Split an OSPF domain into areas and size each LSDB, showing what stub and totally-stubby areas filter out.
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.
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.
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.
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.
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.
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.