OSPF Area Planner
Split an OSPF domain into areas and size each LSDB, showing what stub and totally-stubby areas filter out.
About OSPF Area Planner
Areas are a flooding and SPF boundary: type-1 and type-2 LSAs never leave, so each router only runs Dijkstra over its own area's topology. Stub variants then filter the summaries and externals coming in, which is where most of the LSDB reduction comes from. OSPF scaling problems are almost always LSDB size and SPF frequency rather than router count, and converting edge areas to stub or totally-stubby is the cheapest fix available — often a 10× reduction with a single command.
Formulas
OSPF Area
An area's LSDB holds its own type-1 and type-2 LSAs plus type-3 summaries from every other area and type-5 externals; stub areas drop type 5, totally-stubby areas drop type 3 as well.
- type 1/2
- Router and network LSAs, flooded only inside the area
- type 3
- Inter-area summaries injected by the ABR
- type 5/7
- External routes — type 5 domain-wide, type 7 the NSSA equivalent
SPF scope
SPF cost scales with the intra-area topology, so routers per area is the figure to control.
Worked example
With the default values below, OSPF Area Planner returns LSAs in the Area LSDB of 1,520. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Routers in the domainaccepts 2–5,000 routers
- 120 routers
- Non-backbone areasaccepts 1–200 areas
- 6 areas
- Prefixes originated per areaminimum 1 prefixes
- 250 prefixes
- External (type-5) prefixes redistributedminimum 0 prefixes
- 5,000 prefixes
- Area typeone of Standard area, Stub — blocks type 5, Totally stubby — blocks type 3 and 5, NSSA — type 5 replaced by type 7
- Stub — blocks type 5
Results
- LSAs in the Area LSDB
- 1,520
- Routers per Area
- 20.0
- Type-3 Summary LSAs
- 1,250
- Type-5 External LSAs
- 0
- Reduction vs a Standard Area
- 76.69%
- Design Verdict
- Within the 50-router-per-area design guideline
Step by step
- 1Values used
- 2OSPF Area
- 3SPF scope
- 4LSAs in the Area LSDB= 1,520
- 5Routers per Area= 20.0
- 6Type-3 Summary LSAs= 1,250
- 7Type-5 External LSAs= 0
- 8Reduction vs a Standard Area= 76.69
- 9Design Verdict= Within the 50-router-per-area design guideline
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is OSPF Area calculated?
An area's LSDB holds its own type-1 and type-2 LSAs plus type-3 summaries from every other area and type-5 externals; stub areas drop type 5, totally-stubby areas drop type 3 as well. Areas are a flooding and SPF boundary: type-1 and type-2 LSAs never leave, so each router only runs Dijkstra over its own area's topology. Stub variants then filter the summaries and externals coming in, which is where most of the LSDB reduction comes from.
Why does OSPF Area matter?
OSPF scaling problems are almost always LSDB size and SPF frequency rather than router count, and converting edge areas to stub or totally-stubby is the cheapest fix available — often a 10× reduction with a single command.
What values do I need to enter?
This calculator takes 5 inputs: Routers in the domain, Non-backbone areas, Prefixes originated per area, External (type-5) prefixes redistributed, Area type. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
Why does every area have to touch area 0?
OSPF is a two-level hierarchy with a loop-free assumption: inter-area routes travel via the backbone, and an ABR only advertises summaries it learned there. An area detached from area 0 needs a virtual link or a tunnel, and both are workarounds rather than designs.