Why does OSPF use areas, and what does a multi-area design buy you?
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Areas contain the cost of link-state routing. Type 1 and Type 2 LSAs stay inside their area, so a flap only triggers a full SPF in that area. Other areas see a Type 3 summary change at most, which is a cheap partial calculation, and with summarization at the ABR they see nothing at all.
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What areas reduce
- LSDB size. Routers hold full topology only for their own areas.
- SPF runs. A topology change inside area 1 does not run SPF in area 2.
- Flooding. Type 1 and 2 LSAs never leave their area.
- Routing table size, with summarization at ABRs and stub area types.
The rules
- Every area must attach to area 0, the backbone.
- All inter-area traffic goes through area 0.
- Routers with interfaces in area 0 and another area are ABRs.
How big can one area be
There is no fixed number. Modern routers handle hundreds of routers in a single area. I split when there is a real reason: unstable WAN links I want to isolate, a need to summarize, or small branch routers that should only get a default route.
They will ask next
Is an area boundary on a router or on a link?
On a router. Each interface belongs to exactly one area, and the ABR sits on the boundary. This differs from IS-IS, where the boundary is on the link.
Traps
- Quoting "50 routers per area" as a rule. It was old guidance, not a protocol limit.
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