OSPF Area Planner
Split an OSPF domain into areas and size each LSDB, showing what stub and totally-stubby areas filter out.
Inputs
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
Values used
Routers in the domain = 120 routers; Non-backbone areas = 6 areas; Prefixes originated per area = 250 prefixes; External (type-5) prefixes redistributed = 5,000 prefixes; Area type = Stub — blocks type 5
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.
SPF scope
SPF cost scales with the intra-area topology, so routers per area is the figure to control.
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
How it works
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.
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.