What this note covers
OSPF uses areas to divide a large routing domain into smaller
pieces. This keeps the LSDB and SPF workload manageable, contains the
impact of changes, and defines the roles routers play depending on
where they sit relative to area boundaries.
Why areas exist
The scaling logic
SPF is CPU-intensive and the LSDB grows with the topology. Splitting
the domain into areas means:
- A change floods and triggers SPF only within its own area, not
across the whole domain.- Routers hold the detailed LSDB of their area only; other areas
arrive as summaries.- Summarization becomes possible at area borders.
Cisco recommends no more than ~50 routers per area as a practical
guideline.
Single-area vs multi-area
Single-area
- All routers belong to the same area, which must be area 0
(the backbone).- Fine for small networks; every router shares one LSDB and runs SPF
over the whole topology.
Multi-area
- Routers are split across areas to gain: faster SPF, less
memory to store LSAs, lower CPU impact, and summarization.- Every area must connect to area 0 — the backbone is the hub
that all other areas attach to.
Area 0 — the backbone
The golden rule
All areas must connect to area 0, either directly or (when that’s
physically impossible) through a virtual link that tunnels across
a transit area. Inter-area traffic always passes through the backbone;
areas never exchange routes directly with each other. This prevents
routing loops between areas.
Router roles
Where each role sits
- Internal Router — all its interfaces are in a single area.
Runs one SPF for that area.- Backbone Router — has at least one interface in area 0.
- ABR (Area Border Router) — has interfaces in more than one
area (one of them typically area 0). It’s the only path in and
out of an area.- ASBR (Autonomous System Boundary Router) — connects OSPF to an
external routing domain (another protocol or the internet) and
redistributes routes into OSPF.
The ABR — the hardest-working router
Why ABRs do more
- An ABR maintains a separate LSDB and runs SPF for each area it
belongs to.- It’s the sole entry/exit point for its area — all inter-area
traffic and route information passes through it.- It generates Type 3 (Summary) LSAs to advertise one area’s
networks into another, and Type 4 to point the way to an ASBR.- It’s where summarization and inter-area filtering are configured
(see 10 - Filtering, Summarization & Virtual Links).
The ASBR
The domain's edge to the outside
- Sits at the boundary between OSPF and a different routing domain
(EIGRP, BGP, static, connected, a default route…).- Redistributes external routes into OSPF as Type 5 LSAs
(or Type 7 when it lives inside an NSSA).- Its identity is advertised to the rest of the domain via the ABR’s
Type 4 LSA.
Roles are not mutually exclusive
One router can wear several hats
A single router can be, for example, an internal router in one
sense and an ASBR at the same time, or both an ABR and a
backbone router. The role names describe positions relative to
area and domain boundaries, not fixed hardware categories — a
router’s roles depend entirely on where its interfaces land.