OSPF Area Designer
Describe the routers, their areas and the link costs. It computes the paths, shows where traffic splits across equal-cost links, and checks the area design against the rules that actually stop OSPF working.
Local toolYour network plan stays in your browser. Nothing you enter is uploaded, logged, or sent to any server.
Routers
Links
Design checks
Path between two routers
The rules that actually matter
Area 0 is the backbone and everything hangs off it. Inter-area routing goes through area 0 by design — there is no path from area 1 to area 2 that avoids it. So every non-backbone area needs at least one router that is also in area 0. That router is an area border router.
The backbone must not be in pieces. Two groups of area 0 routers that cannot reach each other within area 0 partition it, and inter-area routing breaks even though every area looks correctly attached. This is checked separately because it is easy to create and hard to see.
Both ends of a link must agree on the area. If they do not, no adjacency forms at all — the interfaces come up, OSPF says nothing useful, and the routes never appear.
Router IDs must be unique. Duplicates cause adjacencies to fail or flap, and the symptom rarely points at the cause.
A virtual link can attach a detached area to the backbone. It is a repair for a design that went wrong, not a design, and treating it as one produces networks nobody can reason about.
Related tools
OSPF Cost Calculator explains where costs come from. BGP Best-Path Simulator covers route selection between autonomous systems.