totally stubby areas cannot contain an ASBR, because that
would require generating Type 5 LSAs — which those areas forbid.
But sometimes you need an external connection (a redistributed route)
inside an otherwise-stub area. NSSA is the workaround.
NSSA (Not So Stubby Area)
Behaviour
- Allows an ASBR inside the (otherwise stub) area.
- Type 5 is still forbidden; instead the ASBR originates
Type 7 (NSSA External) LSAs for its